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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

Winds of Most Concern to Firefighters

The wind conditions that we have covered so far may, or may not, be considered problem winds to the firefighter. These are normal, everyday winds with which the firefighter must deal. Moderate to strong winds present a particular concern because fire behavior is so reactive to wind. Firelines are most often lost, large acreages burned, and property and lives are lost when strong winds fan a fire out of control. This is why we have named the next portion of this unit, "Winds of Most Concern to Firefighters."


Altocumulus cloud formation are indicators of potential winds

Winds Responsible for Losses

Winds most responsible for loss of control lines, property, and life on fires are:

Cold front winds

Cold Front Winds

First let's look at cold front winds. We said earlier that a weather front is the boundary layer between two air masses of different temperatures. Weather fronts center out of an area of low pressure, and they acquire movement as the low-pressure cell moves across the country. The graphic above illustrates a very simple weather map with both a cold front and a warm front. The arrows show the usual direction of winds in relation to these weather fronts. Wind direction is always the direction from which the wind is blowing. As such fronts move through a region, the winds will shift in a clockwise direction. Ahead of a warm front, winds will be out of the northeast and east. Winds ahead of a cold front usually shift from southeast to south, to southwest. As a cold front passes through, winds shift to west, then northwest. The reason for the wind shifts is that air is always flowing in a counterclockwise direction around a low and is crossing the isobars into the center of the low-pressure cell. Winds will be strongest when the frontal boundaries pass through your area, since the strongest pressure gradient exists in these zones. Both types of weather fronts often are not present at the same time. Warm fronts are more common in winter and are seldom seen in the Western United States during summer. Cold fronts are much more common and may be spaced from 1 day to 2 weeks apart. Remember, winds always shift clockwise in direction with the passage of a weather front. They increase in velocity as the fronts pass through an area, and their consistency, strength, and gustiness may vary greatly.

Cold fronts frequently bring thunderstorm activity with possible precipitation. However, during the summer months in the West, cold fronts are often dry, but bring cooler and stronger winds. Cold front winds can easily reach 15 to 25 miles per hour or higher close to the front but blow less strongly well ahead of and behind the front. Typical pre-cold frontal winds would be from 10-14 miles per hour while post frontal winds would be 12-18-miles per hour. If thunderstorms are associated with the front, winds can be much stronger, up to 50 miles per hour, and can rapidly change the usual wind directions.

Foehn winds

Another kind of wind that causes firefighters great concern is the foehn wind. Foehn winds represent a special type of local wind associated with mountain range systems. They occur as heavy, stable air pushes over a mountain range and then descends the slopes on the leeward side as warmer, drier air.

Thunderstorms' downdrafts

Next, we will discuss a local wind condition that is important to all regions of the country. This is the thunderstorm downdraft. Cumulonimbus clouds, called thunderheads, can build over an area at any time there is adequate moisture in the atmosphere and a lifting mechanism to force air to rise. Generally, there are four lifting mechanisms. These will be discussed in the next unit on atmospheric stability and instability. The most usual lifting mechanism for thunderstorms is convection, which is caused by heating from below. Thunderheads begin as small, fluffy cumulus clouds. Moist air that is lifted will cool to its dew point at some altitude to form cumulus clouds. The greater the lifting, the greater the cloud development. The release of latent heat by the condensation of water provides additional energy to develop the cloud.

Towering cumulus clouds may reach to 20,000 feet of thickness. When their tops reach high in the atmosphere, icing of cloud particles can occur. A fully developed cumulonimbus cloud can reach 30,000 to 40,000 feet or more in the west, and to 60,000 to 70,000 feet in the east. Such clouds have stored vast amounts of energy. Not only are there strong indrafts into the base of the cloud, but strong downdrafts occur with the release of its energy. Violent local storms are produced, accompanied by thunder and lightning, perhaps rain, and strong winds.

Whirlwinds

The fourth problem wind is the whirlwind. The most common whirlwind, the dust devil, occurs on hot days over dry terrain when skies are clear and general winds are light. Whirlwinds are an indicator of intense local heating. Strong convection currents or updrafts develop in the areas of intense heating. It is probable that nearly all updrafts have some whirling motion, but usually this is weak and invisible. The stronger the updraft, the stronger the whirl. The whirl becomes visible if the updraft becomes strong enough to pick up dust and other surface materials.

Whirlwinds may remain stationary or move with the surface wind. If it breaks away from its heat source, it may die out, and another whirlwind may develop nearby. In very light wind situations whirlwinds that move, show a tendency to move toward higher ground.

Whirlwinds vary in size from just a few feet to over 100-feet in diameter, and to heights of nearly 4,000 feet. On fires, dust devils are common in an area that has just burned over, since the blackened ash and charred materials are good absorbers of solar radiation and thus encourage local heating.

Whirlwind

Surface winds always shift clockwise in direction with the passage of a weather front. They increase in velocity as the fronts pass through an area.

Wind shifts
Wind shifts with weather fronts

Examples of Foehn Winds

More

Thunderstorm Development

More

Firewhirls and Whirlwinds

The firewhirl, which carries flames and burning materials up into its column, is usually caused by very high fire intensities in local areas. Firewhirls are usually considered more dangerous than dust devils, but both can scatter fire, cause spotting across control lines, and generally increase fire intensity in local areas.

Remember that in order for whirlwinds to develop, certain environmental conditions must be present. These conditions usually include mostly clear, sunny skies; light surface winds; heating with extreme instability of the air near the surface; and on the back side of a shallow ridge, or on lee slopes from the prevailing wind.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 08). 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_8.html. This work is licensed under a Creative Commons License Creative Commons License