Relative Humidity Considerations in Prescribed Burning

Underburning

Daily Temperature/Relative Humidity Cycle

Relative humidity is an expression of the amount of moisture in the air compared to the total amount the air is capable of holding at that temperature and pressure. Each 20^oF rise in temperature (which often occurs during the morning hours on a clear day) reduces the relative humidity by about one-half, and likewise, each 20^oF drop in temperature (which often occurs in early evening) causes relative humidity to roughly double. When a cold front passes over an area, the air behind the front is cooler and drier than the old airmass it is replacing. The result is a drop in both temperature and humidity.

Preferred relative humidity for prescribed burning varies from 30 to 55 percent. Under special conditions, a wider range of relative humidities, as low as 20 percent and as high as 60 percent, can produce successful burns. When relative humidity falls below 30 percent, prescribed burning becomes dangerous. Fires are more intense under these conditions and spotting is much more likely; proceed only with additional precautions. When the relative humidity is 60 percent or higher, a fire may leave unburned islands or may not burn hot enough to accomplish the desired result.

The moisture content of fine dead fuel such as pine needles and dried grasses responds rapidly to changes in relative humidity. However, there is a time lag involved for fuels to achieve equilibrium with the moisture condition of the surrounding atmosphere. Also, previous drying and wetting will influence fuel moisture. Therefore, the relative humidity and fuel moisture must be assessed independently.

For background information on relative humidity, see: Atmospheric Moisture.

Debris Burning

Relative humidity (along with temperature) controls fuel moisture content up to about 32 percent. Liquid moisture such as rain, snow, fog, or dew must contact a fuel for moisture content to rise above 32 percent, and the increase depends upon duration as well as the amount of precipitation.

Recently-cut pine tops have a drying rate that is somewhat independent of relative humidity as long as the moisture content of fresh tops (needles still green) is above about 32 percent. Once this material initially dries to a moisture content below 32 percent, it behaves as a dead fuel and becomes much more responsive to daily fluctuations in relative humidity. The response to changes in relative humidity is much more rapid in fine dead fuels suspended above the ground than in those that have become part of the litter layer. These elevated needles and other suspended dead materials are not in contact with the damp lower litter and are more exposed to the sun and wind.

When burning piled debris, once the larger-diameter fuels ignite, increases in relative humidity have little effect on fire behavior during the active burning phase. Low humidities (below 30 percent), however, will promote spotting and increase the likelihood of fire spreading between piles.

For background information on relative humidity, see: Atmospheric Moisture.