'Humidity' is an eight-letter word that is heard around fire camps and on the fireline almost as often as the more widely known four-letter words. Most firefighters know that humidity has something to do with moisture in the air. If it is low, they expect difficulty in controlling the fire; if it is high, the fire can be expected to burn less aggressively-and perhaps may even go out by itself. Humidity is a very general term, however. A weather specialist may use such confusing expressions as absolute humidity, specific humidity, mixing ratio, vapor pressure, dewpoint, and relative humidity. Each of these terms describes a different view of air moisture, and each has its value. But they all do not have the same importance in wildland fire control-relative humidity overshadows the rest. The following discussion is intended to sort out the meanings of different are moisture terms and show why relative humidity is so important.