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What Is a Dry Thunderstorm?


California Wildfires Rage on in this June 2008 Satellite Image

While wildfires are common in California and the Western United States, they are also devastating weather related events. Many are caused by the dry conditions, high winds, and low humidity.

NASA Earth Observatory
Question: What Is a Dry Thunderstorm?
Wildfires have many causes including
  • lightning
  • volcanic eruptions
  • human causes (Most common - discarded smoking products, campfires, arson, etc.)
  • downed power lines
  • sparks from equipment
Combine the causes with variables such as local topography, wind patterns, humidity, temperature, and fuel sources, and you have the potential for a large wildfire. Although not the most common cause of wildfires, lightning plays a large role in creating devastating fires that can burn hundreds of thousands of acres. Many of the fires are from dry lightning events (or dry thunderstorms).
Answer: A dry thunderstorm is a storm that produces little or no rain. While it may seem contradictory to have a thunderstorm with little rain, it is very common in the western United States where the heat index can be very high. Although a dry thunderstorm is not the only cause to a massive wildfire, long-term drought combined with high spring and summer temperatures can make a lightning strike deadly. These fire weather conditions are the prime ingredients needed to fuel a fire that can rage on for long periods of time.

Much of the reason a thunderstorm can be called dry is due to temperature and humidity below the clouds. It will rain, but the rain (and other forms of precipitation) never reaches the ground. Rain that does not reach the ground is known as virga. Dry thunderstorms can be predicted well in advance of the storm leading officials to warn residents in an area of the possibilities of a wildfire. Incident meteorologists (IMETs) go onto full alert. These specially trained meteorologists look for the fuels that will help a wildfire to spread. IMETs have training in microscale forecasting, fire behavior, and fire operations. They also act as managers who can help to coordinate control efforts. With predictions of wind speed and direction, decisions are made on how to best control and contain wildfires.

The main ingredients for a possible critical fire weather condition are

  • hot temperatures,
  • low humidity,
  • high winds,
  • and a dry fuel source such as brush or trees.
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