Fire weather invades the east

(Photo courtesy of Nick Stasiak)

It’s not very often that you see my weather report say, ‘Red Flag Warning.’ This past Tuesday, the National Weather Service issued a Fire Weather Watch. On Wednesday, Fairfax was upgraded to a ‘Red Flag Warning.’ Everyone from the west coast here at Mason knew just what this meant. The term was pretty odd to the rest of us though. So what exactly does a ‘Fire Weather’ or ‘Red Flag’ alert mean?

When conditions seem to be very dry for a certain length of time, the probability of having brush fires becomes very high. On Wednesday, when the warning was issued for Fairfax, the temperature was right around 55 degrees Fahrenheit. No, that’s not a very warm temperature, but that’s not what causes the fire weather. The real cause is the relative humidity.

Relative humidity is basically the percentage of water vapor in the air. It’s also known as the percentage of the maximum amount of air that can be held at a certain temperature. On Wednesday, as the temperature stayed steady in the mid 50’s, the relative humidity stayed just below 25 percent for most of the day. In turn, the air was extremely dry. Temperature and relative humidity weren’t the only factors though. One more factor plays a big part in the process as well: high wind.

Wind gusts were reaching 25 mph at times on Wednesday. You may be wondering why winds are a factor. Well, when there is a major difference between temperature and relative humidity, such as on Wednesday, then it’s pretty easy for a brush fire to be started. Should a brush fire ignite and the winds pick up, then the fire will spread in no time.

The National Weather Service typically issues a ‘Fire Weather Watch’ when there are conditions within the next 12-72 hours. A ‘Red Flag Warning’ is issued if there are conditions within the next 24 hours, or if fires already began spreading during the watch. Thankfully there were no natural fires at GMU, but if there was one, I know you all would have been prepared following C2M Weather.