Every year in the U.S., an average of 71,000 wildfires burn over 7 million acres of land. Once a wildfire has started, several things, including weather, can influence its spread and intensity.
The types of weather that create favorable conditions for wildfires are collectively referred to as fire weather. They include:
Air temperature has a direct influence on fire behavior. The warmer the air temperature, the more the fuel sources (leaves, grass, branches, logs, etc.) are already heated by the sun, and the less additional heat is needed to ignite a spark.
There's a reason for the expression "Don't fan the flames." Wind increases the supply of oxygen which causes a fire to burn hotter. As it blows over a surface, it also removes moisture/increases evaporation, which dries the fuel source out even more. Finally, wind can increase the spread of fire by blowing hot embers to new areas outside of the parent fire.
Low Relative Humidity
Recall that relative humidity tells us how much moisture (in the form of water vapor) is in the air versus how much moisture the air could hold at it's current temperature. The lower the RH, the quicker the moisture will leave a fuel source and the more readily a fire will start and burn.
Atmospheric stability describes the atmosphere's tendency to either resist or encourage vertical motion. If the atmosphere is unstable, air easily moves upward. This type of environment increases fire activity because the vertical movement and mixing of air (updrafts) and increases the potential for gusty surface winds.
Fire Weather Products
While the above-listed conditions are notorious for fueling fires, the National Weather Service (NWS) will not issue official warnings until certain threshold values--called red flag criteria, or critical fire weather conditions--are forecast to occur. While red flag criteria may differ from state to state, they usually include RH values of 20% or less and winds of 20 mph (32 km/h) or higher.
Once a forecast suggests red flag criteria are likely to be met, the NOAA National Weather Service then issues one of two products to warn the public and area management officials of the potential threat to life and property should fire ignition occur: a Fire Weather Watch or a Red Flag Warning.
A Fire Weather Watch is issued 24 to 48 hours before the onset of red flag criteria, whereas a Red Flag Warning is issued when red flag criteria are already occurring or will occur within the next 24 hours or less.
On days when one of these alerts is in effect, you should avoid outdoor burning activities, such as:
- Burning trash, leaves, brush, and yard trimmings
- Burning outdoor luminary candles (lanterns, tiki torches, etc.)
- Setting off fireworks
- Discarding cigarettes outdoors
- Building large campfires and leaving these unattended.
In addition to issuing fire weather alerts, the National Weather Service deploys specially trained forecasters to locations where large wildfires are active. Called Incident Meteorologists, or IMETs, these meteorologists provide on site weather support (including weather monitoring and daily fire weather briefings) to the command staff, firefighters, and other incident personnel.
Looking for the Latest Fire Weather Data?
Current fire weather information is available through these sources:
- NOAA's Storm Prediction Center Fire Weather Forecasts (a highlight of areas in the contiguous U.S. where a significant threat of wildfires exists during the next 8 days)
- NWS Red Flag Warning Products (a list of U.S. locations with current active red flag watches and warnings)
- USDA Forest Service Large Incidents Map (an interactive map of active fire incidents in the U.S. and Canada)
- InciWeb (a log of national wildfires that includes information such as fire type and total acres burned)