What Is a Dry Thunderstorm? In Plain Words

The glossary definition, in the NWS’s own words

The NWS glossary entry for “Dry Thunderstorm” says:

“Generally a high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as ‘dry lightning’.”

In plain words: the storm still makes lightning. The rain it makes mostly does not reach you — it evaporates into relatively dry air beneath the storm cell on the way down.

Little or no rain at the surface

The NWS Albuquerque fire weather page on dry thunderstorms says “dry thunderstorms” is a phrase used quite often in the wildland fire fighting community, and that it is usually meant to describe thunderstorms that produce little or no precipitation at the surface.

That same page says thunderstorms can be considered dry if they don’t produce any rain at the surface. It also says, 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.

Rain that evaporates before it lands is called virga

The Albuquerque page puts the purest case this way: “dry” thunderstorms produce rain just below the cloud base but, due to a very dry atmosphere below the cloud base, the rain evaporates at some point between the cloud base and the ground. Meteorologically speaking, this is called virga.

The NWS Weather Spotter’s Field Guide says the same thing in its virga section: precipitation falling from the cloud, but does not reach the ground, and the atmosphere below the clouds tends to be very dry and rainfall evaporates before it touches the ground.

Why it matters: lightning can start fires, and there is little rain to check them

The Albuquerque page says: the “drier” the thunderstorm, when combined with dry vegetation (or fuel), the more efficient it is in terms of fire ignitions by cloud to ground lightning.

The NWS Albuquerque monsoon wildfires page says the cloud-to-ground lightning from thunderstorms not only poses a deadly risk to those outdoors, but it can also lead to new wildfire starts or ignitions.

That is the dry part of the problem, in the Albuquerque fire weather page’s words: the storm may produce very little rainfall, and that rain is not effective in checking fire spread after lightning ignition.

By contrast, that page calls a “wetting rain” rainfall amounts that need to penetrate the aerial canopy (or the highest vegetation layer) and reach the surface, in order to moisten all layers of vegetation.

The same page also warns against treating dry versus wet as one fixed line: for fire weather considerations, dry versus wet thunderstorms is a rolling scale, dependent on the type of surface fuel bed and antecedent weather conditions.

Why it matters: gusty winds can come with the evaporating rain

The Spotter’s Field Guide explains the virga wind link this way: the evaporating rainfall cools the air, increasing the air’s density, with gravity pulling the dense air toward the surface, and gusty winds occur in the area of the virga.

The Albuquerque monsoon wildfires page says evaporating rainfall, known as “virga,” can lead to sudden and erratic wind gusts. It also says the combination of lightning and gusty winds from any ongoing virga showers or dry thunderstorms can quickly cause new wildfires to spread rapidly.

The Spotter’s Field Guide also describes a dry downburst this way: little, if any, rainfall accompanies the damaging winds, and sometimes all that is visible is the evaporating precipitation or the dust kicked up off the ground.

Safety, in the NWS’s own words

The national NWS lightning safety overview says: “There is no safe place outside when thunderstorms are in the area. If you hear thunder, you are likely within striking distance of the storm. Just remember, ‘When Thunder Roars, Go Indoors!’”

That page says substantial buildings and hard-topped vehicles are safe options, and rain shelters, small sheds, and open vehicles are not safe.

It also says: if the sky looks threatening or if you hear thunder, get inside a safe place immediately. Once inside, avoid contact with corded phones, electrical equipment, plumbing, and windows and doors. Finally, wait 30 minutes after the last lightning or thunder before going back outside.

The national NWS Lightning Tips page says the same rule in shorter form:

• “NO PLACE outside is safe when thunderstorms are in the area!!”
• “If you hear thunder, lightning is close enough to strike you.”
• “When you hear thunder, immediately move to safe shelter: a substantial building with electricity or plumbing or an enclosed, metal-topped vehicle with windows up.”
• “Stay in safe shelter at least 30 minutes after you hear the last sound of thunder.”

Lack of rain at the ground is not the safety test in those NWS pages. Hearing thunder is.

Why no rainfall number is printed here

The national glossary definition printed above gives no inch amount — it says “little if any precipitation reaches the ground.”

Local NWS pages do give amounts, but they give them for local fuels and local places, and the Albuquerque fire weather page itself says dry versus wet is a rolling scale dependent on the type of surface fuel bed and antecedent weather conditions. That is why this page does not print one nationwide rainfall number for a dry thunderstorm. Use the wording and any local amounts from your own local Forecast Office for the place you are in.