What Is Fire Weather? Definition and Monitoring

Occasionally, on a breezy but otherwise pleasant day, a weather alert will pop up on your cell phone or TV. No, it's no mistake; it's most likely a notification for fire weather—weather conditions favorable for the ignition and spread of wildfires.

Fire weather can occur in any season but peaks during late-summer and fall when drier air and dry fuels (fallen leaves and dormant trees) are commonplace. Although fire weather is experienced globally, it poses a greater risk to locations like the western United States, Australia, Africa, and the Amazon, which are prone to wildfires.

Conditions That Cause Fire Weather

In order to burn, fire needs three ingredients: heat, oxygen, and a dry fuel source. The following weather conditions conspire to supply these, and spell fire danger because of it.

High Air Temperatures

Very warm temperatures increase evaporation, which in turn strips moisture from easily combustible materials, including grasses, shrubs, trees, dead leaves, and pine needles that act as kindling for wildfires. Sun-warmed fuels also ignite faster, since less heat energy is needed to bring them up to their ignition temperature. 

Low Precipitation

Precipitation dampens the surface of fuels to the point that fire cannot ignite. A lack of rain or snow, or in extreme cases, a drought, does the exact opposite; it dries out fuels, thereby allowing them to combust more easily.

Low Soil Moisture

Soil moisture (the amount of water contained by soil) is a good indicator of "fuel moisture," or how full of water living plants are. When soil moisture is low, local vegetation is likely dry and water-stressed, which also means it's more likely to burn. According to a study relating soil moisture to wildfire size in the Southern Great Plains, soil moisture plays such an integral role in fire activity that it outweighs the contributions of warm temperatures and low precipitation.

Low Relative Humidity

When relative humidity (a measure of how much water vapor is in the air) is low, it helps to dry out fuels, making them more flammable. 

Gusty Winds

Should a fire ignite, winds can worsen it in a number of ways. For one, they supply fire with more oxygen, which results in it burning more rapidly. High winds also reduce fuel moisture by increasing evaporation, as well as encourage a fire to spread by physically pushing it and transporting embers ahead of its flaming front.

If you're watching the weather map, look for low humidity and strong, gusty winds to move in after the passage of a dry cold front (a cold front associated with a dry air mass). Critical fire weather is also commonly linked to high pressure in upper levels of the atmosphere, since these weather features can act as "heat domes," bringing clear skies, sinking air, very dry air, and above-average temperatures during the warmer months of the year, of course.

Fire Weather Watches and Warnings

Because fire control so heavily relies on weather, NOAA's National Weather Service (NWS) works in tandem with land management organizations to monitor problematic weather patterns. When several fire weather conditions occur simultaneously, and coincide with dry fuels, the NWS will either issue a fire weather watch or a red flag warning.

Fire Weather Watch

A fire weather watch is issued when red flag criteria might be met in the near future, generally within the next 24 to 72 hours.

Watches give the public and fire crews time to prepare for an elevated fire risk.

Red Flag Criteria

Red flag criteria are threshold wind and humidity values that signal an increased risk of fire danger. Criteria are set by local NWS offices, and vary from region to region, depending on the local vegetation type, topography, drought conditions, and more. At a minimum, the criteria include:

• Winds of 15 miles per hour or greater (measured at a height of 20 feet above ground).
• A minimum relative humidity (usually occurring in the afternoon) of less than 25%.
• A 10-hour fuel moisture (a measure of how much water is held by grass and leaves which take 10 hours to respond to changes in wetness/dryness) of 10% or less.

Red Flag Warning

If a red flag warning is issued, it means that red flag criteria are already being met, or will be met shortly, generally within the next 12 to 24 hours.

Expect any fires that ignite to spread rapidly and become difficult to control or suppress. Under red flag warnings, burn bans will also be in effect.

How Climate Change Is Impacting Fire Weather

If it seems as though you're seeing more red flag warnings today than in years past, blame climate change. Global warming is actually increasing fire weather season length, or the number of days each year when atmospheric conditions are ripe for fire danger. A study in Nature Communications reveals that between 1979 and 2013, fire weather seasons have lengthened by an average of 19% across one quarter of Earth's vegetated areas. Zoom in on western U.S. forests, and you'll find that fire weather seasons there have lengthened by eight days.

This same study also looked at longer-than-normal fire weather seasons. It found that these, too, have become more frequent as a result of climate change—53% more frequent, globally.

Research focusing on California finds that, since the 1980s, the state's increases in autumn temperature and decreases in precipitation amount to a 20% increase in fire weather indices. If recent trends continue, California could see a 25% increase in its autumn fire weather days by 2100.