A dry thunderstorm is a thunderstorm that produces lightning but little or no rain at ground level. The rain does fall from the cloud, but it evaporates in a deep layer of dry air before reaching the surface, a phenomenon meteorologists call virga. The result is arguably the most dangerous weather pattern for wildfire ignition: electrical discharges striking dry terrain with no accompanying moisture to suppress flames. In parts of the western United States, roughly half of all cloud-to-ground lightning qualifies as “dry,” and these strikes are responsible for some of the largest and most destructive fire events on record.
How Rain Disappears Before It Lands
Every thunderstorm begins with moisture rising into colder air, condensing, and eventually falling as precipitation. In a typical storm, rain survives the trip from cloud base to the ground because the air underneath is relatively humid. In a dry thunderstorm, the lower and middle atmosphere are far drier than normal. Raindrops falling through this parched air lose mass rapidly through evaporation, sometimes vanishing entirely thousands of feet above the surface. The visible streaks of rain hanging beneath a cloud but never reaching the ground are virga, and they are the defining visual signature of a dry thunderstorm.
Research on the atmospheric conditions that separate dry thunderstorms from wet ones confirms that the key distinction is moisture content in the layers below the cloud. A study comparing dry and wet high-wind thunderstorms over eastern China found that dry events develop under conditions with lower humidity at both middle-lower and upper atmospheric layers, while wet thunderstorms typically have lower saturation only at middle-lower levels. Dry thunderstorms are also better characterized by steep temperature drop-offs between roughly 850 and 500 hectopascals of altitude, meaning the air cools rapidly with height, which drives vigorous updrafts even when moisture is scarce.1Theoretical and Applied Climatology. Climatology and pre-convection environmental conditions of dry and wet thunderstorm high winds over eastern China
The evaporation process itself has consequences beyond simply removing rain. As water evaporates, it cools the surrounding air, making that air heavier and causing it to rush downward. These downdrafts can produce sudden, intense wind gusts at ground level, sometimes called microbursts or dry downbursts. So a dry thunderstorm can deliver powerful winds and lightning to the surface without a single drop of rain, a combination that is practically tailor-made for starting and spreading fires.
Where and When Dry Thunderstorms Occur
Dry thunderstorms are most common in regions with mountainous terrain, hot summers, and deep layers of dry air at low and middle altitudes. The interior western United States is a global hotspot, particularly the Great Basin, the Sierra Nevada, and the Cascades. A study of central and northern California found that about 46% of all cloud-to-ground lightning during the study period occurred as dry lightning, with activity concentrated at higher elevations above 2,000 meters and peaking in July and August.2IOP Publishing. Meteorological and geographical factors associated with dry lightning in central and northern California The atmospheric setups driving these events typically involve ridging, a dome of high pressure over the continental interior, combined with troughing offshore, which channels moisture aloft from the south or southwest while leaving the lower atmosphere bone-dry.
Australia is another region with frequent dry lightning, particularly across its vast interior and northern rangelands, where summer thunderstorms regularly form over extremely arid land surfaces. Parts of southern Europe, Central Asia, and northern Africa see dry thunderstorms as well, generally in summer when surface heating is intense but low-level moisture is inadequate. The common thread across all these regions is a disconnect between the atmosphere’s ability to generate deep convection (which requires instability and at least some mid-level moisture) and its ability to deliver precipitation to the ground.
Seasonality matters. In the western U.S., dry thunderstorm risk peaks in late June through August, when monsoonal moisture surges intermittently push humid air aloft without wetting the surface layer. Transition seasons carry risks too: the study of California’s dry lightning found that specific synoptic patterns, such as an upper-level ridge over the interior paired with a trough along the coast, repeatedly set the stage for large dry lightning outbreaks.3IOP Publishing. Meteorological and geographical factors associated with dry lightning in central and northern California These patterns can be identified days in advance by forecasters, which provides a critical window for fire agencies to preposition crews and equipment.
Why Dry Lightning Starts So Many Fires
Lightning-caused wildfires are a fact of life in fire-prone ecosystems, but dry lightning is disproportionately responsible. The logic is straightforward: wet lightning delivers rain alongside the electrical strike, soaking fuels and suppressing ignition. Dry lightning delivers the spark with no moisture at all, and often adds wind gusts that fan any smoldering material. The problem compounds in summer, when vegetation has been baking in heat for weeks and surface fuels are already primed to ignite.
