Dry lightning is the phenomenon where a cloud-to-ground strike occurs, but the accompanying precipitation never reaches the surface. It is not a separate type of electrical discharge, but rather a description of the conditions surrounding the strike. This distinction creates a weather event that is particularly dangerous, especially in fire-prone regions.
What Defines Dry Lightning
Dry lightning is the result of a thunderstorm where the precipitation falls from the cloud base but evaporates completely before making contact with the ground. This process of precipitation evaporating in mid-air is known as virga. The visual effect of virga is often a wispy, trailing streak hanging below the cloud base.
A standard thunderstorm often brings heavy rain that can dampen the ground and suppress any potential ignition points caused by lightning. In a dry thunderstorm, the lightning strikes a surface that remains dry, increasing the risk of fire. The presence of virga indicates a layer of very dry air exists between the cloud and the earth’s surface. The official threshold for classifying a thunderstorm as “dry” is generally defined as an event where less than 0.10 inches (2.5 mm) of rain reaches the ground.
Specific Atmospheric Requirements for Formation
The formation of a dry thunderstorm requires a specific vertical atmospheric structure. The storm itself, a cumulonimbus cloud, begins like any other, with warm, moist air rising and condensing high in the atmosphere. This process creates the necessary conditions for charge separation and the eventual production of lightning.
The distinguishing feature is the thick layer of extremely dry air that sits beneath the cloud base and extends down to the surface. As the rain or ice falls from the high cloud base, it encounters this unsaturated air layer. The low relative humidity and high temperatures cause the precipitation to undergo rapid evaporation or sublimation, turning back into water vapor.
This evaporative cooling removes heat from the air, making it denser and heavier. This cooler, denser air then descends rapidly, creating strong, gusty winds known as downbursts or microbursts. High cloud bases ensure the precipitation has a long distance to fall through the dry air, maximizing the chance of complete evaporation before impact.
The Role of Dry Lightning in Wildfire Ignition
Dry lightning is a major natural cause of wildfires, particularly in arid and semi-arid regions such as the Western United States. The danger stems from the combination of a high-temperature ignition source and the absence of moisture to extinguish the resulting spark. When the electrical discharge from the cloud contacts the ground, it transfers immense energy capable of superheating and igniting dry vegetation, known as fuel.
The strike occurs onto fuels that have not been dampened by rainfall. The likelihood of ignition is significantly higher when the live and dead vegetation moisture content is low. This susceptibility is compounded by the strong, erratic winds generated by the storm’s downbursts. These winds can instantly fan a small, smoldering ember into a rapidly spreading flame.
A single dry lightning event can cause multiple ignitions over a wide area, often in remote locations that are difficult for fire crews to reach. These fires can also start as “holdovers,” where the lightning ignites a material that smolders for days or weeks before conditions allow it to erupt into a full wildfire. Lightning-caused fires are responsible for burning the majority of wildland acreage.