THE AIRLINE PILOTS FORUM & RESOURCE
Wildfires |
| Source: Excerpt from The Book " Weather " |
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Wildfires can have a dramatic effect on atmospheric visibility and their development is often related to weather patterns. The term wildfire is used to describe any uncontrolled fire burning through vegetation. Other expressions such as scrub fire and forest fire are used to describe fires in specific habitats. In Australia, all wildfires are known as bushfires. Wildfires occur most often in California, the French Riviera, and parts of Australia. These areas experience periodic drought, high summer temperatures and hot, dry winds, and are covered with highly volatile vegetation. These factors create ideal conditions for fires. Although wildfires have always occurred naturally, usually as a result of lightning strikes, human activity has greatly increased their frequency, with out-of-control burn-off procedures, campfires, and discarded cigarettes being among the most common causes. If a wildfire is not controlled at once, it can become an unstoppable inferno that rages across the countryside, causing loss of life and widespread property damage. FIRE IN THE SKY Smoke from wildfires can rise high into the troposphere, increasing the number of condensation nuclei in the air, and dramatically affecting sunlight. Normally, as light passes through the atmosphere, the colors of the spectrum are dispersed one by one, beginning at the violet end of the spectrum. During a fire, the invisible smoke particles enhance this scattering effect, so that colors toward the red end of the spectrum are scattered just above ground level, creating an eerie orange glow in the sky and intense red sunsets. This situation is likely to persist until the smoke is dispersed by wind or rain. If a fire burns for some time, a pyrocumulus cloud may form. This may produce lightning strikes that trigger further fires. Once they have begun, wildfires may spread erratically. However, a knowledge of how fires interact with weather systems can help firefighters predict some of their movements. If a fire begins and there is no wind, the fire will spread outward in all directions. If a wind then starts to blow across the fire, the fire will form an ellipse, with flames advancing slowly toward the wind, faster on the flanks, and even more rapidly downwind. This downwind fire-front is the most dangerous part of the fire. Any sudden change in wind direction, perhaps associated with the arrival of a frontal system, can create a wider and more threatening front along the side of the ellipse. For this reason, firefighters will seek regular advice from the local weather bureau. To anyone watching the fire, the direction in which the smoke is blowing will indicate the location of the fast-moving firefront. A significant increase in ground-level smoke near the observer is a warning that it is time to leave the area. |
Acknowledgement: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
