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Q: At one time, meterologists never predicted that hurricanes hit Hawaii, only storms. Is the hurricane season that Hawaii experiences a change in the weather patterns or is it simply that the weather is announced differently? Aloha.....

--Valerie, Honolulu

A: Dear Valerie,

We consulted with staff at the National Weather Service (NWS) office in Honolulu, who are in the best position to know the "official" policy over the years. They pointed out that Hawaii has taken 4 direct/almost direct hits by hurricanes in the last 50 years -- Nina in 1957, Dot in 1959, Iwa in 1982, and Iniki in 1992 -- and all were correctly identified in newspaper reports at the time as hurricanes. Furthermore, their personal experience reaching back into the 1980s is that the terms Tropical Depression, Tropical Storm, and Hurricane have been used in forecasts, both by the NWS and the media.

It is possible that recent years' events have sensitized both the media and the public to the importance of such information, resulting in more frequent mention. The NWS Honolulu staff also pointed out that the predominantly wintertime Kona storms can have similar effects, but are "subtropical", rather than "tropical", and therefore don't get the Hurricane label.

Even though we didn't uncover a shift in terminology, it is true that tropical storm activity fluctuates over decades of time as the background atmosphere/ocean system undergoes subtle shifts that favor or suppress tropical storm formation for a number of years at a time. The NWS Honolulu staff report that the late 1980s and 1990s were particularly active, but since then it has been quieter. The central Pacific anomaly is generally the reverse of that in the tropical Atlantic, which appears to be in the midst of a long interval of enhanced tropical storm activity.

Q: I know the moon gets its color from reflecting from the sun; however, why is it that the moon is sometimes a pale yellow, other times a bright yellow, and still other times an orange?

--Sarah, Chelmsford, MA, USA

A: Dear Sarah,

The true color of the Moon, as shown in the multitude of pictures from the Apollo flights, varies from nearly white through shades of gray. Shadows on the moon are very crisp and dark because there is no atmosphere to scatter light. The colors that you see on Earth are the result of scattering by the Earth's atmosphere as the Moon's light travels to your eyes. Just as with the Sun or distant clouds, the atmosphere scatters light out of the line of sight between the distant object and your location. More "gunk" (aerosols, pollutants, water vapor) in the atmosphere causes more scattering. Blue wavelengths are scattered the most and red wavelengths the least, so the original white/gray Moonlight shades through yellows and oranges as the atmosphere's load of gunk builds up. The moon is usually the most red (and dimmest) right at the horizon on a summer evening, where the light's path through the atmosphere is the longest and the gunk is the densest, while a moon overhead on a cold, clear winter night seems nearly unaffected.

Q: What causes the massive storm surge that comes with a hurricane?

--Mike, Mississippi, USA

A: Dear Mike,

There is an excellent web site on this topic:
Storm Surge

To quote: "Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm tide. ... The level of surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast ... will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf ... will not see as much surge inundation, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors severely damage ships, marinas, and pleasure boats."

Q: How deep will water freeze like in a lake? How deep will it freeze in muskkeg like in Alaska, or even hard ground?

--Stan, Texas, USA

A: Dear Stan,

Not too deep in Texas! The colder the air and the longer it is cold, the more heat will be lost by the ground and the deeper the frozen layer. This is how permafrost forms. There the freezing temperatures can be hundreds of feet, but it takes hundreds or thousands of years to form. However, if the winters become less cold, or the summers become warmer, this additional heat will begin to thaw the permafrost from the surface downward. This is happening now in the Arctic.

Lake freezing is more complicated. As water cools, it become denser, so in a lake this cooler denser water sinks and is replaced at the surface by warmer water. This sinking of cooler water and rising of warmer water continues until all the water in the lake reaches 4 degrees Celsius. This is a very special temperature for water because at that temperature further cooling makes the water LESS dense and it stays on top of the lake where it will soon freeze. Because the entire lake is very cold (it's all at 4 degrees C) before any water begins to freeze, the ice cover can get thick very quickly. This is why it takes so long for lakes to freeze in the early winter, but once they do, a relatively short cold spell will form thick ice on the lake. Happy skating!

Q: If humans were to put solar energy panels in antarctica and use them to power freezers to constantly cool the water around the pole, could we slow the melting process and reduce the rate of water levels rising?

--Adam, Sydney, AUS

A: Dear Adam,

I'm glad you're concerned about the warming polar regions and rising sea level. I am, too. But I'm afraid your idea violates thermodynamics. Freezers certainly cool--usually the air in an enclosed space, but heat is also generated in the process. Freezers usually work by compressing a gas and then allowing it to expand in the tubes surrounding the space to be cooled. As it expands, it cools and absorbs heat from the surrounding air. So far, so good. The problem comes, when the gas has to be compressed again. Now it releases heat that must be dealt with. Overall, the freezing and heating are about equal, but add the heat of the compressor motor to run the freezer and you've generated more heat than cold.

Q: I've heard different theories on global warming. One was that when the ice caps melt the sea levels will rise, where as another theory says the sea level would expand onto low lying land. Which of these theories, or others i haven't mentioned, is correct?


A: Dear Melanie,

When ice caps shrink, either by melting or flowing into the ocean, the oceans do get higher but they also flood land next to the ocean. Because the slope of the land is very shallow along most coastlines, a small vertical rise in sea will cause the shoreline to retreat a long ways inland. Remember the oceans are vast, 3/4 of the planet surface. It takes a lot of water (or ice) to change sea level. But the Antarctic ice sheet is big too. If 1% of the Antarctic ice sheet were to suddenly be put into to ocean, sea level would rise about two feet. Along a beach where the slope at the shore is just 1:100, the shoreline would move inland 200 feet--quite a change!

Q: In my 7th grade science class, we are observing the phase changes by heating ice and watching it melt while recording the temperature every minute to graph and analyze the results. I want to make this more meaningful to them. By looking at a graph we learned that the rate of ice melting changes when heated. There were slower and faster time periods of ice melting. My question is do scientists analyze the rate of ice melting to predict how the greenhouse effect will cause ice to melt around the world?

--Therese, Wisconsin, USA

A: Dear Therese,

Your question is a good one. Climate records confirm that when the world is warmer, there is less ice. Melting is not the only process responsible for this connection, but it is a very important one.

There are various ways the heat of a warmer world can get to the ice to melt it. One is radiation from the atmosphere (and clouds). This is how most glaciers lose ice during the summer (and we lose our winter snow). Another very effective way to deliver heat to glaciers is rain. Even cold rain carries with it lots of heat. Some large ice sheets enter the ocean at their margins, and the ocean melts the ice edge. The extreme case is an iceberg that is floating in water. They don't last long as they drift into warmer water.

The best example of how scientists measure the amount of heat and compare it to the amount of melting is in the study of glaciers. Temperature records can be expressed as "positive degree days" (PDD). PDD is the sum of each day the temperature is above the melting point of ice multiplied by the number of degrees above melting for that day. For example, if the very short summer had only three days above melting and the maximum temperatures on those days was 1,1 and 3 degrees above melting (we usually use Centigrade degrees), then the PPD for that summer would be 5. This number is compared with the mass loss due to melting on the glacier, usually expresses as the amount the glacier surface lowered. In general, these two variables are correlated. Warm summers have high PPD values and match large lowerings of the glacier surface.

These types of measurements are done on many glaciers, but extrapolating to all the glaciers in the world is difficult. Satellite measurements are a great help in getting this type of information everywhere. It is research that NASA scientists are working on right now.

Q: What is it like at the edge of our atmosphere and space? What happens at the point where the two meet? How can the Earth's movement through space not affect our atmoshpere?

--Tim, Michigan, USA

A: Dear Tim,

The atmosphere becomes thinner and thinner as you go upward. Down here at the Earth's surface, a cubic foot of air weighs 1.3 ounces (16 ounces to a pound). At 50 miles above the Earth's surface, a cubic foot of air weighs 0.000014 ounces, about 100,000 times lighter than at the surface! The air becomes thinner and thinner as you go up. This is why mountain climbers need to bring along oxygen tanks for climbing Mt. Everest. So, there really is no point where space and the atmosphere meet. The atmosphere simply decreases and decreases until you're left with space with no oxygen or nitrogen molecules.

