Ask a Scientist

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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.