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When you hear the word thunderstorm in the daily weather forecast, you automatically think of thunder and lightning accompanied by rain, often as a drenching cloudburst, and wind, gusty at times. Not all thunderstorms are wet, however, particularly in western North America from the Mexico-Arizona border north into British Columbia and the Canadian Prairies. And that worries those watching for or fighting wildfires. These thunderstorms are called dry thunderstorms.
When I say that no rain is falling from the thunderstorm, what I really mean is that no rain falling from the thunderstorm reaches the earth's surfaces. Through most of the life of any thunderstorm, rain falls within the thundercloud and often descends below the cloud base. But in a dry thunderstorm, the rain never wets the ground below, even if we can see it exit the cloud. What has happened to that moisture? Simply put, it evaporated on its descent to the surface. We can see evidence of this if we look under the towering cloud and see streaks or wispy areas below the base that never reach the ground, a condition know as virga. How far below the cloud the virga extends depends on two main factors: the dryness of the air below cloud base and the size of the raindrops. Small drops falling through dry air disappear quickly through evaporation. Even hail can melt and evaporate before reaching the surface, and larger drops can evaporate if the air is dry enough. Virga from dry thunderstorm over the prairies.Dry thunderstorms are more frequent in the western regions of the continent where the air near the surface is often desert-dry and the cumulonimbus cloud base is high in altitude. That region is also the most susceptible to grass and forest fire dangers. Although dry thunderstorms produce no surface rain, they do produce cloud-to-ground lightning and gusty, erratic winds. And this is what wildfire watchers and fighters fear, for lightning hitting trees can spark a blaze at anytime, but often the rain extinguishes it before it burns out of control. But lightning from a dry thunderstorm will not only set timber ablaze, it can often fan the flames with the associated winds. Large fires may even generate their own thunderstorms, and these are frequently dry. The clouds associated with fire-generated thunderstorms are termed pyrocumulonimbus clouds (pyro meaning "fire"). Here is how they form. The heat generated by the fire, combined with moisture driven from the forest and soil and evaporated from any standing water bodies, can initiate the formation of cumulus clouds above the fire zone. When proper atmospheric conditions surround these clouds, some may erupt into pyro-thunderstorms (fire-generated). Often, any rainfall from a pyro-thunderstorm is quickly evaporated in the dry air above the fire zone and thus never reaches the ground. Go To Page: 1 2
The copyright of the article Dry Thunderstorms in Meteorology is owned by Keith C. Heidorn. Permission to republish Dry Thunderstorms in print or online must be granted by the author in writing.
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