UNIT VI--LOCAL AND GENERAL WINDS
This unit addresses winds, both local and general, and what firefighters should know about them.
Before starting the unit, read the instructions to students on page 2; then the unit objectives on
page 2 of your workbook. When you have finished, return to this text.
See page 3. What is wind? By simple definition, wind is air in motion, especially horizontal,
relative to the earth's surface. We are concerned with winds of two major scales in the
atmosphere--the larger scale general wind and the smaller scale local wind. Collectively, these
winds are measured at three levels in the atmosphere--the winds aloft, the 20-foot surface winds,
and the winds at midflame height. See figure 1.
The general winds or winds aloft are caused by broad scale circulation patterns high above the
earth. This circulation of air throughout the atmosphere is the result of large-scale convective
circulation between the equator and the polar regions, and of the earth's rotation on its axis.
These are sometimes called the gradient winds. In the contiguous United States and Canada,
high- and low-pressure patterns mostly move from west to east due to the prevailing westerlies.
Winds aloft are measured at 1,000-foot intervals, since they can vary considerably at various
As the general air flow nears the earth's surface, it gradually becomes affected by the shape of
the topography and by local heating and cooling over large areas. Frictional drag produced by
the terrain usually slows the larger scale winds and can modify their direction. Next, consider the
smaller scale, local winds. These are produced locally due to heating and cooling or temperature
differences at the earth's surface.
The general winds and the local winds may combine to produce the winds that we experience at
the surface. The measurement of surface winds has been standardized at 20 feet above the
ground in a clearing, or 20 feet above any vegetation.
As surface winds drop closer to the ground, their speeds are reduced primarily due to friction. In
doing calculations of fire behavior, we are concerned with the wind speeds at the level of the
flames. This is referred to as the "midflame windspeed". This unit will discuss how each of these
wind levels are measured and predicted.
All of these wind levels can affect, either directly or indirectly, the behavior of wildfires,
although we are generally not concerned with higher level winds unless fire intensities and
convection columns are very high and long-range spotting becomes a problem. Most weather
forecasts available to firefighters address the general and surface winds because these are more
appropriate to fire danger predictions. Some special fire weather forecasts may predict midflame
(usually eye level) winds, in which case the forecaster has reduced the surface windspeeds for
First, we'll discuss surface or 20-foot winds. These affect the intensity, direction, and rate of