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What's a 'dirty' high-pressure system?

Tracy Humphrey explains how a dirty ridge of high-pressure is bringing foggy mornings to the Central Valley
Credit: ABC 10

CALIFORNIA, USA — A dirty ridge of high-pressure has set up over the area, and it's bringing several mornings of foggy skies. Here's why.  

Recent rainfall courtesy of a cold front Monday has left the valley well saturated. This brings more moisture to the air which makes it easier for air temperatures to reach their dew points, which leads to condensation which creates clouds. When these clouds are trapped at the ground level, we call that fog.  

This is where the high-pressure system comes in. High pressure is a region of sinking air.  Traditionally, a high pressure system would yield clear skies. It brings clear skies, because air dries out as it's compressed and sinks to the ground. This would bring clear skies, which are typically associated with high-pressure systems.

However, because of the recent rains, this high pressure is trapping clouds and fog at the surface. That's why it's called a "dirty high."  

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