PRIMUSr 880 Digital Weather Radar System
Step | Procedure |
9Avoid all rapidly growing storms by 20 miles. When severe storms and rapid development are evident, the intensity of the radar return may increase by a huge factor in a matter of minutes. Moreover, the summit of the storm cells may grow at 7000 ft/min. The pilot cannot expect a flightpath through such a field of strong storms separated by 20 to 30 NM to be free of severe turbulence.
10Avoid all storms showing erratic motion by 20 miles.
Thunderstorms tend to move with the average wind that exists between the base and top of the cloud. Any motion differing from this is considered erratic and may indicate the storm is severe. There are several causes of erratic motion. They may act individually or in concert. Three of the most important causes of erratic motion are:
1.Moisture Source. Thunderstorms tend to grow toward a layer of very moist air (usually south or southeast in the U.S.) in the lowest 1500 to 5000 ft above the earth’s surface. Moist air generates most of the energy for the storm’s growth and activity. Thus, a thunderstorm may tend to move with the average wind flow around it, but also grow toward moisture. When the growth toward moisture is rapid, the echo motion often appears erratic. On at least one occasion, a thunderstorm echo moved in direct opposition to the average wind!
2.Disturbed Wind Flow. Sometimes thunderstorm updrafts block winds near the thunderstorm and act much like a rock in a shallow river bed. This pillar of updraft forces the winds outside the storm to flow around the storm instead of carrying it along. This also happens in wake eddies that often form downstream of the blocking updraft
103. Interaction With Other Storms. A thunderstorm that is
(cont) | located between another storm and its moisture source |
| may cause the blocked storm to have erratic motion. |
| Sometimes the blocking of moisture is effective enough |
| to cause the thunderstorm to dissipate. |
| Severe Weather Avoidance Procedures |
| Table 5- 12 (cont) |
Radar Facts | A28- 1146- 102- 00 |
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