Empty space has no effect (or drag) on the atmosphere. The forces acting on the Earth are primarily the Sun's gravitation and the Moon's gravitation (they both induce tides on the oceans and atmosphere). Space itself exerts no force.

Q: What causes hurricanes to form, and why have we had so many this year?

--Diana, Roanake, VA

A: Dear Diana,

The Earth Observatory has a good, detailed reference on how hurricanes form and intensify. You can find it at the following URL:

The (very) basic explanation is that hurricanes form over tropical waters where sea surface temperatures are warm, humidity is high, pressure is relatively low, and winds throughout the depth of the troposphere (the active weather part of the atmosphere) aren't too strongly varying. Hurricanes get started when surface winds in the tropics converge—come together—at a given location. The winds can't just pile the air up in one place forever, however, so air begins to rise upward from the surface, setting off thunderstorms.

When conditions are favorable, a continual cycle inside the storms of evaporation of water vapor from the ocean surface and its subsequent condensation higher in the atmosphere continually increases the buoyancy of the air. The buoyant air rises, and more air flows in at the bottom. These surface winds fuel more evaporation, which further increases the air's buoyancy, which causes more air to rise, and so the storm strengthens. If these clusters of thunderstorms are located more than 10 or 15 degrees of latitude from the equator, the winds entering and exiting the storm begin to follow curved trajectories due to the Coriolis Force—a product of the Earth's rotation. These curving winds are what gives hurricanes their "spiral" appearance.

Forecasting the exact number of tropical cyclones that will happen in any given year is still a matter of research, but in the last few years researchers have started providing useful estimates. Dr. William Gray (Colorado State University) is the founder of these studies, and the National Hurricane Center (NHC) has recently started issuing outlooks. See for the NHC mid-season update, which has a very nice discussion of the main factors in this year's above-average activity. Summarizing, they cite an active multi-decadal signal (i.e., we're in a decade-long period of increased activity), above-average Atlantic Ocean temperatures, and exceptionally favorable wind and air pressure patterns on the average in the regions where Atlantic tropical cyclones form. What we cannot yet do is to forecast weeks or months in advance where and when specific tropical cyclones will form and what their strength will be.

Q: I have to ask about the data set that shows the heat radiation from Earth into space. It is a color coded graph and the colors are watts per square meter. What I wonder about is the scale going from 85w/m2 to 350w/m2. Since the coldest temperature on Earth is about 225 kelvin which is about 150w/m2 how can a 85w/m2 area exist? Is there is something wrong with the calibration? I think it should start at 185w/m2(~235k) the higher end seems about right.

--Jim, U.S.A.

A: Dear Jim,

The coldest surface temps (antarctic winter) reach about -70C or 203K: but even this doesn't set the low end. The coldest objects on the planet are not at the poles but in the tropics! Deep convective cloud towers can reach 17km altitude and temps of 190K. This is why the fluxes can go so low. sigma T^4 of 190K is about 75 W/m^2. Note that 0.5% of the tropics is covered by clouds of 205K or colder temps, so while 190K is extreme, 225K is way to warm.

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Q: How were the polar ice caps formed, and where did the water that they are made of come from?

--Jessica, Lake Peekskill, NY, USA

A: Dear Jessica,

The polar ice caps formed from snow fall, and the moisture in the snow originally came from the ocean. Water is picked up by the atmosphere and dropped on ice caps. These ice caps are also called ice sheets, which cover most of Greenland and Antarctica, over 2 miles thick in places. The North Pole is an ocean with thin layer of frozen ocean, called sea ice. Snow falling on ice sheets packs into solid ice, and the ice flows back into ocean, some as icebergs and some as water where caps melt at edges in summer. We don't know whether the ice sheets are growing or shinking now, but the laser altimeter on NASA's ICESat(to be launched in the summer of 2002)is designed to tell us whether the ice sheets are getting thicker or thinner.

Q: How can scientists measure fluctuations in carbon sequestration in the forest?

--Akbar, Kuala Lumpur, Malaysia

A: Dear Akbar,

This is a question that a lot of scientists are thinking about now. We can't measure fluctuations in sequestration any where near as well as we need to in order to judge forest impacts on atmospheric CO2 concentrations.

Carbon sequestration in forests can occur by storage in above ground wood (trunks and limbs) resulting from more trees or bigger trees. It can also occur through increases in below ground wood (large roots). Another way is through accumulation of dead material both above ground and below ground and is caused by slower decomposition (or fire suppression) relative to production.

Most measurements are of above ground living wood (because of its direct economic value) and involve many simple measurements of girth and heights. One can also look at tree rings to study past fluctuations. Because these measurements are of individual trees and each tree is exposed to slightly different conditions (soil type, slope exposure, pathogens, etc) you have to make a lot of measurements to get an average for a particular area. It is possible to measure the regrowth of forests using satellite observations. As forests get thicker the sunlight reflected into space changes and we use this signal to estimate changes in forest growth from year to year.

Q: Has the spinning sphere shape of the Earth allowed its atmosphere to flow over the entire Earth creating a planet where life can exist? If the Earth had been cube-shaped, could life exist?

--Johnna, Folkston, GA

A: Dear Johnna,

This is a great thought question for a Ph.D. exam! I would say "probably yes" to your second question. We'll ignore the issue that the Earth, though solid, is not nearly rigid enough to avoid the near-spherical shape. And, let's assume that the masses of solid Earth, water, and atmosphere are about the same on "Cube World" as on Earth. Even though the solid Earth is cubic, the water and atmosphere would still respond to gravity. So, they would puddle on each face of the cube, which is the part of the planet that is closest to its center of mass. Compared to the volume of solid Earth, the other two components are small, so each face would have some ocean, deepest in the center, and then land surrounding that. The atmospheric pressure would be greatest at the ocean's surface and decrease as you climbed toward the edges of the cube. Without doing the calculation, I'm not sure what the pressure distribution would be, but it might well turn out that the edges would have pressures too low to support life. Thus, life forms on each face might have to develop independently. Finally, there are some important parameters left unstated. Does Cube World rotate? How fast? Is the axis of rotation tilted? Where on the cube is the North Pole? Each combination of parameters might give a different environment, some less hospitable than others.

Q: How high above the earth must one go in order to see a curved horizon instead of a flat horizon?

--Bob, Little Rock, AR

A: Dear Bob,

Any time you're above the surface of a smooth sphere you see a curved horizon! In the limit of being far away, the "horizon" is the edge of the disk that the Earth obscures in your field of vision. [Think of the pictures of the Earth taken from lunar orbit.] As you move down toward the surface the apparent size of the disk grows, but fundamentally continues to be a disk (with a curved edge) right up until your nose is touching the ground. In that case the "disk" covers half your 360-degree field of view and appears to be a flat plane. Having a rough surface complicates the picture a little, but compared to the radius of the Earth, even Mt. Everest is pretty small. Furthermore, our normal experience is so close to ground level (again, compared to the radius of the Earth), that the curvature of the horizon is imperceptible.

Q: If we keep burning fossil fuels and natural gases and keep driving our cars every day, and the pollution rate does not decrease, how long will the earth be inhabitable?

--Anthony, Zephyr Hills, Florida

A: Dear Anthony,

You've asked an incredibly important question, but also one that is difficult to answer. We don't have a very good idea of how to define at what point the planet will become uninhabitable because its hard to predict how the earth will respond to the impact of human activities on the climate. From ice core data we know that over the past 450,000 years there has been a natural oscillation in atmospheric CO2 levels and average global temperature, with warmer temperatures occurring with higher CO2 levels.

In this time period, until the start of the industrial revolution, atmospheric C02 varied between 200-300 parts per million (ppm). Now, with the increased burning of fossil fuels and deforestation, this level has surpassed 370 ppm, a condition that the earth has not experienced in the last half a million years.

So the task is to understand how the earth, as a system, will respond to this sudden large perturbation, and a tremendous amount of research is currently being done to address this question. To do this one must view the earth as a whole, and look at the interactions between the atmosphere, oceans and land. The oceans play an important role in the global carbon cycle because CO2 is very soluble in seawater, there is 50 times as much carbon dissolved in the ocean as exists in the atmosphere. Most of the research involves computer models that simulate the circulation in the atmosphere and ocean, and the interactions between them to try to understand the long-term effects of increased C02.

