Wednesday, April 8, 2015

If the spring and fall equinoxes are alike, why isn't the weather?

Consider typical temperatures around the vernal equinox, about two weeks ago.  Now consider the temperatures typical around September 21, the autumnal equinox.  At both these times the days were equal, the sun rose to the same noon height, following the same path across the sky.  

The amount of solar energy any place on earth is receiving on any day is the result of two things: the daylength and the directness of the sun's rays.  The longer the day and the more direct the rays (the higher the sun gets in the sky) the more energy we receive.  [You can prove the importance of directness by shining a flashlight on a flat surface: the energy is concentrated in a smaller area if the rays are perpendicular, but smeared out, hence weaker, as the angle declines.]  

If your location receives the same amount of solar energy on these two dates, why is the spring equinox so much colder than the fall one?  Consider two pots on the stove.  [Don't try this at home!]  The burner under one is turned on and turned up gradually to "medium."  The burner under the other has been on "high," but is gradually turned down to "medium."  When both pots are on "medium," each is receiving the same amount of energy at that moment, but the pot that was on "high" is still cooling off and so is the hotter of the two.  So also the earth: it stores heat during the summer months and takes time to cool off during fall.  

This is especially true in areas with large bodies of water nearby: almost no other substance is as good as water at storing large amounts of heat with little increase in temperature.  (Consider how long it takes a pot of water to get hot on "high."  And how long a pan of water stays hot!  --far longer than an empty pan.)  This is the reason that coastal and lake regions have less extreme temperatures in summer and winter: the water absorbs heat that would otherwise make us miserable in July, but remains warm enough to keep local temperatures higher than they otherwise would be in winter.

More than two weeks into fall, some soccer parents are in short sleeves, 
but two weeks into spring there's still snow on the ground here.

For the same reason, the warmest part of a day is usually an hour or so after the time the sun reaches its highest point. 

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