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Summary: I don't understand it either, but suggest 2 types of explanations: "upper lows" and lunar cycles |
In contrast to fall- winter where huge low pressure systems wind up offshore and then clobber the coast as they slog inland. (see our October weather articles); in summer, a period of rainy weather often comes when the (surface) atmospheric pressure is high, and no (surface) "low" is lurking offshore.
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Upper 'lows" during summer, weather people often talk about "troughs" or "upper level lows" 'causing' the rain. Example Left: A jetstream 300Kp pressure map with the upper low at blue arrow; note clouds (upper), and rain (lower) are on 'edge' of close set jetstream contours. (both figs date from Aug 8th 2000) |
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"enhanced" (color coded) IR satellite imagery. I find these
useful as shows which clouds are really "bad" weather Note the "summer storms" are associated with jet stream contours |
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Here is the actual rainfall upon which I have superimposed the moon phase--- Looks like pretty good fit.
Of course both are not exclusive. But it is hard to understand why full moon tides bring rain and dark don't.
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Our weather is usually seen as determined in winter by the strength and location of the Aleutian low (right), and the Pacific High in summer (left) |  |
By July the days are getting shorter and the Arctic is cooling off, even though the interior US may be scorching. That translates into the high weakening and moving south. The Jetstreams reflect the change quickly.
Resources used-----
Upper jetstream map from California site; lots good material here; including the color coded enhanced IR image, and 1st year text book exercises.
Lower map from environment Canada precipitation predictions map
