Wildland Firefighting Practice Exam

The term that describes the ratio of the actual water vapor present to the maximum amount of water vapor that air can hold at a given temperature is called relative humidity. Relative humidity is expressed as a percentage, indicating how much moisture is currently in the air compared to how much moisture the air can hold when it is saturated. This measure is crucial in meteorology and firefighting since it affects fire behavior; higher relative humidity typically indicates a lower likelihood of fire spread, as moist air can help suppress ignition.

While absolute humidity reflects the actual amount of water vapor in the air without considering temperature, and specific humidity measures the mass of water vapor relative to the mass of the air, these do not provide the ratio needed to assess moisture's impact on fire conditions as effectively as relative humidity does. The mixing ratio, which is the mass of water vapor relative to the mass of dry air, serves a different purpose and also does not correspond directly to the concept of saturation that relative humidity conveys.