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Unit 6: Local and General Winds

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Wind Effects  ::   General Winds   ::   Local Winds   ::   Winds of Concern   ::   Wind Input   ::   Exercises

Objectives:

Upon completion of this unit you will be expected to:

  • Explain the relationship between general, local, surface and midflame winds.
  • Give the origins or causes of six kinds of winds, local or general.
  • Given locations on a topographic map, describe typical slope and valley winds at those locations for various times of day.
  • Show the usual winds that result with a cold front passage.
  • Illustrate the first three stages of thunderstorm development, and describe the weather conditions associated with each stage.
  • Explain how windspeeds are measured, and how wind predictions are applied to fire spread calculations.
  • Determine midflame windspeeds when given the 20-foot surface winds, fuel type or model, and terrain features.

Introduction:

This unit addresses winds, both local and general, and what firefighters should know about them.

Winds and Their Effects on Fires

Wind is air in motion, especially horizontally, 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.

Winds that affects wildland fires
Winds that affect wildland fires.

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 altitudes.

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.

Effects of Wind on the Intensity, Direction, and Rate of Spread of Wildland Fires

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 you. First, we'll discuss surface or 20-foot winds.

Winds affect the intensity, direction, and rate of spread of wildland fires by:

  1. Angling the flames to preheat fuels by radiation.
  2. Angling convection currents to preheat fuels ahead of a fire.
  3. Providing a fresh supply of oxygen to the fire.
  4. Speeding up moisture exchange between the air and the fuels.
  5. Carrying burning embers and firebrands to cause short range spotting.
  6. Causing fires to burn erratically.

 

Direct Effects of Winds on Fire

Winds aloft have direct effects on fires in regard to:

  1. Convection column development
    Convection column development
  2. Smoke transport
    Smoke Transport
  3. Spotting distance and direction (long range)

 

Wind is the most variable, thus, the most difficult element of weather to predict. It is also one of the most critical factors to fire behavior.

Because wind is so critical to fire behavior, we try to obtain the best wind observations and forecasts possible. Predicting what the winds will be at various times and places is no easy task, even for the experienced meteorologist. This is why the firefighter usually relies on wind predictions from weather forecasts. There are times, however, that by being especially observant of the terrain and past weather patterns, the firefighter is in a good position to adapt a wind forecast to his site. This is especially true where local winds repeat themselves under similar weather conditions. An example of this could be the slope and valley winds in a large valley.

There are other weather situations in which personnel in the field, by recognizing the indicators, are able to anticipate wind changes and take appropriate action. It is not our intent to make you a forecaster of winds, but you should be aware of situations when certain winds can occur.

Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, November 07). Unit 6: Local and General Winds. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_6__Local_and_General_Winds_1.html. This work is licensed under a Creative Commons License Creative Commons License