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Weather Talk: Vernal Equinox and radiation

Today is the Vernal Equinox and incoming solar energy is the same as it is at the Autumnal Equinox in the fall. But average high and low temperatures today in Fargo are 38 degrees and 21 degrees whereas average high and low at the time of the fal...

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Today is the Vernal Equinox and incoming solar energy is the same as it is at the Autumnal Equinox in the fall. But average high and low temperatures today in Fargo are 38 degrees and 21 degrees whereas average high and low at the time of the fall equinox is 69 degrees and 45 degrees.

The sun is the source of heat and the solar radiation our atmosphere received is the same on the two dates. The difference is what happens to that radiation.

In September, when fields are still largely covered in crops, all but about 25 to 33 percent of that energy is absorbed and contributes to heating the air. When the ground is covered in snow, which is usually is, to some degree, in March, about 50 to 100 per cent of the incoming radiation (and heat potential) is reflected by the snow-covered ground back into space.

However, during that rare March when the fields are neither snow-covered nor crop-covered, almost all of the incoming radiation is absorbed and we get unusually warm weather like we did this year. 

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