As a SoCal transplant to the Midwest I have had two very different experiences with weather.  Growing up, I learned a few immutable climate axioms.

  1. If there is sunshine its going to be warm, if it is cloudy it is going to be cold.
  2. If there are clouds plus precipitation its going to be even colder.

The beauty of these principles is that you can look out your window and know how to dress.  The tragedy of these principles is that they are totally false.

In Chicago, sunshine means on average that it is going to be colder.  Especially in the winter.  Cloudy days are on average warmer than sunny days.  And precipitation, especially snow, means its going to be even warmer than if it was just cloudy.

Eventually I think I figured out why.  The true axioms of weather are instead

  1. How warm or cold it will be depends almost entirely on where the air is coming from.
  2. Whether there are clouds and how much precipitation there will be depends a lot on where the air is coming from.

#1 is obvious but would be completely lost on a SoCalian because the air pretty much comes from the same place all the time.  It blows in from over the Pacific.  This being constant, the only thing left to determine the tiny fluctuations in temperature is whether or not the Sun has yet to burn off the marine layer (essentially fog that comes along with water blowing over the ocean.)  Naturally therefore sunshine=warmer.

(A variation on this which still confounds the SoCalian is the occasional Santa Ana condition where the air is blowing offshore from the deserts.  This keeps the marine layer at bay (sunshine) and the desert air is of course warm.)

In Chicago, the air can be coming from just about anywhere Westish.  Air coming from Canada: cold.  Air coming from Missouri:  warmer.  Now, sunshine means that the excess moisture that would have been in the air to form clouds fell to the ground before the air arrived.  That is more likely to have happened if the air is coming from Canada because in Canada it is colder and that means the the air can hold less moisture.  Hence the correlation sunshine=cold.

Indeed, clouds indicate that the air has moved from some place warmer, where the air could hold moisture.  Finally, precipitation means that it was so warm wherever the air is coming from that there is so much moisture in the air that by the time it reaches cold Chicago, it has to fall out of the sky onto me.

File this one under Blogging Something I Know Nothing About, yes. But after the first few times I ran outside in shorts in January because the sky was crystal blue, I have come to depend heavily on this theory.  And its working pretty well, at least until I move to Hawaii.

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