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VIRGA AND DRY THUNDERSTORMS


The sweltering heat of summer also brings thunderstorms to the Inland Northwest and instability and moisture rises. Sometimes showers and thunderstorms develop with a limited amount of moisture in the air. This leads to a fantastic sound and light show due to the thunder and lightning, but invisible rain. The rain evaporates as it falls into a layer of dry air near the earth's surface. This is called virga. As falling precipitation evaporates it moistens the dry air from the top down. If the precipitation lasts, the air becomes moist enough to block evaporation and the rain or snow reaches the ground.

Rain is always good news to firefighters battling wild land blazes. But the threat of summer thunderstorms in the West often puts firefighters on the alert. Even though the rain doesn't make it to the ground, flashes of lightning streak from the clouds to the ground. This lightning can start fires in dry woods with no rain to extinguish or slow the blazes. Lightning accounts for a large number of the forest fires in the Western US.


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