tailieunhanh - BIOMES OF THE EARTH - GRASSLANDS Part 4

Quá trình này sản xuất cắt gió, một lực lượng tồn tại khi gió thổi chiều cao đặc biệt trên con đường của gió dưới nó và ở một tốc độ lớn hơn. Gió cắt đặt cột của gia tăng không khí quay, vì vậy không khí là leo thang trở lên. | GRASSLAND CLIMATES the cloud and the speed or direction of the wind may change at different heights inside the cloud. This process produces wind shear a force that exists when the wind at a particular height blows across the path of the wind below it and at a greater speed. Wind shear sets the column of rising air rotating so the air is spiraling upward. The rotation begins in the upper part of the cloud below the level of wind shear. The rotating center of the cloud is then known as a mesocyclone. The word cyclone describes air that rotates in the same direction as the Earth counterclockwise in the Northern Hemisphere. Most mesocyclones rotate cyclonically counterclockwise but the reason for this is unclear and occasionally there are mesocyclones that turn in a clockwise direction anticyclonically . Gradually more and more of the inside of the cloud begins to turn and the rotation extends downward. At this stage the mesocyclone is up to five miles 8 km across. Eventually the rotation may extend to the air immediately below the cloud. Air that is drawn into the up currents now starts turning as it approaches the cloud so the mesocyclone consists of air that is spiraling upward to where it is swept into the anvil and removed. Because air is being removed the atmospheric pressure inside the mesocyclone is low and as air enters the spiral its pressure drops. The reduction in pressure allows the air to expand causing it to cool and its water vapor condenses. Condensation in the rotating air beneath the cloud base makes it look as though the cloud itself is descending. Its rotation is clearly visible from a distance and fragments of cloud can be seen moving across it. The rotation continues to extend downward and as it does so it becomes narrower. Visible because of the condensation it produces the rotating column of air extends below the storm cloud as a funnel cloud widest at the top and tapering toward the lower end. Air accelerates as it enters the spiral and the .