رعد و برق خشک: عامل آتش‌سوزی جنگل

Dry lightning is the most insidious form of the atmosphere's most dramatic spectacle. Unlike the lightning that accompanies heavy rain — where the same storm that produces the fire-starting spark also produces the rain that extinguishes it — dry lightning strikes the ground from thunderstorms whose rain evaporates before reaching the surface, delivering ignition to tinder-dry landscapes without the compensating moisture that would suppress the resulting fire. In the arid and semi-arid regions of the world, including the Mediterranean, dry lightning is the primary natural cause of wildfire — and in a warming climate that is simultaneously producing more thunderstorms and drier landscapes, the threat of dry lightning fire is intensifying in ways that challenge fire management, ecosystem resilience, and human safety.

What Makes Lightning "Dry": The Virga Mechanism

All thunderstorms produce rain — the condensation and ice processes that generate lightning also produce precipitation. What distinguishes a dry thunderstorm from a wet one is not the absence of rain at cloud level but the fate of that rain as it falls. When a thunderstorm forms over a deep layer of hot, dry air — as is common in summer over continental interiors and mountain regions — the rain that falls from the cloud base enters an environment where the air's capacity to absorb moisture far exceeds the moisture content of the falling rain. The rain evaporates, a process that cools the surrounding air (evaporative cooling) and produces the visible wispy streaks of precipitation that never reach the ground — a phenomenon called virga.

The height of the cloud base is a critical factor. In moist environments (tropical maritime air, for example), cloud bases are typically 500–1,500 metres above the surface — a short distance through which rain falls quickly and arrives at the ground largely intact. In dry environments, cloud bases can be 3,000–5,000 metres above the surface, creating a deep layer of dry air through which rain must fall. At the typical fall speed of raindrops (approximately 5–10 m/s), the transit from a 4,000-metre cloud base to the surface takes 6–13 minutes — ample time for evaporation to consume all but the heaviest rainfall. The result is a thunderstorm that produces full electrical activity — the charge separation and lightning generation are unaffected by whether the rain reaches the ground — but delivers no rain to the surface.

The downdrafts produced by evaporative cooling during virga are an additional hazard. As rain evaporates into the sub-cloud air, the cooling creates negatively buoyant air that accelerates downward, producing gusty surface winds that can exceed 80 km/h. These "dry microbursts" arrive at the surface as hot, dry gusts that fan any fires ignited by the lightning — a cruel combination in which the storm produces both the ignition source (lightning) and the wind that drives the resulting fire, without the rain that would suppress it. The dry microburst is the atmospheric equivalent of an arsonist with a bellows.

The Fire Connection: From Spark to Conflagration

Not all lightning strikes produce fire. The critical factor is the duration of the current flow — specifically, whether the lightning stroke includes a "long continuing current" (LCC) component. A typical lightning return stroke lasts only a few milliseconds — too brief to ignite most fuels. But approximately 20–30 percent of cloud-to-ground strokes include an LCC component — a sustained current flow of 50–500 milliseconds following the initial stroke — that delivers enough energy to ignite fine fuels such as dry grass, pine needles, and dead leaves. In conditions of extreme fire danger (fuel moisture below 8–10 percent, temperatures above 35°C), even shorter continuing currents can produce ignition.

The fires that result from dry lightning ignition are often distinctive in their pattern. A single thunderstorm complex can produce dozens to hundreds of ground strikes across a landscape in a matter of hours, creating multiple simultaneous ignition points spread across a wide area. Unlike human-caused fires, which typically start at a single point and spread from there, dry lightning fires can overwhelm firefighting resources by producing dozens of simultaneous fires across remote, mountainous terrain. The 2008 California dry lightning siege — in which a series of dry thunderstorms produced approximately 12,000 lightning strikes across northern and central California over three days — ignited more than 2,000 fires, many in remote forested areas, that burned for weeks and consumed over 500,000 hectares.

The timing of dry lightning fires compounds their danger. They typically occur during the hottest, driest period of the fire season — midsummer to early autumn — when fuel moisture is at its lowest and fire behaviour at its most extreme. They often occur in remote areas (mountainous terrain favours the elevated heating that triggers dry thunderstorms), where detection is delayed and access for firefighting is difficult. And they often occur in the afternoon and evening, when temperatures are highest and relative humidity is lowest, allowing fires to establish and grow before the cooler, more humid conditions of night provide any moderation. By the time a dry lightning fire is detected and firefighting resources are deployed, it may have grown beyond the point where initial attack can succeed.

Meteorological Forecasting: Predicting the Invisible Storm

Forecasting dry lightning events requires identifying the atmospheric conditions that produce thunderstorms with high cloud bases over deep, dry sub-cloud air. Meteorologists use several diagnostic tools: the Haines Index (now largely replaced by the Hot-Dry-Windy Index), which combines measurements of atmospheric dryness and instability to estimate fire weather potential; upper-air soundings that reveal the moisture profile from surface to cloud level; and satellite imagery that shows convective development over terrain while surface observations confirm the absence of rainfall. The combination of a high lifted condensation level (indicating a high cloud base), steep mid-level lapse rates (indicating instability sufficient for thunderstorm development), and low surface humidity creates the forecast signature for dry lightning potential.

