Eddies will form on the lee side of ridges and whirlwinds may form on unstable days. 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. As a result, this slope is warmer than slopes facing a northerly direction. The warmer slope results in lower relative humidity, higher temperatures and rapid loss of moisture.
The period that fires will ignite and burn will also be longer on south-facing slopes. Box canyons are ravines that end at or near the ridge top. Wind is also a major factor in transporting firebrands—pieces of burning fuel, like twigs, leaves or small embers—ahead of the main fire. This causes spotting—the ignition of new fires ahead of the fire front. Under very dry and windy conditions, profuse spotting from certain eucalypt forests will result in the ignition of a broad area ahead of the fire.
This creates a firestorm environment with highly chaotic and turbulent winds further driving fire in all directions. The mass ignition can result in the formation of large fire-whirls, or fire tornadoes. If enough fuel is being consumed within the fire area, the heat generated creates an extremely strong updraught of air that can result in a pyrocumulunimbus cloud, or flammagenitus.
In extreme cases this thunderstorm-type cloud can produce lightning strikes, which have been known to start new fires. Things you need to know about bushfire behaviour The primary influences upon how bushfires move through the landscape are humidity, topography, wind and temperature.
Mark Finney: This entire building is devoted to fire. Narrator: Forest Service scientist Mark Finney showed us how they unlock the mysteries of fire. Finney: So, this is the burn chamber right here. Everything we do in here is designed to look at how fires behave. Narrator: On this day, he and a dozen of his colleagues are preparing for a test on a large tilting burn bed of precisely cut cardboard.
Narrator: The burn table brims with instruments that measure pressure and temperature, taking samples times a second. And there are cameras everywhere.
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