LESSON 3 - EFFECTS OF WEATHER, TOPOGRAPHY AND FUELS ON FIRE BEHAVIOR

3.1 Wind

The movement of air across the earth`s surface has an impact on the fire environment before and after ignition has taken place.

Wind can influence fire behavior in a variety of ways.

Wind is a crucial factor affecting the rate of spread and direction of a fire. Wind can influence fire behavior by:

Moving moist air away from fuels, causing them to dry out faster.

Moving moist air over fuels causing an increase in moisture content.

Carrying burning embers that have been lifted aloft by convection air and starting spot fires ahead of the perimeter.

Bending the convection column, which promotes pre heating of unburned fuels in front of the fire.

Bringing a continuous supply of oxygen to the fire.

Winds occur at various levels above the earth`s surface

a) General Wind - High, large scale winds that are moving from one pressure system to another. Global winds that are unaffected by terrain conditions.

b) Local Wind - Winds located at the surface and up to several hundred metres, influenced by local temperature differences.

c) Surface Wind - Winds affecting surface conditions.

Even when winds on the earth’s surface are relatively light, winds can be many times stronger several hundred meters above the surface where convection may carry burning embers.

Wind greatly influences both direction and rate of spread. As a general rule if the wind speed doubles, the rate of spread will more than double.

This sketch illustrates how the shape of a fire is affected by wind speed:

Without wind on level ground, fire will burn in a circular pattern. An increase in wind speed will result in an elliptical shape burn pattern.

> Proceed to Diurnal Effects on Weather >