Forest fires, or wildfires, change local weather patterns by adding heat and smoke to existing weather patterns. Fire needs three things to burn: heat, oxygen and combustible material, like vegetation. Dr. Terry Clark has devised a computer model that helps firefighters make better decisions about how to fight forest fires.
Convection
A large, hot fire can generally heat air near the ground surface so that the rises. A convection column of rising air forms may carry smoke and embers thousands of feet in the air, as high as 25,000 feet. Surface air rushing in to replace the rising air creates strong in-drafts that may intensify and move the fire forward. These have the name forward bursts or hairpin vortices. This relationship between fire, convection, and winds explains why larger wildfires “create their own weather.”
Water vapor
Smoke combining with the water vapor makes acid rain. As the convection column builds and cools, the water vapor may condense, releasing latent heat into the column. This reinforces the convection. In a convective thunderstorm, with lightning and rain a similar pattern producing downbursts sometimes occurs. Such downbursts can, in turn, speed the spread of the fire in a dangerous and unpredictable manner.
Heat
Forest fires get their heat from debris and small vegetation, often in areas, where drought occurs. Fires travel uphill quickly. If a fire starts on a hill, wind will push it upward, causing it to spread and feed the fire. Dry conditions worsen the effect. With more blazes burning the fire gets hotter, the fire bigger and harder to fight.
Examples
Smoke, dry lighting and no rain contributed to the Yellowstone fires in the summer of 1988. Winds blew two smaller fires together forming one large fire. In some ways, local weather patterns intensified these fires making them last longer and grow bigger. The lack of rain played a large part in their spread.
When conditions are right, some fires can act as a tornado sending flames 100 feet in the air and torching anything in its path. This is the scenario of the October 19-20, 1991 fire in Oakland, California. Smoke can add cloudy skies to locations that have foggy conditions creating an inversion layer. Winds from the fires can cause normally, cool conditions to become hot.
Winds change local weather patterns. Large fires create their own winds. If the winds are high enough, they become firestorms. Where firestorms occur, the weather conditions are dry and the winds high.