However the models are only as good as the knowledge of the processes that are put into them and there are feedbacks in the systems that are still not understood well enough to be modeled correctly. One of the big questions involves what is referred to as the "missing sink" of carbon. The amount of excess CO2 in the atmosphere is less than the amount that has been emitted into the atmosphere from fossil fuel burning, even accounting for the amount of carbon that the oceanic and terrestrial systems have taken up. This means that there must be another sink for the C02 that we are not aware of. The issue of the missing carbon sink underscores the fact that we do not fully understand fundamental aspects of how the earth is currently operating, which undermines our ability to predict how it will behave in the future. Current models suggest that a doubling of atmospheric CO2 will increase the global temperature by 1.5-5.5°C. However this is a global average and to understand its impact on humans, it also necessary to know what will be the regional patterns of this response. Certainly coastal areas will be heavily impacted by rising sea levels as glaciers and the polar ice caps melt. How well humans will be able to adapt to the changing conditions will depend on how fast the changes occur among other things.

Q: What causes the high (in 1971,82, 83, 84, 95, 96) and low (in 1963, 65, 67, 76, 78, 85, 97)fluctuation of the annual percipitation in Northwest Oregon (Willamette Valley)?

--Heidi, Marina, California

A: Dear Heidi,

There are two main processes that affect the long term climate variability in the Northwest USA: El Niño events and the Pacific Decadal Oscillation (PDO). El Niño events, and their counterpart, La Niña events, are part of a natural oscillation of the coupled ocean-atmosphere system in the tropical Pacific referred to as ENSO (El Niño-Southern Oscillation). While the dominant effects are seen near the equator, these events can also impact the climate further away. Spring and fall seasons tend to be drier in the NW Pacific during El Niño events, and La Niña events, which usual follow directly after El Niño events, cause wetter falls and winters. El Niño events typically start in the fall and last through to the following spring. The last three biggest El Niño events occurred in 1972/73, 1982/83 and 1997/98.

Precipitation in the NW Pacific is also impacted by the PDO, which is a type of oscillation like ENSO, but the events last much longer, persisting 10 or more years as opposed to ENSO events which typically are over in less than a year. Also the main signatures of the PDO are seen in the North Pacific/North America region, not in the tropics. A cool and wet phase of the PDO existed during 1947-1976, which would tend towards wetter conditions in Oregon. Since 1977 the PDO has been in a warm and dry phase, which would tend towards drier conditions in Oregon.

While not all the anomalous years that you listed fit the patterns outlined above, a more detailed comparison between NW Pacific precipitation and the ENSO cycle and the PDO shows a correlation between them.

Q: Can you please give me more information about the radiation levels on Earth. Just on the three types: Reflected Solar Radiation and Longwave Radiation and Net Radiation. Are there significent changes of these levels? Is radiation increasing on Earth from the ozone and the microwave, man-made radiation?

--Michelle, Melbourne, Victoria, Australia

A: Dear Michelle,

Just to make sure we're all on the same page, the "radiation" being discussed here is the radiant energy in various parts of the electromagnetic spectrum, which we experience as radiant heat (longwave), light (shortwave), various radio waves (including microwave, AM, FM, etc.), and cosmic rays. The Sun is so hot that much of the radiant energy it emits is in the form of visible light. About 40% of the sunlight striking the Earth is reflected by the Earth system (including clouds), and the rest is absorbed by the ocean, land, and atmosphere (in order of importance). In turn, different parts of the Earth system pass around the heat by conduction (into and out of the solid earth); transport (by ocean and wind currents); conversions among gaseous, liquid, and solid water (evaporation, condensation, sublimation, deposition, freezing, melting); and emission and absorption of radiant energy. Finally, the Earth emits as much radiant energy back to space as it receives from the Sun. This emission to space occurs mostly in the longwave band because the Earth system is relatively cool.

The point of the concern over the increase in CO2 and the other "radiatively active trace gases" (which includes water vapor) is that they change the way in which the solar energy captured by the Earth is redistributed on its way back to space. In isolation, increases in these gases act to slow the rate at which radiant energy returns to space, heating the earth's surface and atmosphere. What we do not yet know with precision is how other parts of the Earth system react when this heating tendency is inserted into the real Earth system. In particular, the response of clouds is still an active area of study. One major barrier to understanding is that the energy budget is highly complex and variable in space and time.

The direct human contribution of radiant energy, including microwave signals, is tiny compared to the total energy budget. Our release of CO2 is much more important.

Q: Was the Gulf of Mexico created by a giant meteor that Impacted the earth? And if so, does this strange oil leak that seeps out of this gulf have anything to do with this event? could this place be the site of origin of the Microbial soup theory?

--Antonio, Santa Rosa, CA U.S.A.

A: Dear Antonio,

The Gulf of Mexico is too large to have been formed by a meteor impact. However, the Chicxulub crater, which is believed to be the largest meteorcrater on the earth, is found off the Yucatan Peninsula in the Gulf of Mexico. The Chicxulub crater is 150-300 km in diameter, whereas the Gulf of Mexico is more than 1500 km in diameter. The Gulf of Mexico already existed when the Chicxulub meteor hit. The origins of the oil in the Gulf of Mexico have to do with its geological history. Oil is formed from unoxidized organic material, organic material being decaying plant and animal matter. During the Jurassic Period (140 million years ago) the entire Gulf of Mexico dried up, as it evaporated the water became depleted in oxygen and layers of black shale were deposited. Geological heating of these shales over millions of years created the oil.

The type of environment that first led to the creation of life is still debated. Some scientists have suggested this could have occurred at hydrothermal vents. Hydrothermal vent environments are in some ways similar to the seeps found in the Gulf of Mexico, but they are usually found in areas of the ocean with active volcanic activity. In contrast to the gulf seeps hydrothermal vents emit very hot water (300řC, three times hotter than boiling water!) and it is this extreme heat that is thought to have been the catalyst for reactions leading to the formation of life. While we will probably never be able to locate exactly where this happened, we can be fairly certain that it did not occur in the Gulf of Mexico. The earliest evidence of life on earth is 3.5 billion years ago, and at that time the continents and oceans had a very different configuration, and the Gulf of Mexico did not exist as it does today.

Q: With all the talk of global warming and the cause (many blaming human release of various gases), didn't the eruption of Mount Pinatubo produce a higher percentage of the harmful gases that cause global warming than humans ever did or will?

--Julie, Niagra Falls, NY U.S.A.

A: Dear Julie,

There is no question that Pinatubo and similar volcanic eruptions spew massive amounts of gunk into the atmosphere. ["Gunk" is a technical term for "gases and particulates that you wouldn't want to take home."] However, the particular materials ejected normally have the net effect of cooling the Earth, rather than warming it. The volcano only has a global effect when the eruption is powerful enough to send its plume into the lower stratosphere, in regions where there is relatively little mixing and turbulence. In a short time the heavier particles fall out and most of the gases chemically convert to aerosols. However, the remanents can persist for years and spread over most of the globe, acting principally to reflect sunlight and thus cool the Earth. In the case of Pinatubo the cooling lasted for about a year, during which time the stratospheric contamination was visible as faint pink shading just after sunset on the clearest evenings.

Q: Where did Earth's atmosphere come from?

--Ronald, Plainfield, NJ U.S.A.

A: Dear Ronald,

The atmosphere, as well as the ocean and solid earth, is the result of a long process of accretion, first from the original disk of tiny fragments that surrounded the early Sun, and subsequently from meteors and comets. As well, the formation of a free atmosphere depended on the formation and release of gaseous molecules, such as molecular oxygen (O2), as opposed to the constituents being bound in solid molecules, such as silicon dioxide (SiO2). A few gases were so light that the molecular or atomic forms have largely escaped from the atmosphere, including hydrogen and the well-named rare gases. Finally, the present composition of the atmosphere is the result of long-term interactions between the atmosphere and the biosphere. For example, it is thought that molecular oxygen only appeared when plants started giving it off as a by-product of photosynthesis.

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Q: I am a freshman at Dr. Michael M. Krop High School. There is a thin, halo-like, rainbow around the sun. The sun itself, is giving off a very light, white light. This rainbow-halo was spotted between 12:00 and 1:00 o'clock, when the sun was very high in the sky. I, as well as the entire school of students, would like to know what this is.

