Thursday, April 16, 2015

Blue Skies, Dry Air, Breezy Days Are Rite Of Spring In Northeast. Here's Why

Pretty much blue skies yesterday in an afternoon
view looking southwest from South Burlington, Vermont
toward still snowcovered Whiteface Mountain in New York.  
Yesterday was one of those bright, brisk, beautiful, blue sky and breezy April days here where I live in Vermont. (How's that for alliteration?)

You tend to get a number of days like this during the early and mid spring in much of southern Canada, the Northeast and the Great Lakes region.

Humidities in these air masses are very low, so the air has a very clear, haze-free quality to it. With plants not yet growing yet, the dead stuff from last year dries out quickly, raising the risk of fires.

There was a red flag warning in much of New England yesterday because of this risk. But otherwise, it was a perfect day.  

This kind of weather happens because areas of high pressure, the kind that was over the Northeast yesterday, tend to have dry, sinking air that discourags clouds. So it probably would have been a pleasant day no matter what time of year that high pressure system decided to come through.

However, the air was especially dry yesterday, which came about for reasons in addition to the overhead weather pattern.

Trees in the region, and in southern Canada, where the air mass came from, haven't leafed out yet. Leaves emit moisture into the air, so had everything been in bloom, the humidity might have been a little higher than yesterday's desert-like levels.

There might have even been a few extra clouds decorating the blue sky had things been leafy.

On top of that, the Great Lakes, and all the lakes in southern Canada and in our region, were either colder than the air, or still frozen.  This set up is common in the spring, as the air responds to the strengthening rays of the sun faster than water does.

Warmer air over colder water tends to prevent rising air currents from bodies of water, such as lakes. So they are not able to add any appreciable moisture to the air.  Less moisture equals fewer clouds, and more blue sky. Which is what we had yesterday.

This is in contrast to New England's incredibly cloudy late autumns and early winters. Then, the water in the Great Lakes and other bodies of water is warmer than the air, since the summer weather in the previous months had just warmed them up

When the water is warmer than than the air, updrafts tend to form and moisture in the water lifts up into the air, tending to form lots of clouds.

But it's spring, so the lakes don't do the moisture thing, since they're so cold.

This time of year, we in the Northeast have to rely on larger scale storm systems to supply the April showers that will (hopefully!) bring us those May flowers.

Luckily, there are several chances of showers over the next week or so in the Northeast. Nothing huge, but enough to keep encourage those trees to bud and early season flowers to bloom.

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