|MadSci Network: Earth Sciences|
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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