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.

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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.