How weather affects fire behavior

Flames erupt in dry conditions in Lake Riverside Estates in September. Anza Valley Outlook/Diane Sieker photo

Weather plays a significant part in the intensity and severity of wildland fires. Changes in humidity, wind and temperature can improve or delay containment efforts. Commonly referred to as fire weather, these conditions may spell disaster for firefighters.

Nick Nauslar of the National Oceanic and Atmospheric Administration’s Storm Prediction Center described fire weather as, “Any sort of weather that ignites and/or helps spread fire. Stronger winds; low, relative humidity; unstable atmospheric conditions and thunderstorms, all fall under the umbrella of fire weather.”

Lightning strikes are the most common weather cause for wildfires. Dry thunderstorms can create lightning strikes miles away from the storm clouds. Several of Anza’s recent fires have been started by lightning strikes to the ground and dry fuels.

Seasonal Santa Ana winds can also cause power lines to spark, which can ignite dry brush nearby. Wind determines the rate and direction of fire spread. Stronger winds supply more oxygen to the fire, preheating fuels and blowing embers and sparks ahead of the flames.

Fires will follow the direction the wind blows, and the faster it is blowing, the faster the flames spread. Gusts can also elevate flames into trees. Wildfires can create their own wind patterns, which also affects how the fire spreads.

Firefighters mop up a wildland fire in the hills east of Temecula in 2018. Anza Valley Outlook/Diane Sieker photo

Hot temperatures and lower relative humidity make available fuels such as grass, weeds and brush more receptive to ignition. In hot weather, the sun is more intense, heating and drying out fuels such as sticks and other flammable debris.

Large fires can create their own weather, and pyrocumulonimbus clouds can produce thunderstorms. A large fire vortex observed at the 2018 Carr Fire near Redding, caused damage equivalent to an EF-3 tornado with estimated winds up to 143 miles per hour.

Relative humidity is a measure of how much moisture is in the air. Humidity varies with temperature. As temperature increases humidity decreases. This factor affects fuel moisture content and its flammability.

Fire requires fuel, oxygen and heat to ignite and grow. Wildfires are more likely to break out in hot seasons or climates.

Meteorologists examine weather patterns and may declare red flag watches or warnings. These warnings indicate a high degree of confidence that weather and fuel conditions meet the “Red Flag Event” criteria for a given fire weather zone. It typically comes into effect for conditions below 15% relative humidity combined with gusty winds.

When red flag warnings are declared, Anza residents need to be aware that many weather conditions can cause an increase in wildfire danger.

Diane Sieker can be reached by email at