Why is frost more likely in valleys?

The National Weather Service has issued Freeze Warnings for Jo Daviess, Stephenson, Boone, Carroll, Whiteside, Ogle, Lee, DeKalb, and Green County for early Saturday morning as temperatures dip into the 20s and 30s.

You may have heard us in recent weeks forecast cooler temperatures in rural valleys. Typically, they are the coolest spots at night around here. The reason is the air at higher elevations becomes heavier and sinks as night wears on, pooling in the lowest elevations. In Meteorology this is known as negative buoyancy. The cool air descends hills along the same terrain as rain or snow would run off. When the air reaches the valley floor, it will keep cooling due to continued radiational heat loss as the night wears on.  Cold air pools can be very deep in mountainous areas but can be as small as just a few feet around here. In short, cover your plants if you’re in a valley!

Interestingly, any obstruction the cold air encounters entering a valley causes the air to stop, much like water running into a dam. In this example, a stand of trees may block the wind, causing a frost pocket.The temperature may be several degrees colder in this shaded area than the valley floor itself.

In addition, valleys with ponds or rivers can provide fog. -ES

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Posted under cold blast, fog, weather geek

This post was written by qni_it on April 20, 2012

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