How dry the vegetation is when lightning hits turns out to be a powerful predictor of whether a fire actually starts. A study isolating the effect of live fuel moisture content in California found that when vegetation moisture drops below a critical flammability threshold, the likelihood of fire from a lightning strike is about 1.8 times higher statewide and roughly 2.5 times higher in shrublands compared to when moisture stays above that threshold. When moisture falls further, to 10% below the threshold, the risk ratio climbs above two.4Geophysical Research Letters. Dry Live Fuels Increase the Likelihood of Lightning‐Caused Fires In plain terms, the drier the brush, the more likely any given strike will start a fire, and dry thunderstorms arrive precisely when fuels are at their driest.
Machine learning models developed for Tasmania reinforce the connection between vegetation type, fuel moisture, and dry lightning ignition. The models found that fuel characteristics and moisture content associated with particular vegetation types are among the strongest predictors of whether a dry lightning strike will actually produce a wildfire, offering a way to map which landscapes are most vulnerable in advance.5Geophysical Research Letters. Modeling the Probability of Dry Lightning‐Induced Wildfires in Tasmania: A Machine Learning Approach
The Holdover Problem
One of the most insidious traits of lightning-ignited fires is that they do not always flare up immediately. A lightning bolt can ignite organic material in duff, a rotten log, or a tree’s root system, and that smoldering ignition can persist for hours or even days before conditions align to produce visible flames. This delay is called holdover time, and it creates serious headaches for fire detection and response.
A global database of holdover times for lightning-ignited wildfires shows that the distributions are heavily skewed toward short delays: the first hour after the strike is the most common ignition window, and across most datasets, more than half of fires become detectable within 24 hours. But there is a long tail. Median holdover times across different datasets range from about 1.4 hours to over 53 hours, and in boreal regions the second day after the strike is sometimes the most common detection window rather than the first.6Earth System Science Data. A global database on holdover time of lightning-ignited wildfires A dry thunderstorm can produce thousands of lightning strikes across a landscape in a single evening, and some of those ignitions may not show themselves for two or three days, long after the storm has cleared and fire detection crews have stood down.
Holdover fires are especially treacherous because they can be extremely difficult to locate. A smoldering stump in a remote wilderness area produces minimal smoke, no visible flame, and no heat signature detectable by satellite until conditions change. A burst of wind, a rise in temperature, or a drop in humidity can convert a quiet holdover into an active fire in minutes. This means that the total fire impact of a dry thunderstorm event is not fully known until several days after the event itself, and fire agencies in the western U.S. and boreal Canada have learned to extend aerial patrols for days following large dry lightning episodes.
Detecting Virga and Improving Forecasts
Predicting dry thunderstorms requires identifying storms that will generate lightning but fail to deliver rain. Forecasters use a combination of atmospheric soundings (vertical profiles of temperature and moisture), satellite imagery, and radar to assess whether the air beneath developing thunderstorms is dry enough to evaporate rain before it reaches the surface. The challenge is that the boundary between a “wet” and “dry” thunderstorm is not sharp: a storm might deliver measurable rain at one point and virga half a mile away.
Advances in ground-based remote sensing have improved the ability to detect virga in near-real time. A tool called the Virga-Sniffer, developed during a field campaign in the tropical western Atlantic, combines vertically pointing cloud radar with ceilometer measurements to automatically identify precipitation evaporation streaks. The tool can handle multilayer cloud situations, where rain might fall from one cloud layer and evaporate before reaching a lower layer, and its modular design allows it to be adapted for different environments and sensor configurations.7Atmospheric Measurement Techniques. The Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations While developed for climate research rather than fire forecasting specifically, tools like this point toward a future where virga detection could be integrated into fire-weather warning systems.
The U.S. National Weather Service already issues “dry thunderstorm” forecasts as part of its fire weather outlooks, using parameters like the Haines Index and, more recently, the Hot-Dry-Windy Index to assess the combined threat of ignition and fire spread. The basic forecast question is whether a thunderstorm’s rain will reach the surface. When models indicate mid-level relative humidity below roughly 30 to 40 percent and cloud bases well above 10,000 feet, forecasters raise the probability of dry lightning. These forecasts have improved over the past two decades, but false alarms remain common because the difference between a trace of rain and none at all can depend on micro-scale variations that global and regional models do not resolve.
When Fires Create Their Own Thunderstorms
Large, intense wildfires can generate their own thunderstorms, known as pyrocumulonimbus clouds (pyroCb). These fire-generated storms are a feedback loop: the fire heats the air violently, driving a convective column that can punch into the upper troposphere or even the stratosphere. If enough moisture is present, the column forms cumulonimbus clouds that produce lightning, which can ignite new fires downwind of the original blaze. Because the parent fire has already dried out the surrounding landscape and the atmosphere is typically turbulent and dry at lower levels, many pyroCb lightning strikes qualify as dry lightning.
