Temperature Considerations in Prescribed Burning

Underburning

The average instantaneous lethal temperature for living tissue is about 147^o F. Air temperatures below 60^o F are recommended for winter underburns because more heat is needed to raise foliage or stem tissue to lethal temperature levels. When the objective is to control undesirable species, growing-season burns with ambient air temperatures above 80^oF are recommended. These conditions increase the likelihood of reaching killing temperatures in understory stems and crowns. Of course, the overstory pines must be large enough to escape injury. Larger trees have thicker bark and their foliage is higher above the flames, which allows more room for the hot gases to cool before reaching the crowns.

Temperature strongly affects moisture changes in forest fuels. High temperatures help dry fuels quickly. When fuels are exposed to direct solar radiation, they become much warmer than the surrounding air. Moisture will move from the warmer fuel to the air even though the relative humidity of the air is high. Temperatures below freezing, on the other hand retard fire intensity because additional heat is required to convert ice to liquid water before it can be vaporized and driven off as steam. Consequently, it does not take much moisture under these conditions to produce a slow-moving fire that will leave unacceptably large areas unburned.

For background information on temperature, see Temperature.

Debris Burning

Cleared areas are often burned when ambient air temperatures are high. There is no overstory present to worry about and surface heating from direct sunlight usually increases the mixing height, which helps disperse the smoke. It is particularly important to use an ignition pattern such as center firing when ambient air temperatures are high. This tactic draws the heat into the cleared area and prevents heat damage to trees in adjacent stands.

For background information on temperature, see Temperature.