--Krishton, Miami, FL U.S.A

A: Dear Krishton,

You hit it on the head - it was a halo. It results from the refraction of sun or moon light by hexagonal plate-like ice crystals that are approximately 30 microns in size. Larger-sized particles give a greater separation of colors than smaller-sized particles. Clouds that contain such ice particles are less common in subtropical locations such as Miami than in areas further north, so it's possible that most of the students have never seen such a phenomenon. Note the related answer on this site on "moon dogs." The halo results from randomly oriented crystals, whereas the sun dogs and moon dogs are specifically the result of horizontally oriented crystals. Near local noontime the viewing angles strongly favor the halo.

Q: I was reading the web article about utilising satellite imagery of glaciers to monitor changes in climate. It is a very good set of pages. I was wondering what is done in such research to account for rock fall on to glaciers covering up the ice. It would seem to me that this would give a inaccurate reading of the actual amount of ice that is changing, especially because of the fact the as a glacier melts it would be expected that more rocky material would be exposed at the surface. As rock that had previously been trapped in the ice would get exposed as ice melts. How is such a discrepancy accounted for or is just assumed minimal and ignored?

--Nicholas, Unknown

A: Dear Nicholas,

You are correct that debris on a glacier affects the accuracy of measuring changes in a glacier terminus from space. Rocks and other material that are included in the ice tend to accumulate on the surface as the glacier melts down. This, as you've suggested, is a greater problem on a retreating or stagnant glacier, and is not generally a concern on an advancing glacier. Sometimes, even on the ground, it is difficult to distinguish the glacier tongue from the surrounding morainal material. The accuracy improves when we have multiple years of satellite images to study. In spite of such problems, the accuracy of the measurements have been validated by field measurements and can be as high as plus or minus 30 meters (the size of a Landsat pixel).

Q: I have a question concerning phytoplankton. What happens when they die, in that how does carbon get recycled?

--Wajeeha, Unknown

A: Dear Wajeeha,

Phytoplankton are grazed by zooplankton which package their waste products into fecal pellets that sink through the water column. Most of these particles decompose or are consumed by other orgamisms in the surface layer, hence recycling the carbon and nutrients. Only about 1% of sinking organic material reaches the seafloor.

Q: I've seen recent news reports suggesting that global warming could "switch off" the warm waters of the Gulf Stream - resulting in a colder climate for North west Europe. Is there any evidence that this is happening now, and what are the best predictions for this effect in future years.

--Phil, Manchester, UK

A: Dear Phil,

There is no adequate computer model to address this question. The Gulf Stream is unlikely to stop, but its course could be altered. In the far North Atlantic Ocean one of the complicating factors is as follows:

The thermohaline circulation carries water from the surface in polar regions to the deep ocean, where it circulates as bottom water before being recycled to the surface thousands of miles away. The name indicates that the circulation is driven by temperature (thermo) and salinity (haline) differences between different masses of ocean water. Geological evidence suggests that this circulation has fluctuated significantly in the past, perhaps on timescales of less than a century. It is currently a matter of research as to the conditions that might provoke such a transition. A warming trend should favor a reduction in the thermohaline circulation, but it is very hard to assess its strength and the necessary computer models for forecasting are still relatively primitive.

Q: How long does it take for light from the sun to reach the earth?

--Byron, Dothan, Alabama

A: Dear Byron,

Working in round numbers and English units, the Sun is about 93 million miles away, and the speed of light is roughly 186 thousand miles per second, so the travel time of light from the Sun to the Earth is about 8.3 minutes.

Q: What is the sensitivity, (percipitation density) required for rain/moisture to be "seen" by Doppler radar?

--R., Ontario, CA

A: Dear R.,

There are many different models of weather radar that have Doppler capabilities. That is, in addition to sensing the reflected power from a volume, it can also use the Doppler effect to interpret the phase shift contained in the returned energy, providing information about velocities along the radar beam. The most common Doppler radar in the United States is the National Weather Service's Weather Service Radar - 1988, Doppler (WSR-88D). These systems blanket the country. Sometimes still referred to by its development name (Next Generation Radar, or NEXRAD), the WSR-88D is a key tool for real-time analysis of weather situations. It has two sensitivities, "clear air" and "precipitation." As the name implies, clear air mode is able to pick up echoes from dense clouds, before precipitation ever starts. This capability is used directly to track the clouds, and also to gain Doppler velocity information over a wider area than would be possible from precipitation alone. Under the right conditions clear air mode will pick up echoes from other airborne objects, most notably large concentrations of dust and/or insects, and large flocks of birds. However, the Doppler signal from self-mobile objects (insects, birds, etc.) is not useful for studying wind conditions!

Q: Why leaving Earth's atmosphere, does it appear to be smooth sailing, passing from blue sky, to space, when re-entering the earth's atmosphere, you could essentially burn up? Where does the heat and fire come from, when it is not there in the beginning?

--Ashlee, Austin, TX

A: Dear Ashlee,

The heat in the reentry phase is due to friction between the spacecraft and the air. In the case of the space shuttle, the de-orbit burn reduces the forward speed by about 200 mi/hr (300 km/hr) to 17,100 mi/hr (27,400 km/hr). Some 30 minutes later the orbit has decayed from an altitude of about 250 mi (400 km) to 400,000 ft ( 122,000 m) and the atmosphere becomes dense enough to exert significant drag. About 18 minutes later the shuttle is descending through 180,000 ft (55,000 m), having slowed to 8300 mi/hr (13,000) km/hr). All that extra kinetic energy has been converted to heat. The same concept applies to the meteors you see streaking across the night sky.

When a space vehicle is launched, enormous amounts of energy are required to lift it away from the surface. Thus, lift-off is a somewhat slower process, and the boost phase can be designed so that the really high speeds needed to achieve orbit are not attained until the vehicle is above the level at which friction drains away energy as wasted heating. Even so, the ride isn't exactly smooth sailing! The space shuttle exceeds the speed of sound about 44 seconds after launch, and supersonic flight up through the remaining layers of the atmosphere can be pretty bumpy. Also, the standard shuttle acceleration is about three times normal gravity. Needless to say, *everyone* stays in their seats with their seat belts fastened until they reach orbit!

Presumably the fiery reentry could be avoided by using enough retrorocket power to slow the spacecraft. However, the cost of lifting the fuel and rockets into orbit is so great that the use of atmospheric friction to achieve the reentry deceleration is absolutely standard.

Q: How does the earth spin on its axis and does the speed affect weather?

--David, Birmingham

A: Dear David,

The Earth's spin is mostly an accumulation of the spin that was contained in the huge number of chunks that went into building up the Earth at the beginning of its existence. Major effects since then have been tidal forces caused by the Sun and Moon and a slow transfer of energy to the Moon's orbital speed.

The effect of the Earth's rotation on the weather is two-fold. First, the daily cycle of the Sun drives local weather circulations, most notably seabreeze/landbreeze circulations and the afternoon maximum in thunderstorms over land. The speed of rotation governs how long these circulations have to become established and evolve. The second effect of the Earth's rotation is that the rotation causes a Coriolis acceleration, causing things moving horizontally to curve to the right. The strength of the Coriolis acceleration depends on the speed of the Earth's rotation, and a stronger Coriolis acceleration makes large-scale weather systems more important compared to local or region circulations (seabreezes and monsoons).

One standard question on Ph.D. exams in Meteorology is to ask what would happen to the weather and climate if the solar input or rotation rate were changed by some specific large amount!

Q: How did earth get its name?

--Clive, Unknown

A: Dear Clive,

A little dictionary work shows that "earth" comes from the Ino-European root that means, well, "earth." Lots of early civilizations had the view that the universe consisted of the sky, the air, and the dirt and rock surface on which they lived. The same word sufficed as a collective term for the entire surface and for what we would also call dirt or soil. The name stuck as a more complete picture of our home planet emerged.

Q: What is the temperature of the exosphere?

--David, Piedmont

A: Dear David,

The exosphere is the outermost layer of the earth's atmosphere, starting at 500-1000 km above the surface. Unlike the ocean's surface, the "edge" of the atmosphere is fuzzy, gradually thinning to the vacuum of space. At these altitudes the air is so tenuous that temperature has to be defined in terms of the energy of individual molecules. With each molecule on its own, so to speak, daytime energies reach well over the equivalent of a thousand degrees as the Sun's rays energize the molecule. Then at night the molecule radiates its energy away and its energy level plunges to within a few degrees of absolute zero.

