In number 4 thunderstorm downdrafts will be cooler

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strength of the downdrafts, other surface winds at the time, and on terrain features. In number 4, thunderstorm downdrafts will be cooler and somewhat moister than surrounding air. This is not to say that the relative humidity will be high, but merely higher than otherwise. Let's take another look at the mature thunderhead. See figure 20. Thunder-storms 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. See page 22. 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. 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.
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  • Spring '04
  • MIchealJenkins
  • weather forecasts, windspeed, midflame windspeed

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