All thunderstorms produce lightning but not all thunderstorms produce rain that reaches the ground. For people in humid climates it may seem somewhat unusual for thunderstorms to develop but have very little rainfall occur. For the western interior U.S., low rain thunderstorms are common in the warm season. Some would think that thunderstorms would not ignite forest fires since the rain would put out the fire. This is not always the case because some lightning strikes occur away from the immediate vicinity of the thunderstorm and not all thunderstorms have significant rainfall with them. What causes a low precipitation thunderstorm? Low precipitation thunderstorms have characteristics of a dry lower troposphere and a high cloud base. Because the lower troposphere has a large dewpoint depression, rising air will take longer to become saturated. Thus, the cloud bases will be high based. A high cloud base and a large low level dewpoint depression result in the thunderstorm having a relatively small amount of moisture to condense out and a long vertical distance for raindrops to evaporate before reaching the surface. If the forest below is dry, a cloud to ground lightning strike can easily start a fire. Forest fires are a natural process and are needed to clear excess vegetation, regenerate new growth, and add nutrients to the soil. However, fires that occur in areas of man's domain are looked on as destructive and must be stopped. As years pass, some areas become increasingly susceptible to fire as old vegetation accumulates. This increases the likelihood of massive forest fires when a prolonged dry period occurs. |