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Q: I have noticed that in January when I am looking at the sky on a very cold night, the stars and moon are very clear and bright. Why is this? Does it have something to due with the weather being very cold? In July, when it is hot at night, they are very difficult to see.

--Haley, Louisville, KY USA

A: Dear Haley,

Most of the answer to this question is contained in the answer to the question about the sky being blue. A soggy, hazy night in the summer, particularly near a big city, provides lots of gunk in the air (that's a technical term!). The gunk both makes it hard to see the object by blocking its light and reflects nearby light, reducing contrast between the object and the surrounding sky. Objects in the sky are the most clearly visible when the air is the driest and cleanest. "Dry" ensures that the particles that do exist in the air are as dessicated, and therefore as small, as possible. So, the stars are most visible on clear, cold winter nights featuring air that has a history over clean regions, such as the Arctic. High altitude (mountains) helps because there is less atmosphere between the viewer and the objects.

Q: I recently saw two moon dogs with a full moon. They looked just like sun dogs, but with the moon instead - like vertical rainbows - green/blue/red etc. Are they rare? This is the first time I've ever seen them. Any other info about them would be greatly appreciated also.


A: Dear Caroline,

Good eyes! Yes, they are "sun dogs, but with the moon." Yes, they're fairly rare because the moon is only available to produce them part of the time and because you have to be outside at night to see them. The book "Halos, Rainbows, and Glories" by Robert Greenler is an excellent source of information on all sorts of atmospheric optical phenomena. Sun and moon dogs are more formally referred to as parhelic and parlunic spots, respectively. They result from the refraction of sun or moon light by hexagonal plate-like ice crystals that are approximately 30 microns in size and horizontally oriented. The spots are located on the halo that (sometimes) also forms around the sun or moon, and the relative strength of the halo and the spots gives insight into how uniformly the ice crystals are horizontally oriented.

Q: Will the sun grow in some million years and burn earth?


A: Dear George,

By studying numerous stars similar to the sun in size, astronomers believe that the sun has a long, stable lifespan ahead. Eventually, some 6 billion years in the future, the hydrogen that the sun is burning (actually fusing into helium) will be exhausted and it will start consuming helium, forcing it into a red giant phase. The diameter of the sun will swell enourmously and the earth will become uninhabitable at that point. It's a worthy goal for humankind to treat the earth and each other gently enough that our descendants are free to worry about this issue!

Q: The sky generally appears blue. This means that only blue light is reaching the Earth from the sun. Then why doesn't everything around us also appear blue?

--David, Melbourne, Victoria, Australia

A: Dear David,

As a matter of logic, I would say that you should look at the sun to determine the color light that it's sending us. The truest answer is visible when there's least interference - such as at noon on a dry, cloudless day in high mountains, or, even better, from space. It's white with a slight yellowish tinge. The tiny aerosol particles that the atmosphere contains are of such a size that they preferentially scatter short (blue) wavelengths out of the original path of the sunlight. So, most of the time the sun appears yellowish and the rest of the sky appears blue. A really long, gunky path through the atmosphere, such as sunset over a polluted city, leaves a dim, red sun that you can view with the naked eye. In this case the rest of the sky might appear white or reddish because the blue light was mostly scattered away before getting to the part of the atmosphere in your vicinity.

Q: Why is it that monsoons only occur in southeastern Asia and the Indian subcontinent areas? Is there a special situation there unlike any other place on earth?

--Otto, Chicago, IL

A: Dear Otto,

The monsoon was first named as such in the Indian Ocean area (the origin of the word is Arabic for season), and denoted the broadscale shifts in wind between the boreal summer and winter seasons. These winds are driven by continental-scale land-sea temperature contrast between the southern Asia landmass and the Indian Ocean. Summertime heating forces ascending motion over the land, and consequently inflow off the Indian Ocean. Wintertime cooling forces descent and outflow toward the ocean. Subsequently, scientists realized that such large-scale seasonally varying circulations occur in many tropical locations, including southwestern North America, central South America, West Africa, and northern Australia. The particular expression of the monsoon in each area depends on the configuration of land and water and the influence of global-scale wind patterns. These govern, for example, whether there is just a shift in the wind, as in North America, or an actual reversal of the flow, as in Asia. In recent years the occurrence of a well-defined summer maximum in broadscale precipitation has been taken as a more reliable indicator of monsoons than the wind patterns.

Q: Do solar flares afect photosynthesis? If so, then how?

--Andrew, Atkinson, Maine

A: Dear Andrew,

Solar flares do not have an appreciable effect on photosynthesis. Photosynthesis is the process by which plants convert the energy in light into the chemical energy in sugar. Plants have a marvelous system which uses chlorophyll and pigments of varying colors to collect the energy in light. But all these pigments collect energy only in the visible part of the spectrum. Solar flares may emit a tiny bit more visible energy than normal, but the difference between the total visible light from the full disk of the sun with or without a flare is extremely small.

A great website for further information is NASA's Solar Flare Theory site.

This doesn't mean that solar activity doesn't affect us on Earth. Solar flares are occasionally associated with coronal mass ejections which throw off storms from the sun that contain enough energy to interfere with our communications, television, radio and even electrical power grids. In 1989 a solar storm caused a power outage in Canada and the northeastern US that left 8 million people without power.

Another note about pigments in plants: Green plants appear green because they all contain the pigment chlorophyll. Chlorophyll appears green because it absorbs the colors of light that are NOT GREEN! Chlorophyll REFLECTS green light and that is the color that we perceive with our eyes. Other pigments absorb and reflect different colors of light. The carotenes reflect yellow and orange light. Anthocyanins reflect red light. In the fall, as the leaves begin to die, the chlorophyll fades and reveals the yellows, oranges and reds that give us "fall" color. The yellows, oranges and red colors are actually there all summer, but it is only after the green fades that we see the other beautiful colors in leaves.

Q: Why does the hole in the ozone layer of the atmosphere remain over the south polar region? It seems that if CFCs and other pollutants are responsible for the depletion of ozone then there should be many ozone holes around the world, especially the northern hemisphere.

--Jennifer, Charleston, SC

A: Dear Jennifer,

The chemical cycle that destroys ozone in the stratosphere depends on sunlight and a heterogeneous reaction, that is, a reaction among gas-phase molecules that is greatly accelerated when it happens on the surface of solid particles. During the long, dark Antarctic winter (the Northern Hemisphere's summer) the circulation in the stratosphere isolates the air over Antarctica. This concentrates the loss of heat to space, cooling the temperature to the point that otherwise-rare stratospheric clouds form. Of course, the cloud particles are all ice crystals. When the sun re-appears at the beginning of the Antarctic spring the (solid) ice crystals, sunlight, and chemicals start working. The air mass over Antarctica continues to be relatively isolated from the rest of the atmosphere for another month or more, allowing the ozone depletion to become highly visible. Once the stratosphere over Antarctica warms enough, the wintertime circulation breaks up and blobs of low-ozone air mix toward the equator. As well, the stratospheric clouds stop forming and the chemical cycle largely stops. The Arctic appears to suffer a similar process, but the asymmetries of the land masses around the Arctic prevent the air mass over the Arctic from becoming as isolated as is the case over the Antarctic. No other regions of the stratosphere support the formation of stratospheric clouds for enough time to appreciably affect the ozone.

Q: While researching how plants affect the average temperature of an ecosystem I found lots of information on how trees affect the carbon cycle and therefore can slow global warming, but I'm looking for more immediate responses to plant life. After reading your site I see that plants can cool the air by increasing evaporation. Could you please clarify this interaction?


To summarize, the interaction between vegetation and its surroundings is complicated because the actual physical processes are taking place at space scales that are much too small to represent in computer models. Qualitatively, denser vegetation usually has the effect of increasing the flux of latent heat (water vapor) from the surface and decreasing the flux of sensible heat (temperature). As well, it distributes the frictional effect of the surface across a deeper layer of the atmosphere.

The issue of the interaction between vegetation and the atmosphere is a very hot topic in atmospheric sciences. It is important for studying and simulating local and regional weather and climate, surface and subsurface hydrology, and natural and agricultural vegetation. Such work is frequently indexed under "land surface process studies." The web site lays out some of these issues in the context of introducing a particular numerical model of vegetative cover, and it contains additional references.