A study of five pyroCb events in British Columbia during a catastrophic fire season in 2021 found striking variation in lightning output. One large pyroCb over the Sparks Lake fire generated persistent overshooting cloud tops and produced about 5,600 total lightning strikes, while the remaining four events had minimal to no observed lightning. The ratio of cloud-to-cloud versus cloud-to-ground lightning in the Sparks Lake pyroCb was significantly higher than in other high-based thunderstorms, meaning more of the electrical energy stayed aloft rather than striking the ground.8Journal of Geophysical Research: Atmospheres. Impact of Smoke Aerosol Loading on Lightning Characteristics of Pyrocumulonimbus Compared With Other High‐Based Thunderstorms Even so, the cloud-to-ground strikes that did occur landed in a landscape already in crisis, and the interaction between smoke aerosol, convective intensity, and lightning production remains an active research question.
PyroCb events are becoming more studied as wildfire seasons intensify. They inject massive quantities of smoke aerosol into the upper atmosphere, which can persist for months and affect regional air quality far from the fire itself. The 2021 British Columbia event and the 2019-2020 Australian bushfire season both produced pyroCb plumes tracked by satellites for weeks, crossing oceans and continents.
The Wind and Health Dimensions
Dry thunderstorms are primarily discussed as a fire hazard, but they carry other risks that get less attention. The evaporative cooling that drives downdrafts can produce sudden wind gusts exceeding 60 miles per hour at the surface. These “dry microbursts” are a hazard to aviation and can damage structures, down power lines, and kick up dust storms. In arid regions, a dry thunderstorm can generate a haboob, a wall of dust hundreds or thousands of feet tall that reduces visibility to near zero and deposits fine particulate matter across populated areas.
Thunderstorms in general have measurable health effects that go beyond physical injury from lightning or wind. A study in New York State found that thunderstorms and power outages were each independently associated with increases in respiratory-related emergency visits, but the effect was substantially stronger when both occurred together. During pollen seasons, the combination produced the highest risk, with respiratory visits rising by roughly 44% compared to days with neither thunderstorms nor outages.9PubMed Central. The Joint Effects of Thunderstorms and Power Outages on Respiratory-Related Emergency Visits and Modifying and Mediating Factors of This Relationship The mechanism involves thunderstorm outflows fragmenting pollen grains into respirable-sized particles and driving them to ground level. Dry thunderstorms, with their strong downdrafts and dusty environments, are particularly good at this kind of atmospheric stirring.
Ecosystems Shaped by Lightning Fire
It is tempting to view dry thunderstorms purely as a hazard, but the fires they ignite have been shaping ecosystems for millions of years. Many plant communities in fire-prone regions are not just tolerant of fire but dependent on it. Serotinous cones that open only after exposure to heat, thick bark that insulates living tissue, and underground root systems that resprout rapidly after burning are all evolutionary responses to recurring fire. A comprehensive review of evolutionary fire ecology notes that there is now overwhelming evidence that fires are very old in the geological record and that many plants and animals have adapted to the particular fire regimes they experienced throughout their history.10PubMed Central. Evolutionary fire ecology: An historical account and future directions
Dry lightning has been the primary natural ignition source in many of these ecosystems, particularly in western North American forests where summer lightning is abundant but rain is scarce. The fire regimes that shaped ponderosa pine savannas, chaparral shrublands, and boreal spruce forests all owe much of their character to dry thunderstorms. Frequent low-intensity fires maintained open forest structures, cycled nutrients back into the soil, and cleared understory growth that would otherwise fuel catastrophic crown fires.
The trouble with the modern situation is not that dry thunderstorms exist or that they start fires. It is that a century of fire suppression has allowed fuels to accumulate far beyond historical levels, and warming temperatures have extended the fire season and dried out vegetation earlier and longer. Dry thunderstorms now strike landscapes that are both drier and more fuel-loaded than at any point in recent centuries. The result is fires that are more intense, harder to control, and more damaging to communities and ecosystems than the lightning fires that once played a constructive ecological role. Fuel moisture is dropping, fire seasons are lengthening, and the atmospheric conditions that generate dry thunderstorms are not becoming rarer. For fire managers, the challenge is less about preventing dry lightning and more about restoring landscapes to a condition where they can absorb it the way they did for millennia before human intervention disrupted the cycle.