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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

Local Winds

Local winds are small scale convective winds of local origin caused by temperature differences. Local terrain has a very strong influence on local winds, and the more varied the terrain, the greater the influence.

Convective winds are all winds - up, down, or horizontal - that develop as a result of local temperature differences.

Ways that Local Winds Develop

Some ways in which local winds develop are:

  1. Convection from daytime heating.
  2. Unequal heating and cooling of the surface.
  3. Gravity, including downdrafts.

Common Local Winds

Winds of local origin, convective winds, can be as important in fire behavior as the winds produced by the large-scale pressure patterns. In many areas, they are the predominant winds in that they overshadow the general winds. If their interactions are understood and their patterns known, local convective winds can be predicted with reasonable accuracy.


Common local winds

Some common local, convective winds are:

1. Land and Sea Breezes

In unit 4, we discussed the surface properties that cause land surfaces to become warmer than water surfaces during the daytime. As a result of this local-scale temperature and pressure difference, a sea breeze begins to flow inland from over the water, forcing the warm air over the land to rise and to cool adiabatically. In the absence of strong general winds, this air flows seaward aloft to replace air which has settled and moved toward shore, and thus completes the circulation cell. The surface sea breeze begins around midmorning, strengthens during the day, and ends around sunset.

The land breeze at night is the reverse of the daytime sea breeze circulation. At night, land surfaces cool more quickly than water surfaces. Air in contact with the land then becomes cooler than air over adjacent water. Again, a difference in air pressure develops over the land and the water causing air to flow from the land to the water. The air must be replaced, but return flow aloft is likely to be weak and diffuse and is diminished in the prevailing general winds. The land breeze begins 2 to 3 hours after sunset and usually ends shortly after sunrise.

Another combination of convective winds results in slope winds. Slope winds are local diurnal winds present on all sloping surfaces. They flow upslope during the day as the result of surface heating, and downslope at night because of surface cooling. Slope winds are produced by the local pressure gradient caused by the difference in temperature between air near the slope and air at the same elevation away from the slope.

During the daytime, the warm air sheath next to the slope serves as a natural chimney and provides a path of least resistance for the upward flow of warm air. The layer of warm air is-turbulent, increasing in depth as it progresses up the slope. This process continues during the daytime as long as the slope is receiving solar radiation. When the slope becomes shaded or night comes, the process is reversed.

A short transition period occurs as a slope goes into shadow: the upslope winds die, there is a period of relative calm, and then a gentle, smooth downslope flow begins. Downslope winds are very shallow and may not be represented by a 20-foot surface windspeed. The cooled denser air is stable, and the downslope flow tends to be quite smooth and slower than upslope winds. The principal force here is gravity. Downslope winds usually continue throughout the night until morning, when slopes are again warmed by solar radiation. The times during which winds change from downslope to upslope and vice versa can depend on aspect, time of year, slope percent, current weather conditions, and other lesser factors.

2. Slope and Valley Winds

3. Thunderstorm downdrafts

4. Whirwinds

Slope and Valley Winds

More

Surface Winds

More

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_5.html. This work is licensed under a Creative Commons License Creative Commons License