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Name: Joanne P.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 1/30/2004


Question:
In the summer, the sky seems to be bright blue. But in winter, the sky seems to be most often white. Why is this? What makes the sky in the summer a different color than in the winter? Does it have anything to do with the weather; such as temperature, cloud cover, or precipitation?


Replies:
Joanne,

First some background on this subject.

Ice crystals, water vapor, and particles (pollen, pollution, etc.) scatter light, mostly red light, leaving the blue wavelengths for us to see. The greater the amount of water and particles in the air, the more the blue wavelengths of light are scattered also. On a very hazy or humid day the sky looks pale blue or even almost white in an extreme case, since most wavelengths of sunlight are being scattered almost equally. That is also why during a clear day (at any time of year) the sky straight above you is bluer than the sky at the horizon; the light coming down to you from straight above you has passed through less air and therefore less scattering of the light in all but the blue wavelengths has occurred. The light coming from the horizon has passed through more air and so more of the blue wavelengths have also been scattered out.

Now we get to your questions.

"What makes the sky in the summer a different color than in the winter?" There are at least two reasons.

First, during winter, in very cold air, ice particles and supercooled water droplets can form easily. They are not dense enough to form a cloud or to be visibly seen, but they are very good at scattering light of all wavelengths (all colors), something like on a humid summer day, which can make the sky less blue and even make it appear almost white.

Still, the sky above you will be more blue than the sky at the horizon, but not as blue as during the summer.

Second, during winter, the troposphere (the lower layer of air where our weather occurs) is not as deep as it is in the summer (meaning that there are fewer ice crystals, water droplets, and particles to scatter light); therefore less red light is scattered out and the sky does not look as blue.

"Does it have anything to do with the weather; such as temperature, cloud cover, or precipitation?"

Temperature is only indirectly responsible; more ice crystals and supercooled water droplets can occur in winter's cold temperatures. Certainly, if it is cloudy, the sky will appear white. If it is snowing, which it can do even when it is partly cloudy or even if it is clear but extremely cold (the latter are actually ice crystals formed in cloud free air instead of from clouds), the snow or ice crystals will scatter a lot of light, making the sky less blue.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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