The temperature of the ground also affects the movement of air currents, as explained previously. Prolonged high temperatures also affect the endurance and efficiency of the firefighters. Moisture in the form of water vapour is always present in the air. The measurement of that moisture is called humidity and is always expressed as a percentage.
Warm air absorbs more moisture and produces a lower humidity. As air cools, the humidity increases. The amount of moisture in the air affects the moisture in the fuel. This is an important factor in firefighting, since wet and most green fuels will not burn freely.
Air is usually drier during the day than at night. Fires, then, burn more slowly at night, under normal circumstances, because the fuels absorb moisture from the damp night air. This partially explains why a fire burns out of control in the afternoon and yet may be controlled by the same crew at night. Every effort should be made to control a fire before burning conditions build up the next day.
Ravines and gullies will form paths for the flow of air and may change direction of the fire. In narrow ravines, heat will dry out fuels on the opposite side and they will readily ignite.
Intersecting drainages and sharp turns will cause turbulence. Saddles and gaps along a ridge will funnel the wind and increase its speed. Winds will also be gusty and spotting is more likely. Fires will tend to burn toward them and increase in intensity and rate of spread. This change can be abrupt. Barriers are anything natural or man-made that can stop or slow down the spread of fire. Examples are: fields, roads, streams, lakes, swamps if wet rocky outcrops, and old burns.
They should be considered in planning a prescribed burn or in control of a wildfire. They can also be barriers to equipment. A thick stand of trees will also act as a barrier to the wind. It will be forced over the top causing gusty conditions. An opening in the stand will channel the wind much the same way as saddles.
The direction a slope faces determines how much radiated heat it will receive from the sun. Slopes facing south to southwest will receive the most solar radiation.
Santa Ana and Diablo winds typically run downslope, says Nauslar. Rapidly spreading downhill fires also played a major role in last winter's Thomas Fire. It coincided with the longest Santa Ana wind event on record, according to a recent analysis of the fires led by Nauslar.
Fires that burn quickly downhill are particularly worrisome because communities are often situated at the base of the mountains. Last fall, it was a downslope fire that pushed into Santa Rosa and drove the destruction of Coffey Park and other local communities. Firefighters are keeping careful tabs on changes in fire behavior to help prevent tragedies like what happened in Santa Rosa. Climate change is expected to continue to increase nighttime temperatures and the types of extreme weather that has primed fuels this year for faster, bigger fires.
Search-Icon Created with Sketch. KQED is a proud member of. If the slope is sufficiently steep, all of the air is coming in from the downhill side. Narrator: With air fanning the fire exclusively on the downhill side, the flames get pushed into the slope, putting them in contact with more fuel. The tilted, climbing fire transfers a lot of heat to the trees and brush ahead of the flames, preheating and drying them, making them more combustible.
Narrator : This is what happened when the Camp fire started. The wind funneled the flames into a steep canyon, and they rapidly accelerated uphill, right toward Paradise.
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