National Weather Service United States Department of Commerce

Relative Humidity

Relative humidity is the ratio in percent of the amount of moisture in the air at a given temperature, compared to the amount of moisture that the air could hold if saturated (100%). The daytime humidity will be the minimum value expected during the 12 hour period. The nighttime forecast will be the maximum value during that 12 hour period, usually. The minimum humidity typically occurs at the time of the maximum temperature, and maximum humidity typically occurs at the time of minimum temperature. The forecast will generally be in a small range (5 to 10%) that is expected in the zone.

It should be understood that forecast humidity accuracy often depends on accurate temperature forecasts. For example, if the maximum temperature is under forecast (too low). The respective humidity will be too high.

Low relative humidities below 30 to 35 percent tend to increase active fire behavior. Relative humidities below 25 percent will restrict any kind of prescribed burning.