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Frost Pockets
Thu Oct 28, 2004
Listen in RealAudio 
Frost Pockets, coming up on The Weather Notebook.
The first frosts of the season are the scourge of gardeners and farmers, and those living in hilly terrain are especially threatened when that first cool blast of autumn arrives. Hi, I’m Bryan Yeaton for The Weather Notebook.
At night, the surface radiates heat, and the under clear, long autumnal nights, surface air can become very cold, particularly at higher elevations that were colder than the terrain below when the sun set. Since cold air is also heavier air, it flows downhill just like water—often following the same paths.
In fact, terrain height differentials as small as three feet can cause cold air to flow downslope, even in calm conditions. Usually, cold-air drainage flows gently downhill at five to ten miles per hour, but in steep terrain with extreme cooling --- such as off high mountain glaciers or snowfields --- downslope winds can hit nearly 30 mph.
Eventually, the cold, dense air stops flowing and pools in terrain hollows and depressions, or fills a streambed or closed canyon with cold air.
When below-freezing air fills a small depression, microclimatologists call it a frost pocket. Frost pockets are generally the first spots to be hit by late summer or autumn frosts, and are also the last to see frosts leave in the spring.
Frost pockets not only suffer early killing frosts, they also have shorter average growing seasons, since cooler air collects within them even when above freezing. As a result, neighboring zones may contain distinctly different micro-environments that encourage more cold-tolerant plants.
Commercial growers as well as recreational gardeners must be aware of the potential for frost pockets in planting their land, particularly for frost sensitive plants such as tomatoes and geraniums.
Thanks to our contributing writer, meteorologist Keith Heidorn. Our show is produced by the Mount Washington Observatory, and funded by Subaru of America.
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