Insolation during the 2017 Eclipse

On an ordinary day, the insolation — the amount of sunlight hitting a given spot on the Earth — is proportional to the sine of the Sun's altitude. When the Sun is 30° above the horizon, the sunlight energy per square meter is half of what it is when the Sun is directly overhead. This relationship is the reason that the tropics are hot and the poles are cold. Combined with day length, it's also the reason for the difference in temperature between the seasons at temperate latitudes.

As this animation shows, the Moon's shadow dramatically, if temporarily, affects insolation in the continental United States during the total solar eclipse of August 21, 2017. The effect is readily apparent to observers in the path of totality. As the umbra passes overhead, the temperature drops by several degrees. The cooled column of air within the shadow cone can even influence cloud formation and the speed and direction of the wind.

The insolation map in the animation combines solar altitude with obscuration, the fraction of the Sun's area blocked by the Moon during the eclipse. It ignores a number of other factors, including atmospheric scattering, refraction, and cloud cover, that also play a role in the amount of sunlight that reaches the ground.