H-GOOD QUESTION

Jun 5, 2013 8:54 PM by John Reger

GQ: What is relative humidity and how does it affect wildfires?

With the early start of fire season this year, you hear the term "relative humidity" when the professionals talk about weather conditions. That prompted tonights Good Question: What is relative humidity and how does affect wildfires? I asked KSBY Chief Meteorologist Dave Hovde.

"It's the amount of moisture that the atmosphere can contain at any given pressure and temperature," said Hovde. "So 40% relative humidity means the air is holding 40% of the amount water vapor it can. At 100% you're getting a cloud, fog, something like that. They've done studies on this sort of thing when it comes to controlled burns. They done a study and discovered that if relative humidity gets high, the chances of a controlled burn going uncontrolled are extremely low, almost non-existent. But as relative humidity gets dryer look what happens: the chances of a fire getting to be uncontrolled, causing spot fires, is nearly guaranteed, under 25%. Here's a recent fire forecast: We always watch as temperatures go up and relative humidities go down. If you have dry fuel moistures, like we do locally, you add in some wind you get red flag conditions which means any time a fire starts it has the potential to grow uncontrollable."

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