Atmospheric Moisture

Moisture escapes into the atmosphere through evaporation from water bodies and soil, and through transpiration from vegetation. The dew-point temperature and the absolute humidity represent the actual moisture in the air, while the relative humidity indicates the degree of saturation at a given temperature. Absolute humidity varies in space and time for several reasons; however, relative humidity does not necessarily change in the same manner, because relative humidity is very dependent upon air temperature. The temperature effect frequently overrides the absolute humidity effect; therefore, relative humidity usually varies inversely with temperature.

Atmospheric moisture is a key element in fire weather. It has direct effects on the flammability of forest fuels, and, by its relationship to other weather factors, it has indirect effects on other aspects of fire behavior. There is a continuous exchange of water vapor between the atmosphere and dead wildland fuels. Dry fuels absorb moisture from a humid atmosphere and give up their moisture to dry air. During very dry periods, low humidity may also affect the moisture content of green fuels. When atmospheric moisture condenses and falls as precipitation, it increases the moisture content of dead fuels, and, by replenishing soil moisture, it provides for the growth of green vegetation.

Moisture influences surface temperatures, including surface fuel temperatures, by controlling radiation in its vapor state and by reflecting and radiating when it is condensed into clouds. The heat energy released in condensation provides the energy for thunderstorms and the violent winds associated with them. Moisture is also necessary for the development of lightning, which in many mountainous areas is a cause of wildfire.

• Sources of Atmospheric Moisture
• Evaporation
• Dew Point
• Absolute Humidity
• Relative Humidity
  • Variations in Relative Humidity Near the Ground
  • Variations in Relative Humidity With Height
• Measuring Humidity