The Mystery of Lightning-Induced Fires in Rainy Weather: Explained

During thunderstorms, lightning strikes often precede rain, leaving the question: can lightning still ignite fires even when rain is present? This article explores why lightning can cause fires even in the midst of a storm, introduces the phenomenon of dry thunderstorms, and explains the role of fuel moisture content in wildfire behavior.

How Lightning Can Ignite Fires in Rainy Weather

Lightning strikes can indeed cause fires even during rain due to several factors. The primary reason is the high temperature of a lightning strike. When lightning strikes, it can reach temperatures of up to 30,000 degrees Fahrenheit (16,600 degrees Celsius), which is hot enough to ignite dry vegetation, trees, and structures.

Additionally, rain may not fall directly at the location of the lightning strike. Dry thunderstorms are a significant factor here. These storms produce lightning but little to no rain reaching the ground, especially in arid regions. The ground can also be very dry in certain conditions or regions, allowing a lightning strike to ignite flammable materials like grass, leaves, or wooden structures.

Fire spread is another critical factor. If a fire starts from a lightning strike, it can spread quickly, particularly in windy conditions or dry environments. This rapid spread can result in large fires despite nearby rain.

Dry Thunderstorms: The Hidden Threat

Thunder and lightning are produced by thunderstorms, but it does not always rain during these storms. There are different types of thunderstorms, including:

Rain-Producing Storms

The most common type of thunderstorm is the rain-producing storm, where thunder, lightning, and rain occur together.

Dry Thunderstorms

These storms are particularly common in arid regions and are known to produce lightning without any significant precipitation reaching the ground. Dry thunderstorms can lead to wildfires without any accompanying rainfall. This phenomenon is also known as a “dry strike” where lightning hits the ground several miles ahead of the rainstorm.

As a lookout, I have observed numerous dry strikes. Sometimes, the rain never reaches the area where the strike happened, leaving dry ground and vegetation susceptible to ignition. For instance, forest fuels that are less than 6mm in diameter, like leaves and twigs, can only burn if the fuel moisture content (FMC) is below approximately 16%.

Fuel Moisture Content: The Key to Wildfire Behavior

In my experience, 16% FMC represents a critical threshold for bushfire behavior. When fuel moisture content is above this point, a fire struggles to sustain combustion. However, below 12-15% FMC, a fire becomes more reliable and thorough, but may not be ideal for ecological outcomes. Below 10% FMC, wildfires become difficult to control due to the high readiness of all fine fuels to ignite.

Lightning that strikes in areas where the FMC is already low (below 16%) can initiate combustion because the lightning generates sufficient heat to overcome the moisture in the fuels. Consequently, if the rain arrives later and increases the FMC, it can extinguish the fire, leaving only a small patch of land burned.

In summary, while rain often accompanies thunderstorms, it is not a guaranteed occurrence. Lightning can cause fires under specific conditions, particularly in dry areas or during dry thunderstorms. Understanding the role of fuel moisture content is crucial for predicting and managing forest fires.

Keywords: lightning strikes, dry thunderstorms, fuel moisture content, dry storms, fire behavior