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The Dangers of Dry Thunderstorms
June 27, 2023 at 12:35 PM EDT
By WeatherBug Meteorologist, Alyssa Robinette
A dry thunderstorm. Sounds like a contradiction, right? Well, meteorologically speaking, it is possible, and even quite frequent in some areas, to have dry thunderstorms. Here, we examine dry thunderstorms and why they can be dangerous.
Dry thunderstorms can have several meanings and can be considered on a rolling scale. Most often, thunderstorms can be considered “dry” if they produce no rain at the surface. This occurs when “dry” thunderstorms produce rain just below the cloud base but there is a very dry atmosphere below the cloud base, causing the rain to evaporate at some point between the cloud base and the ground. This is also known as virga.
The phrase “dry thunderstorms” is used most frequently in the wildland fire fighting community. The “drier” the thunderstorm, the more efficient it is in terms of fire ignitions by cloud-to-ground lightning when combined with dry vegetation. For the fire weather community, a dry thunderstorm may be used to describe a storm producing very little rainfall, such that it is not effective in checking fire spread after lightning ignition. As a result, thunderstorms are also usually classified as “dry” if they produce rainfall amounts of one-tenth inch or less.
There are some exceptions to this criterion. Any “dry” versus “wet” thunderstorm classification is dependent on the type of vegetation and previous weather conditions. If it has been hot and dry for a long period of time, rainfall amounts will need to be larger to saturate the vegetation completely and lessen the chances of lightning fire ignition. More rainfall is also required if there are significant trees or a closed forest canopy since the rain will have to penetrate the canopy to reach the vegetation below.
Fire weather meteorologists also use the phrase “mixture of wet and dry” to describe thunderstorms which produce a diverse rainfall pattern. This means that the main rain accompanying the thunderstorm produces a “wetting rain,” or rainfall that wets all layers of vegetation, in a small area, while much lighter amounts are found around the periphery of the storm.
Other factors that affect fire ignitions due to thunderstorms include storm motion, the duration of any rainfall which occurs and weather immediately following the passing thunderstorms. Storms that possess a large wetting rain can move quickly and still start fires, generally due to rainfall duration in any one location can be very brief and rain may not effectively penetrate the vegetation. Storms are generally more effective at igniting fires if atmospheric conditions immediately following the storm passage become drier and breezier and the vegetation is especially dry preceding the storm.
Dry thunderstorms are most common across the Western U.S. in late spring or early summer. However, if the preceding winter was dry then followed by very little rainfall during the early spring period, the period mid-April through May is also susceptible to dry thunderstorm effects, particularly fire ignitions. A few episodes of “dry” thunderstorms can also affect the fall period.
In addition to starting wildfires, there is an increased risk for people to get struck by lightning during dry thunderstorms. People often think that they are safe from thunderstorms if it is not raining where you are at. However, if you can see lightning or hear thunder, you are too close and are NOT safe. Remember that you should wait at least 30 minutes from the last rumble of thunder before you head outside.
Regardless of “dry” or “wet” classification, it is never safe to be outside during a thunderstorm. You should always seek safety in an enclosed building or car. When indoors, stay away from plumbing and electricity. This includes staying off phones with cords and not taking a bath or shower.
Source: NWS
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Image: Monsoon thunderstorm with lightning. (John D Sirlin via Shutterstock)
Dry thunderstorms can have several meanings and can be considered on a rolling scale. Most often, thunderstorms can be considered “dry” if they produce no rain at the surface. This occurs when “dry” thunderstorms produce rain just below the cloud base but there is a very dry atmosphere below the cloud base, causing the rain to evaporate at some point between the cloud base and the ground. This is also known as virga.
The phrase “dry thunderstorms” is used most frequently in the wildland fire fighting community. The “drier” the thunderstorm, the more efficient it is in terms of fire ignitions by cloud-to-ground lightning when combined with dry vegetation. For the fire weather community, a dry thunderstorm may be used to describe a storm producing very little rainfall, such that it is not effective in checking fire spread after lightning ignition. As a result, thunderstorms are also usually classified as “dry” if they produce rainfall amounts of one-tenth inch or less.
There are some exceptions to this criterion. Any “dry” versus “wet” thunderstorm classification is dependent on the type of vegetation and previous weather conditions. If it has been hot and dry for a long period of time, rainfall amounts will need to be larger to saturate the vegetation completely and lessen the chances of lightning fire ignition. More rainfall is also required if there are significant trees or a closed forest canopy since the rain will have to penetrate the canopy to reach the vegetation below.
Fire weather meteorologists also use the phrase “mixture of wet and dry” to describe thunderstorms which produce a diverse rainfall pattern. This means that the main rain accompanying the thunderstorm produces a “wetting rain,” or rainfall that wets all layers of vegetation, in a small area, while much lighter amounts are found around the periphery of the storm.
Other factors that affect fire ignitions due to thunderstorms include storm motion, the duration of any rainfall which occurs and weather immediately following the passing thunderstorms. Storms that possess a large wetting rain can move quickly and still start fires, generally due to rainfall duration in any one location can be very brief and rain may not effectively penetrate the vegetation. Storms are generally more effective at igniting fires if atmospheric conditions immediately following the storm passage become drier and breezier and the vegetation is especially dry preceding the storm.
Dry thunderstorms are most common across the Western U.S. in late spring or early summer. However, if the preceding winter was dry then followed by very little rainfall during the early spring period, the period mid-April through May is also susceptible to dry thunderstorm effects, particularly fire ignitions. A few episodes of “dry” thunderstorms can also affect the fall period.
In addition to starting wildfires, there is an increased risk for people to get struck by lightning during dry thunderstorms. People often think that they are safe from thunderstorms if it is not raining where you are at. However, if you can see lightning or hear thunder, you are too close and are NOT safe. Remember that you should wait at least 30 minutes from the last rumble of thunder before you head outside.
Regardless of “dry” or “wet” classification, it is never safe to be outside during a thunderstorm. You should always seek safety in an enclosed building or car. When indoors, stay away from plumbing and electricity. This includes staying off phones with cords and not taking a bath or shower.
Source: NWS
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Image: Monsoon thunderstorm with lightning. (John D Sirlin via Shutterstock)