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The Earth’s Atmosphere Contents Part 7

Tầng bình lưu: từ độ cao trên tầng đối lưu đến khoảng 50 km, nhiệt độ tăng theo độ cao đạt đến 0 °C. Ở đây không khí loãng, nước và bụi rất ít, không khí chuyển động theo chiều ngang là chính, rất ổn định. | 282 Chapter 10 Thunderstorms and Tornadoes Focus on an Observation THUNDERSTORM ROTATION - ----------- ---------- Rotating clouds at the base of a severe thunderstorm often indicate that the storm is about to give birth to a tornado. But how do the clouds develop rotation Figure 3 illustrates how rotating vortices can develop near the surface. Notice that there is wind direction shear as surface winds are southeasterly aloft they are westerly. There is also wind speed shear as the wind speed increases with increasing height. This wind shear causes the air near the surface to rotate about a horizontal axis producing narrow tubes of spiraling air called vortex tubes. A strong updraft of a thunderstorm may then tilt a rotating tube and draw it into the storm as depicted in Fig. 4. This situation sets up two spinning vertical columns of air one rotating clockwise and the other counterclockwise. As air is drawn more quickly into the storm the spiraling columns spin faster. If the thunderstorm has a more complicated structure as most do additional rotating air columns may form. This phenomenon normally induces the southern flank of the storm to rotate in one direction usually counterclockwise when viewed from above and the northern flank in the other direction usually clockwise. Hence the thunderstorm rotates. FIGURE 3 Spinning vortex lubes created by wind shear. FIGURE 4 The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm producing two rotating air columns that are oriented in the vertical plane. Severe Weather and Doppler Radar Most of our knowledge about what goes on inside a tornado-generating thunderstorm has been gathered through the use of Doppler radar. Remember from Chapter 5 that a radar transmitter sends out microwave pulses and that when this energy strikes an object a small fraction is scattered back to the antenna. Precipitation particles are large enough to bounce microwaves back to the antenna. As a consequence the .