Q: How much CO2 is released from forest fires?

--Joy, Corvallis, OR

A: Dear Joy,

According to one reasonable estimate (see reference) the total is roughly 20% of the CO2 emitted by all sources, distributed as 14% from burning savanna, 5% from burning tropical forest, and 1% from burning temperate and boreal forest. These numbers exclude burn-offs of agricultural waste (another 8%) and use of wood and charcoal for fuel (totalling 5%), but cannot distinguish between controlled and uncontrolled burns of the natural vegetation. The total carbon released by savanna and forest fires is around 2360 Teragrams per year (roughly 80 tons per second). As you might imagine, there is a wide variety of estimates for these statistics because there is a great deal of uncertainty in making the calculation. Estimates must be made for the total area burned, the organic matter per unit area, the fraction of the organic matter that is above ground, the burning efficiency, and the average carbon content of the organic matter. This issue is a key part of the global carbon cycle, which refers to the transport and storage of the many different forms of carbon in the earth-atmosphere-ocean system.

To take the question a step further, savanna and forest fires are also an important source of huge quantities of aerosols (tiny particles), as well as many important trace gases, such as bromine.

Andreae, M. 0. In Global Biomass Burning: Atmospheric, Climatic, and Biospheric Implications; Levine, J. S., Ed.; The MIT Press: Cambridge, MA, 1991. Quoted in:

Q: There seems to be good evidence that the earth is warming and the ocean level rising. This may possibly be, in part, due to human activitiy, and the changes will certainly produce significant impacts on we humans. To put this in a larger perspective, though, I'm curious about how significant (or insignificant) the apparent changes are relative to some of the temperature changes the earth has gone through before humans were around to either be a factor in the process or be affected by the changes.

--Carl, Winton, Minnesota

A: Dear Carl,

It is hard to give a precise answer on the state of the past climate because only the last few decades of data have been sufficiently comprehensive to give true global averages of the various atmospheric parameters - temperature, precipitation, cloudiness, and so on. Instrumental records go back another century in some locations. "Proxies" for weather data can be used to provide fragmentary glimpses of still earlier climate. Reaching progressively further back in time, researchers study records of cherry blossom flowerings in Japan and canal freeze-ups in the Netherlands, tree ring records and ice cores, and ocean-sediment cores and assessments of fossil populations in rock layers. Such work is known as paleoclimatology.

Geologists have identified a number of major ice ages during the earth's history, during which the climate was much colder than it is today. The last such episode peaked about 21,000 years ago, at which point the climate wasn't simply different - the current sites of many major cities were covered by thick ice sheets. More recently the period 1450 - 1890 AD was unusually cold, at least across North America and Europe. In contrast, the North Atlantic climate had been warm enough a century earlier to allow the Vikings to establish agricultural settlements in southern Greenland. A more significant warming appears to have occurred in the mid-Cretaceous era (120-90 million years ago), when warm-weather vegetation, dinosaurs, and sea life (particularly corals) flourished at much higher latitudes than would now be possible.

The outstanding feature of the current situation is the rapidity with which changes are likely to occur. It appears that the changes mentioned above happened over centuries or millenia, while the current changes will be happening over a few decades, which is an instant, geologically speaking. As a consequence, ecosystems, human activities, and even the earth atmosphere/ocean/cryosphere system will find themselves far out of balance with the new climate and numerous adjustments, both large and small, will be occurring for a very long time.

For additional information on paleoclimatology and its perspective on global warming, go to, which is provided by the National Geophysical Data Center.

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Q: Why does only one side of the moon face the earth?

--Daniel, Johannesburg, South Africa

A: Dear Daniel,

The original rotation of the moon was braked by the friction of tidal forces induced by the earth on the moon. Any time two celestial bodies (planet, moon, or star) lie in each other's gravitational fields, the side of each closest to the other experiences a stronger gravitational attraction than the side that is farthest away. When a body is rotating, the locations of maximum and minimum gravitational force continually change, driving increases and decreases in elevation of whatever solid, liquid, and gaseous surfaces there are - tides. Tides on earth are primarily driven by the moon and sun. We are most familiar with oceanic tides, but the atmosphere and solid earth also have tides. Similarly, when the moon was rotating faster than once a month (the period needed to keep the same side to the earth at all times), it experienced tides.

The IMAGE satellite program has an excellent web site that addresses various aspects of this question, as well as many other moon-related questions.

Q: I suppose that if the earth's temperature (and energy) will increase the violence of meteorological phenomena will also increase, because (tell me if I am wrong) of the delta temperature between polar regions (where the ice melting absorbs energy) and the rest of the world where temperature is increasing. But I wonder - when the ice is completely melted, will the earth reach a more stable situation?

--Diego, Rome, Italy

A: Dear Diego,

Most numerical models of climate indicate that polar regions are the most sensitive to global warming. The vast expanses of the tropical oceans already have two efficient processes in place for exporting incoming solar energy to higher latitudes - ocean currents and the Hadley circulation in the atmosphere. Ocean currents carry massive amounts of warm water toward higher latitudes and cool water back toward lower latitudes. In the Hadley circulation, heating near the Equator drives rising motion (mostly inside intense thunderstorms), which forces warmer high-altitude winds toward the Poles and cooler low-altitude winds toward the Equator (the tradewinds). Tropical land areas contribute to the Hadley circulation. At midlatitudes, large amounts of heat continue to be carried poleward by ocean currents and the familiar high and low pressure systems and fronts. Thus, the polar regions have the weakest dynamical controls on the average temperature. Even with the usual caveats about incomplete data and regional variability, the polar regions do seem to be showing average increases that are larger than in other regions.

Despite the reduction in equator-to-pole temperature gradient, there has been speculation that warming might drive more extreme events. The various concepts require considerable work before a definitive statement can be made.

The no-icecap scenario is a fascinating thought experiment, fraught with numerous uncertainties. The increase in sea level might facilitate the penetration of warm currents into the Arctic Ocean, reinforcing the loss of ice. Ice-free polar oceans might provide massive amounts of additional moisture for most of the year, driving heavy snows over bordering land areas. Changes in polar surface cover and the accompanying changes in polar cloudiness could have profound effects on the regional temperatures by modifying how efficiently heat radiates to space during the long polar nights. It is not clear that these possibilities point to more placid conditions, at least in polar regions. At lower latitudes the arguments for greater occurrence of extreme events continue to apply, if they're valid in the first place. To summarize, we are not yet in a position to know how storminess will change as the climate changes.

Q: Is there any evidence of global warming and/or climactic global changes affecting the monsoon seasons in Asia?

--Joel, Eugene, OR

A: Dear Joel,

The available datasets make it clear that the Asian monsoon system has significant fluctuations over periods of a decade or longer. However, attributing those fluctuations to particular causes is a messy business. We know that the monsoon can exhibit a wide range of behaviors for a given average state of the atmosphere. On the other hand, we know that the average state of the atmosphere has significant fluctuations. And, when the average state of the atmosphere changes, the typical progression of the monsoon likely changes. For example, during the mid-Holocene era (some 6,000 years ago) the earth's orbital parameters were slightly different than they are now, causing the seasons to be stronger than they are now. Paleoclimate data show that the main regions of monsoon precipitation shifted northward in both Asia and Africa during that time.

We do know that the tropical Indian Ocean has warmed over the last three decades, which likely has an important effect on the Asian Monsoon. We also know that the correlation between the Asian monsoon and the El Ni¤o-Southern Oscillation has decreased during that time. Unfortunately, the current data records are too short to untangle the natural variability of the monsoon from the effect of climate change, whether natural or anthropogenic.

The alternative is to perform studies with numerical models of climate (similar to weather forecasting models, but designed to represent longer timescales). Results from such model studies are decidedly mixed, with tentative indications that global warming might cause the Asian summer monsoon to be somewhat stronger and the winter monsoon somewhat weaker. Even if this result holds up, the magnitude of the change is an open question.

Q: Why does the color of the moon change? What makes the moon look red sometimes?

