The term "dry lightning" is heard quite a bit during dry, hot months. Residents of western U.S. states are very familiar with the term. Summer time news stories are full of wild fires started by dry lightning. But what is dry lightning? Why are they notorious for producing wildfires?
In fact, the term dry lightning is a misnomer since lightning is never wet. The "dry" in dry lightning is defined as lightning produced in a rainless storm. This is in contrast to "wet" storms, where lightning strikes while rain is falling and hitting the ground. In these cases any spark caused by the lightning is quickly put out by the rainfall. High winds are less damaging due the increase in moisture at the earth's surface.
In the case of dry lightning, rain falls through very dry air which evaporates the moisture before it hits the ground. This form of precipitation that never hits the ground is known as "virga". When lightning strikes it may cause a spark. This can result in a fire in the area of the storm. Even in cases where a storm produces some precipitation, the lightning may still be considered "dry". Up to one-tenth of an inch of rain is still considered a "dry" storm.
Mid-western states tend to have far more dry lightning incidents due to the fact that the storms start much higher up in the atmosphere. This produces a larger dry zone in which the rain evaporates and returns to the clouds. The effect of this is to keep the storm producing more rainfall and lightning. The main factors that produce dry lightning are hot temperatures and low humidity. Most western and mid-western states fit these conditions perfectly in the summer months.
The phenomenon of dry lightning produces another weather effect that exacerbates the fire problem. As the rain evaporates at a higher altitude, dry micro-bursts of cold air rush down. This produces strong gusts of surface wind that help fan the flames. Lightning strikes that set fire to dry wood or brush quickly turn into massive forest fires as the winds breathe life into the flames.
Some protection against dry lightning can be achieved with a little preparation. Keeping wooded areas free from excess dry wood and brush will limit the fire damage. Reducing flammable material around houses and keeping lawns trimmed low will further cut back on residential fires from dry lightning storms. Lastly, forming firewalls or firebreaks around homes will help slow the spread of fire from one property to another. When in doubt, always check the local forecast and take precautions like these if dry lightning is on its way.