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The use of red flags to signal danger dates back to at least the 1700s. For example, spotting a red flag on a beach warns you of hazardous surf or dangerous rip currents. However, when it comes to weather, a 'red flag warning' points to a different kind of risk: fire.
What is a Red Flag Warning exactly?
It essentially signals that the conditions are perfect for a fire to ignite and spread: high temperatures, low humidity, and strong winds.
The National Weather Service, part of the National Oceanic and Atmospheric Administration, has detailed guidelines for determining when a red flag warning is necessary.
According to the NWS website, a red flag warning is issued when 'Relative humidity of 15 percent or less is combined with sustained surface winds or frequent gusts of 25 mph or more. Both conditions must occur together for at least three hours within a 12-hour period.' Dry thunderstorms, which produce little rainfall, are another major risk factor. Other considerations include 'the first significant lightning event after a hot and dry spell,' a cold front bringing strong winds, sudden wind shifts, and prolonged drought conditions.
As CBS News reports, red flag warnings are most frequent from February to April and from October to mid-December. These warnings are often preceded by a 'fire weather watch,' indicating that conditions conducive to a red flag warning could develop in the coming days.
What actions should you take during a Red Flag Warning?
If you receive a red flag warning on your weather app, make sure not to discard a cigarette or match out of your car window, as this could easily 'ignite dry grass along the roadside and spark a wildfire,' warns the NWS. Also, avoid mowing a dry lawn or driving a vehicle through tall, dry grass, as sparks from machinery might set the ground on fire, according to the International Association of Fire Chiefs [PDF].
Be sure to follow any local rules or bans regarding personal fire setups. If you're permitted to have an outdoor fire, always keep a close eye on it; and make sure to thoroughly put it out—ashes and charcoal included—by dousing it with plenty of water.