--Eric, Bowie, Maryland

A: Dear Eric,

The atmospheric optics rules that you know about for the sun also apply to the moon (and any other object at great distances from the observer) - the sun is just a lot brighter than anything else. The tiny aerosol particles that float in the air are of such a size that they preferentially scatter shorter ("blue") wavelengths of light out of the line of sight between an observer and an object, compared to longer ("red") wavelengths. The longer the path through the atmosphere and/or the heavier the load of aerosols, the more scattering that takes place, causing the object to appear more red and less blue. When the object is directly overhead, the path through the atmosphere is as short as possible, and it gets progressively longer as the object sinks toward the horizon. Heavier aerosol loading can be caused by blowing dust, fires, ordinary pollution, and so on, as well as humid conditions that tend to plump up several classes of aerosol particles. So, just as with the sun, you see a dim, red moon on the horizon in conditions of gunky, humid air, and a bright, whitish moon well overhead on clear winter nights.

By the way, the optics rules also let the moon create the same optical phenomena as the sun, including rings around the moon, "moon dogs," and rainbows. However, since the moon is much dimmer than the sun, all of these optical effects are much dimmer.

Q: Since photosynthesis is the reaction of CO2 and H2O forming sugar and O2, isn't the rate of photosynthesis determined by the concentration of CO2 in the atmosphere? With currently increasing CO2 concentrations, shouldn't photosynthesis be speeded up? Is there evidence for this?

--Harold, Madison, Wisconsin

A: Dear Harold,

Laboratory studies show that higher concentrations of CO2 promotes plant growth, which is driven by the rate of photosynthesis, for some, but not all species of plants. For example, wheat benefits greatly, while corn sees almost no change. In the wild, the rate of photosynthesis is governed by a number of factors. Some are direct, including the concentration of CO2, the ambient temperature, and the availability of sunlight. There are other indirect factors that affect a plant's ability to support photosynthesis, including availability of moisture, temperature, genetics, soil condition (PH, salinity, water saturation, tilth, nutrient availability), disease, and damage by parasites and herbivores. If any of the factors other than CO2 is limiting a plant's photosynthesis rate, adding more CO2 will be relatively ineffective in provoking additional photosynthesis.

Q: What is the latest status of the Antarctic iceberg B-15? What effect will it have as it leaves the Ross Icesheet?


Designated "B-15" by the National Ice Center after its separation from the Ross Ice Shelf in mid-March 2000, this iceberg is one of the largest ever recorded. Despite holding together in one piece for nearly seven weeks, B-15 broke in half in early May.

Before fracturing, the berg measured 295 by 37 kilometers (170 miles long by 25 miles wide), and had a total area slightly more than 10,000 square kilometers (4250 square miles). For reference, B-15 was about as wide as Long Island, and by comparison would extend from New York City past Montauk Point to Martha's Vineyard.

Using satellite and ocean tide information, Professor Doug MacAyeal of the University of Chicago constructed some models to predict the motion of B-15 while it is in the Ross Sea. These tidal models accurately predicted the berg to rotate to a slightly NW-SE orientation. Now that it is in two pieces, the eastern portion has assumed the NW-SE orientation while the western portion is more flush against the existing iceshelf. Also, smaller bergs have calved off the main B-15 halves.

More images of the original B-15 and the current B-15A and B-15B icebergs can be viewed at the University of Wisconsin's Antarctic Meteorology Research Center iceberg images web site.

The first part of May also saw the calving of three large icebergs from the Ronne Ice Shelf of Antarctica. The bergs were designated A43A, A43B, and A44 by the National Ice Center. With the loss of these three icebergs, the front of the Ronne Ice Shelf returns to about the same position it occupied in the 1950s.

According to Dr. Ted Scambos of the National Snow and Ice Data Center, "The recent calving of the large bergs from the Ross Ice Shelf and the Ronne Ice Shelf reflects the normal cycle of Antarctic ice shelves slowly advancing and periodically calving new bergs. These events are not necessarily indicative of regional warming as are the events we see along the Antarctic Peninsula where ice sheets are tending to disintegrate rather than calve large bergs."

For more information on these icebergs and where they may move next, and on iceshelves in general, see the following websites:
The National Snow and Ice Data Center's iceshelves pages
The National Ice Center (Click on "Icebergs")

Q: I am having trouble visualizing why high noon can vary at my location as much as a half hour during a calendar year. The reasons given, namely the tilt of the axis and the elliptical nature of the Earth's orbit, seem to be insufficient to account for that large a variation. Also the extremes of the fluctuation do not seem to align with perigee or apogee or equinoxes or solstices. Is there more to it or am I missing something?

--Anssi, Vancouver, BC, Canada

A: Dear Anssi,

In Chapter 1 of this science mystery, our perceptive reader noticed that the time that a sundial shows ("Sun time") varies in the course of a year as much as 15 minutes ahead or behind the time that a clock shows. In Chapter 2, a little library work showed him that the variations in Sun time are due to two separate effects, namely the tilt (or "obliquity") of the Earth's axis and the elliptical orbit that the Earth takes around the Sun. For the Earth's present orbital parameters both effects are important, with the first effect being slightly larger. Now in Chapter 3 he is trying to puzzle out how this all fits together. The short answer is that his sources are correct and he is correct -- it's not obvious. The first key insight is that even though these effects are small, the changes are cumulative. So, changing individual Sun days by a few seconds shifts the relationship between Sun and clock time by minutes over several weeks.

The rest of the story gets more technical, but there's a surprise ending! It takes 23 hours, 56 minutes, and 4 seconds for the Earth to revolve exactly once on its axis (a "sidereal day"). During this day, the orbital motion of the Earth makes it appear that the Sun has moved (mostly) eastward in the sky, requiring another 3 minutes, 56 seconds of rotation to again point at the Sun (on the average). Because of the Earth's tilt, part of this apparent motion is north-south, reducing the amount of extra rotation needed to account for the eastward motion. Apparent north-south motion is largest at the equinoxes and zero at the solstices, so the Sun appears to move eastward the fastest at the solstices and the slowest at the equinoxes. The variations in orbital velocity produced by an elliptical orbit cause faster apparent eastward motion at perigee (closest approach) and slower at apogee (largest separation). A second key insight is that fast changes in apparent Sun position mean a longer Sun day, since it takes a little longer for the Earth to rotate the extra distance to face the Sun, and periods with slow changes have shorter Sun days.

The final chapter for this mystery depends on the key insight that the obliquity effect provides two speed-up/slow-down cycles each year, since there are two solstices in a year, while the elliptical orbit effect only has one. The winter solstice and perigee happen to nearly coincide (December 21 and 31, respectively), providing reinforcing longer-Sun-day effects and the largest cumulative slowdown compared to clock time at that season. Six months later there is near-balance between the summer equinox's increase and the apogee's decrease in Sun day length. So, at the end of the story the seemingly obscure variations in sundial readings actually provide a tangible illustration of the Earth's motions in space. Cool!

By the way, these variations in Sun time and the Sun's annual north-south migration are frequently combined into the "analemma," sometimes called "the astronomer's odd figure-8," that appears on many globes and sundials. You can read more about Sun time by visiting web sites such as or

Q: In God's grand design, doesn't "global warming" make good sense? We raise the co2 level, the warming raises the growing seasons to use the co2 and produces the corresponding O2. I know this is overly simplistic but is there a positive scientific perspective? Warm air holds more moisture which will water the greater number of plants and so on.

--Al, Fort Wayne, Indiana

A: Dear Al,

Let's start with the really big picture. What are the direct climatic changes due to increasing average global temperature--generally referred to as "global warming"? Scientists are fairly confident that most areas would see increased temperatures year-round, although some areas will actually have decreased temperatures due to particular local/regional circulations. If temperatures rise unchecked, storm tracks and global air and ocean circulation patterns will certainly change, but there is very little agreement about what precisely to expect. More hurricanes or fewer? Enhancement or suppression of the Asian Monsoon? More floods and droughts or fewer?

At the next level of detail, warmer summers and winters will have a wide variety of effects. In our homes and offices, for instance, less energy would be used for heating, and more for cooling. Large regions of Canada and Russia might achieve a sufficiently long growing season to allow for more crop farming. At the same time, tropical pests would move into higher latitudes. In fact, ecosystems in general would go through periods of massive adjustments. The navigation season would lengthen for waters that experience seasonal ice, such as the Great Lakes and the margins of the Arctic Ocean. The snow season would be shorter, reducing water storage and winter recreation opportunities in the mountains. Major melting of glaciers and icecaps would occur. The swimming pool industry would boom and the market for snow shovels, road salt, and parkas would decline. It has been suggested that warming will promote more rain in moist areas, but even if true, the increased evaporation that warming should cause might actually lead to drier conditions. And how important might the rainfall and evaporation changes be compared to the unknown changes in large-scale storm patterns?

