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Unit 6: Local and General Winds

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Thunderstorm Development

Stages of Cumulus Cloud Development

Stage 1 is most often called fair weather cumulus. These are clouds with horizontal bases, rounded surfaces, and with little vertical development. The winds associated with this stage are light indrafts into the base of clouds.

The second stage is towering cumulus. These are clouds of vertical development, towering and cauliflower like in appearance, with clear-cut tops. These have stronger indrafts into the base of the clouds, perhaps affecting the surface winds.

Stage 3 is the Cumulonimbus or thunderstorm. These clouds have high vertical development and anvil-shaped tops composed of ice crystals. The bases of mature thunderstorms are ragged from downdrafts and virga. Downdrafts that reach the ground result in cool, gusty surface winds that can be experienced within about 5 to 6 miles or so of a thunderhead. Surface wind velocities will often be 25 to 35 miles per hour but can reach as high as 50 to 70 miles per hour. When you observe cumulus clouds near your fire, you should recognize and report them as being in one of these stages.

Stage 1 Stage 2 Stage 3
Stage 1 Stage 2 Stage 3

Stage Cloud Name Appearance Associated Winds
1 Fair weather cumulus Clouds with horizontal bases, rounded surfaces, and with little vertical development. Light indrafts into base of clouds.
2 Towering cumulus Clouds with vertical development, towering, cauliflower appearance and clear-cut tops. Stronger indrafts into base of the clouds, perhaps affecting surface winds.
3 Cumulonimbus High vertical development, and anvil shaped tops composed of ice crystals. Bases are ragged from downdrafts and virga. Downdrafts that reach the ground exist, resulting in cool, gusty winds that can be experienced within five to six miles of thunderstorm.

Thunderstorm
Thunderstorm activities

Let's take another look at the mature thunderhead. (See figure above) Thunderstorms may remain stationary or move with or across prevailing winds. The top of the cloud may fracture and drift in the direction of winds aloft. Indrafts can continue into the base on the windward side, as downdrafts are pouring from the base elsewhere. Downdrafts will usually reach the ground and flow in all directions; however, local and general winds tend to mix and modify the downdraft winds. Thunderstorm winds will be experienced for greater distances on the ground if they are combined with a prevailing surface wind.

There are several things to watch for that will indicate when downdrafts from a thunderstorm have begun. First, you may see a small roll cloud developing on the downwind side of the cloud base. You may see virga hanging from a ragged, dark base. Virga is actually rain that falls part way to the ground. Then you might observe a dust cloud, as the first gusts spread out over the countryside. Depending on your proximity to a thunderstorm you may experience varying weather conditions. The important thing is that you be prepared for the worst, should it occur. Remember, thunderstorm winds can easily reach 30 to 60 miles per hour.

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_10.html. This work is licensed under a Creative Commons License Creative Commons License