MadSci Network: Earth Sciences

Re: Can a solar eclipse cause clouds to form, if so how?

Date: Sat Dec 14 00:27:43 2002
Posted By: Denni Windrim, Director of Education, Sylvan Learning Centre
Area of science: Earth Sciences
ID: 1039101531.Es

Clouded eclipses are quite common because a clouded sky is quite common. Why 
is it that we marvel at a day when not a cloud can be seen, and how often do 
we complain about "ruined weekends", even though the sky has been cloud-
covered during our working hours?

Your question was "Can a solar eclipse cause clouds to form?" In short, no. 
First, the maximum period of totality for a solar eclipse is 7 minutes 31 
seconds, which occurs when the sun is at its most distant from Earth (summer 
in the Northern Hemisphere) and the moon is at its closest. This combination 
of events is very rare, and consequently solar eclipes average out to about 
4 - 5 minutes. As well, in an ideal situation, the moon's shadow is a 
maximum of 270 km in diameter. Because the penumbra is so small and races so 
quickly across the Earth's surface, there is no perceptable effect on the 
temperature of the earth's surface, or on the atmosphere immediately in 
contact with it. People have reported a noticeable drop in temperature as an 
eclipse goes total, but this has a great deal to do with the sudden absence 
of direct solar radiation on the skin, in much the same way that we feel 
chilly when a cloud obscures the sun. 

Having said this, I suspect it may be possible for ground haze to form if 
the eclipse is of long duration, the terrain is such that it dissipates heat 
rapidly, and the air mass directly in contact with the ground is at dew 
point. In such a case, the very tiny amount of cooling could be enough to 
cause fog to form. However, when we reach this point, we're at the end of a 
long chain of optimal events, and as any craps player knows, throwing four 
pairs in a row is not worth betting on.

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