The chain of connections goes on. One major issue is that higher temperatures combined with the melting icecaps and glaciers would cause a rise in sea level, estimated to be 0.5 meters in the next 100 years (and more after that) if global warming continues as expected. People living in coastal areas worldwide will have to respond, most notably in low-lying areas such as Florida; Venice, Italy; the Netherlands; and the hundreds of low islands and atolls in the global oceans.

Are these effects "good" or "bad"? Some are pretty obvious, but these types of judgements are a societal issue, not scientific. Scientists can say that global warming is a package deal--you get everything listed above plus a whole lot more that we still lack understanding of. Many of the effects that the warming will bring will last a long time once it gets started and there's no way to turn it off once the chain of events begins. So one might conclude these are "bad" effects because today's ecosystems and human activities are highly attuned to the current climatic state. In short, the potential for negative effects in significant parts of the global community makes it hard to speak favorably of global warming.

Q: Is it possible to flow water to the Sahara Desert from the Atlantic and, therefore, change the climate for good in Africa?

--Isidro, Barcelona, Spain

A: Dear Isidro,

There is archeological evidence which tells us that the Sahara was relatively green during the last Ice Age. This evidence includes rock paintings of animals that are typical of a modern-day savannah. Since the last Ice Age, the Sahara has become one of the great deserts of the world because the atmospheric circulation patterns changed as the ice sheets retreated to the extreme polar regions. It is highly likely that a single project to soak the desert with water would only make a difference for a few years, during which time the atmospheric circulations would again dry out the area. Alternatively, you could pump water for as long as you wished to sustain lush vegetation. In either case, the financial, political, and technical hurdles seem beyond solution: Remove the salt from millions of tons of ocean water, pump it thousands of kilometers across many international boundaries, and distribute it more or less evenly on millions of hectares of land. Compared to these strenuous efforts, the natural process for producing rainfall that most of us enjoy seems positively simple!

Q: Where does the heat required to activate the "warm pool" in the Indonesian area come from? Since the sun doesn't change its radiation patterns to specifically enhance the temperature of the Indonesian area in its daily transit, it seems likely that some other source of energy must exist. Perhaps subterranean volcanoes?

--Robert, USA

A: Dear Robert,

The heat that warms the ocean around Indonesia comes from the sun. Areas along the equator receive abundant exposure to the sun's rays, which eventually are stored as heat in the ocean. Unlike the eastern Pacific, which usually has strong upwelling of cold, deep currents (driven by strong trade winds) that keep the surface waters cool, the Indian Ocean and Western Pacific warm pool don't have similar upwelling mechanisms for cooling the surface waters as efficiently, so the waters are much warmer there.

While there are thermal vents on the Indian Ocean floor, the amount of heat they release is negligible compared to the heat from the sun's rays.

  Ask a Scientist

Editor’s note: The Earth Observatory no longer supports the ‘Ask a Scientist’ feature. These pages provide an archive of previous questions and answers.

previous questions previous questions

Q: I had a student ask me if the Southern Hemisphere has a jet stream similar to the one in the Northern Hemisphere. This is not a field in which I am well versed. Any help?

--John, Greenwood, SC

A: Dear John,

Indeed, the Southern Hemisphere also has similar jet streams. In this country, we refer to the air mass moving over North America in the upper troposphere as THE jet stream, but among atmospheric scientists a jet stream refers to any strong, narrow current concentrated along a quasi-horizontal axis relative to the planet's surface, with strong vertical and lateral wind shears. At minimum, a jet stream is moving 30 meters per second, although velocities of 500 km per hour have been observed near the upper boundary of the troposphere. Typical jet streams in Earth's atmosphere are thousands of kilometers long, hundreds of km wide, and several km deep.

Jet streams are the product of Earth's coriolis force, as well as temperature and pressure gradients in the atmosphere--mainly horizontal temperature gradients that are particularly strong across cold and warm fronts. The jet streams in the Southern Hemisphere will typically be weaker because the equator-to-South Pole temperature gradient is less in the Southern Hemisphere.

Q: What is meant by the temporal resolution of a satellite?

--Wendy, USA

A: Dear Wendy,

When referring to satellites, one will often ask, "What is its resolution?" By "resolution," folks generally are asking for one of three possible things: (1) temporal resolution, (2) spatial resolution, or (3) spectral resolution.

Temporal resolution refers to the frequency at which a satellite "sees" a given point on Earth."Temporal" means, "pertaining to Earthly time." The MODIS instrument sees the entire surface of the Earth every 1-2 days, whereas ASTER will take 5 years to see the entire surface. So, MODIS has a much higher temporal resolution than ASTER. Yet, because MODIS will "see" the poles much more frequently than it will see a given point on the equator, we say its temporal resolution is higher at the poles.

Spatial resolution refers to the detail at which a satellite sensor "sees" the Earth; or the size of its individual pixels (picture elements) in its viewing "footprint" on the Earth's surface. More precisely, spatial resolution is the area of a single data point on Earth's surface measured by a satellite. For example, a single MODIS pixel on the surface at its highest resolution is 250 square meters (about the size of a city block); whereas ASTER sees the Earth at a much higher resolution of 15-square-meter (the size of your backyard) pixels.

Spectral resolution refers to the number of wavelengths of the Electromagnetic Spectrum in which a given satellite sensor "sees" the Earth. MODIS collects images at 36 different spectral wavelengths over a broader span of the EM spectrum (from visible light through the infrared portion). MISR collects images at only 4 different spectral wavelengths, mostly in the visible region. Therefore, MODIS has a much higher spectral resolution.

Q: Would you please tell me why the reactions occur on the surface of the ice particles that accelerate the Ozone destruction cause by stratospheric Chlorine?

--Cathy, USA

A: Dear Cathy,

The ozone depletion phenomenon over Antarctica is actually fairly complex. It's difficult to explain it briefly and simply, but I'll try: During the Antarctic winter months, air cools dramatically over the South Pole because there is no sunlight to heat the ozone in the air. This air descends, because it is denser, allowing more air to move poleward from middle latitudes to replace it. The Earth's Coriolis force causes the poleward-moving air to be deflected eastward forming a strong wind jet--called the "polar night vortex"--that peaks in wind speed from May to June. This vortex tends to isolate the Antarctic stratospheric air mass from mid-latitude air and it inhibits the transport of warmer air from the middle latitudes into that region. Thus, during the Southern Hemisphere's winter months, when it is always night at the South Pole, the air inside the vortex gets cooler, to about -90 degrees Celsius. Even though the stratosphere is very dry, clouds can form under these very cold conditions. These clouds are mostly made up of nitric acid trihydrate crystals and water ice.

The surfaces of the ice crystals allow chlorine nitrate and hydrochloric acid (HCl)--what we call chlorine "reservoir" gases--to chemically combine to form nitric acid (HNO3) and chlorine gas (Cl2). This reaction also takes place while the molecules are in their gaseousphase, but very slowly. Surface reactions are usually faster than gas-phase reactions because bond strengths are lowered for materials that adhere to a surface. So, the nitric acid stays bound to the ice crystal while the chlorine gas escapes. This is important because if the nitrogen compounds were also released, they would begin to tie up the chlorine again. Things change quickly with the onset of spring and the return of sunlight to the region. The chlorine gas "photolizes," and the chlorine atoms react with ozone to form chlorine monoxide. If enough chlorine monoxide is formed, it reacts with itself to form the dimmer Cl2O2, which itself photolizes into chlorine atoms and molecular oxygen, leaving the chlorine again free to attack ozone.

This catalytic ozone destruction cycle continues until all the ozone is gone. What stops the process is that after the ozone is gone, the chlorine begins to attack methane (you need a lot of chlorine atoms to do this, so all the ClO has to be gone, which means all the ozone has to be gone.) The methane- chlorine reaction ties up chlorine into HCl, which is fairly inert. Later in the spring, the vortex breaks up and the chemically perturbed Antarctic air is dispersed to middle latitudes. I might add that about 7/8 of the chlorine in the stratosphere is man-made from CFC's.