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The Earth’s Atmosphere Contents Part 3

Tham khảo tài liệu 'the earth’s atmosphere contents part 3', khoa học tự nhiên, vật lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 94 Chapter 4 Humidity Condensation and Clouds FIGURE 4.18 Cirrus clouds. with latitude. Large temperature changes cause most of this latitudinal variation. For example high cirriform clouds are composed almost entirely of ice crystals. In subtropical regions air temperatures low enough to freeze all liquid water usually occur only above about 20 000 feet. In polar regions however these same temperatures may be found at altitudes as low as 10 000 feet. Hence while you may observe cirrus clouds at 12 000 feet over northern Alaska you will not see them at that elevation above southern Florida. Clouds cannot be accurately identified strictly on the basis of elevation. Other visual clues are necessary. Some of these are explained in the following section. CLOUD IDENTIFICATION High Clouds High clouds in middle and low latitudes generally form above 20 000 ft or 6000 m . Because the air at these elevations is quite cold and dry high clouds are composed almost exclusively of ice crystals and are also rather thin. High clouds usually appear white except near sunrise and sunset when the unscattered red orange and yellow components of sunlight are reflected from the underside of the clouds. The most common high clouds are the cirrus which are thin wispy clouds blown by high winds into long streamers called mares tails. Notice in Fig. 4.18 that they can look like a white feathery patch with a faint wisp of a tail at one end. Cirrus clouds usually move across the sky from west to east indicating the prevailing winds at their elevation. Cirrocumulus clouds seen less frequently than cirrus appear as small rounded white puffs that may occur individually or in long rows see Fig. 4.19 . When in rows the cirrocumulus cloud has a rippling appearance that distinguishes it from the silky look of the cirrus and the sheetlike cirrostratus. Cirrocumulus seldom cover more than a small portion of the sky. The dappled cloud elements that reflect the red or yellow light of a setting sun make .