How can we get frost when the freezing level is higher than our home?

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by Jeff Renner

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KING5.com

Posted on November 14, 2012 at 7:34 PM

How can we get frost when the freezing level is higher than our home?

It's important to cover a basic definition first. The freezing level refers to expected air temperatures NOT affected by the ground. Specifically, the freezing level refers to the altitude at which the temperature is expected to be freezing. Temperatures where we experience them are at the ground, where temperatures are affected by variations in the presence (or absence) of ground cover. Areas covered by trees or plants will cool more slowly, while areas largely barren of cover (rocky areas, for example) will cool more quickly. That has an impact on the air immediately adjacent to the ground. This is why your car may be covered by frost when the observed air temperature is above freezing; metal or glass cools much more rapidly than air. Another factor can be the fact that the density of air (it's 'heaviness') increases as it cools; cold air will tend to slide downslope into valleys or depressions. Official air temperatures are measured about 6 feet above the ground; air temperatures may cool another three to five degrees on clear, windless nights below that level.

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