The challenge for forecasters is that the transition from "wet" to "dry" thunderstorms is not binary but continuous — a thunderstorm may produce significant rain at its core while its periphery yields only virga and dry lightning. This spatial variability means that a single storm complex can simultaneously suppress fire in one area (under the rain core) and ignite it in another (under the dry periphery). Radar reflectivity data, which measures precipitation intensity, can help distinguish wet from dry sectors of a storm, but the resolution and coverage of radar in the mountainous terrain where dry lightning is most common are often insufficient for this discrimination. The forecasting of dry lightning remains one of the most difficult challenges in fire weather meteorology.

Geography of Dry Lightning: Where It Strikes

Dry lightning is predominantly a phenomenon of continental interiors, mountain regions, and areas where hot, dry air masses underlie thunderstorm development. The western United States — from the Sierra Nevada to the Rockies — is the global epicentre of dry lightning fire, with the combination of high-elevation terrain (which triggers thunderstorm development), deep dry air masses (which evaporate precipitation), and fire-prone vegetation (which provides the fuel) creating conditions that produce thousands of dry lightning fires annually. The Great Basin, the interior valleys of California, and the forested mountains of Idaho, Montana, and Oregon are particularly affected.

In the Mediterranean, dry lightning is less frequent than in western North America but increasingly significant. The combination of hot, dry summer air masses and thunderstorm development over the mountainous terrain of the Mediterranean basin produces dry thunderstorm conditions, particularly in midsummer when surface temperatures are highest and the atmosphere is driest. Greece, Turkey, and the western Mediterranean experience dry lightning events primarily when continental heat builds to extreme levels and isolated thunderstorms develop over mountain ranges without producing rainfall that reaches the valley floors. The Greek mountains — Pindus, Olympus, Rhodope — experience these conditions several times each summer, and the fires that result contribute significantly to the annual burned area.

Australia, southern Africa, and the boreal forests of Canada and Russia also experience significant dry lightning fire. In Australia's interior, the combination of extreme heat and occasional thunderstorm activity over the desert and savanna produces dry lightning fires that can burn millions of hectares of grassland and bush. In the boreal forests, summer dry lightning ignites fires in the vast, largely unmanaged forests of Siberia and northern Canada — fires that are allowed to burn because no human infrastructure is threatened, but that release enormous quantities of carbon and smoke that affect air quality across the hemisphere.

Detection and Response: Fighting an Invisible Ignition

Lightning detection networks — systems of electromagnetic sensors that triangulate the location of lightning strokes by detecting the radio-frequency radiation they produce — are the primary tool for identifying dry lightning fire risk. These networks, which include the National Lightning Detection Network (NLDN) in the United States and the ZEUS and EUCLID networks in Europe, locate lightning strokes to within a few hundred metres and classify them by type (cloud-to-ground versus intracloud) and polarity. By overlaying lightning stroke data with precipitation data from weather radar, forecasters can identify areas where lightning has occurred without significant rainfall — the signature of dry lightning — and direct aerial and ground reconnaissance to those areas to detect fires while they are still small.

The challenge of detection is compounded by the phenomenon of "holdover fires" — fires that are ignited by lightning but do not produce visible smoke for hours or even days after the strike. In heavy fuels (large logs, deep organic soil), a lightning strike can initiate smouldering combustion that persists beneath the surface, invisible to aerial detection, until it breaks through to the surface and produces the visible smoke that triggers a fire report. Holdover fires have been documented up to two weeks after the lightning event that caused them, creating a detection problem that requires sustained surveillance of areas where dry lightning has been detected long after the thunderstorm has passed.

Firefighting response to dry lightning events requires a fundamentally different strategy from response to single-point human-caused ignitions. Because dry lightning produces multiple simultaneous fires, the principle of rapid initial attack that guides most firefighting operations — detecting each fire quickly and suppressing it before it grows large — breaks down when the number of new fires exceeds the available initial attack resources. During major dry lightning events, fire managers must triage — assessing which fires threaten values (structures, communities, watersheds) and prioritising those for initial attack while accepting that fires in remote, low-value areas may be allowed to burn or may receive only monitoring until resources become available.

Climate Change and the Intensifying Threat

Climate change is altering the dry lightning fire equation in ways that are predominantly negative for fire management. The relationship operates through two primary mechanisms: more thunderstorms (warming increases the energy available for convection, producing more frequent and more intense thunderstorms in many regions) and drier fuels (warming increases evapotranspiration, drying vegetation earlier in the season and to lower moisture levels). The combination — more ignition sources in a more flammable landscape — is projected to increase dry lightning fire occurrence by 30–100 percent in fire-prone regions by the end of the century, depending on the emissions scenario and regional climate response.

Research published in the journal Science in 2014 estimated that lightning frequency in the contiguous United States would increase by approximately 12 percent per degree Celsius of warming — a significant increase that translates to thousands of additional lightning strikes per year in already fire-prone regions. If a proportionate fraction of these additional strikes are dry lightning (occurring in dry conditions over dry fuels), the corresponding increase in fire ignitions could substantially increase annual burned area. Subsequent research has refined these estimates but confirmed the direction: a warmer atmosphere is an electrically more active atmosphere, and more lightning means more fire.

The Mediterranean basin, already one of the regions most affected by climate-driven changes in fire regime, faces particular risk from the dry lightning intensification. As summer temperatures continue to rise and heat waves become more frequent and severe, the atmospheric conditions that favour dry thunderstorm development — extreme surface heating beneath elevated moisture from monsoonal or frontal sources — will become more common. Greece's fire seasons of 2021 and 2023, which included significant dry lightning fire activity alongside the dominant human-caused ignitions, may be indicative of the intensifying role of dry lightning in the Mediterranean fire regime.