Relative humidity

Relative humidity describes the amount of water vapor in the air. It is defined as the ratio of the amount of water vapor present in the air compared to the greatest amount possible at the same temperature. Relative humidity of 100 percent means the air is fully saturated and no moisture will be drawn from other materials. A relative humidity of 50 percent means the air contains half the water it could hold, and so on. Warm air can hold more moisture than an equal volume of cooler air.

When relative humidity is low, the air draws moisture from surrounding objects including fuels. As they dry, they become easier to ignite and burn. When the humidity is higher, moisture is removed from fuels slowly and may even be returned to the fuels. This makes them less likely to burn and fires at high humidity tend to burn slower and are more easily controlled. A general rule-of-thumb is that as the temperature goes up 20° F, relative humidity decreases by half. Relative humidity drops substantially following the passage of a cold front.