Thank the ‘dirty high’ for the clouds, drizzle, and fog

Posted at 6:35 PM, Dec 13, 2014
and last updated 2014-12-13 18:35:52-05

WEST MICHIGAN — It’s not too often we talk about high pressure systems in a negative way…since most of these systems dry the air mass out, create sinking air or subsidence, and usually give us sunshine. This weekend, despite the high pressure ridge over the Great Lakes, the high itself is dirty. That simply means it fails to clear our skies out and we’re left with a deck of low level stratus clouds, fog, and drizzle.

In the case of this dirty high, we can thank a subsidence inversion for setting up and trapping moisture at the surface. It simply means that warm, sinking air aloft creates warmer temperatures above the ground. When the air aloft is warmer, we refer to that as a temperature inversion. Different from normal, the air usually cools as you leave the surface. Long story short, moisture (clouds/fog/drizzle) can get trapped below this inversion and leave us with gloomy days. It’s especially difficult to break the temperature inversion this time of year with a very low sun angle and weak strength. That, coupled with little/no wind, and the dull, gray, cloudy skies tend to stay locked in place. At least temperatures made the 40s Saturday and will approach 50 Sunday and Monday.

The image attached to this story shows the upper level Jetstream and the huge bubble (or ridge) over the Great Lakes. The active weather the past few days has been on the West and East Coast simultaneously! Once the ridge lifts out, we’ll see rain chances increasing later Monday and Tuesday with cooler temperatures by Wednesday.

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