THE ATMOSPHERE, WEATHER, AND CLIMATE 



23 



ground-level counterpart of rain. Frost is the ground- 

 level counterpart of snow. It forms when atmospheric 

 water vapor crystalizes directly upon the ground or 

 when dew is frozen. 



STORMS 



The three major storms are tropical cyclones, thunder- 

 storms, and tornadoes. The tropical cyclone influences 

 a large geographic area, but the other two are local 

 storms. 



Tropical cyclones of North America, called hurri- 

 canes, are formed either near the southwest coast of 

 Mexico, or east and slightly south of the West Indies. 

 They are found in the Pacific and Atlantic areas, re- 

 spectively. 



A generalized hurricane life history is as follows: 

 Hurricanes originate in the northeast trade winds 

 when a local irregularity develops in the winds or 

 when the southeast trades break through into the 

 northeast trades. In either case, there is an inter- 

 ruption in the normal northeast flow of air, resulting 

 in a "Low." Winds then enter the "Low" in a 

 counterclockwise swirl, forming a cyclone; warm air 

 rises within the "Low"; and the rising air condenses. 

 The heat released by condensation causes adjacent 

 air to become warmer and lighter, which, in turn, 

 causes the cyclone to rotate faster and the air to rise 

 higher in the "Low." This entire process leads to a 

 progressive increase in momentum and winds up to 

 150mph (occasionally, up to 200 mph) develop. Ul- 

 timately, if it is of average size, the hurricane is 

 about 400 miles in diameter and causes widespread 

 damage in its path. However, the central 20 miles, 

 the "eye" of the hurricane, does not possess these 

 damaging winds; rather, it is a zone of calm or little 

 wind activity. During its formation, a hurricane is 

 moving, usually at speeds of 10 to 15 mph; as it 

 matures and travels northward, it often picks up 

 speed, sometimes moving over the sea or land at 

 speeds above 60 mph. 



A tornado (whirlwind) is the most destructive 

 weather phenomenon. It is fortunate that tornadoes 

 are local storms of restricted width and length. The 

 diameter of destruction is limited to about 1000 feet. 

 Tornadoes follow a sinuous path, rarely more than 

 300 miles, before the life cycle is completed. Most of 

 those in the United States form from late spring to 

 early summer and affect the Mississippi and Mis- 

 souri valleys. They originate from tremendous air 

 instability that is associated with thunderstorms on 



hot, humid days. When conditions are right for 

 tornado formation, suddenly from a blanket of deeply 

 rounded cumulus clouds there appears a twisting fun- 

 nel which grows to the ground. During this process, 

 the terrifying roar of the tornado already has started. 

 A waterspout is a tornado over water. The whirling 

 funnel of air carries water instead of dust. 



A full-grown tornado travels at speeds of 20 to 40 

 mph, but the traveling speed does not cause the 

 damage. Most damage is the product of the whirling 

 and rising winds and very low pressure. The wind 

 speed, which may exceed 300 mph near the center, 

 flattens things in its path. The low air pressure of 

 the center causes closed dwellings to explode because 

 the normal air pressure within the building is much 

 greater than the pressure of the tornado. 



Thunderstorms are somewhat rare along the Pa- 

 cific Coast, but they occur throughout the United 

 States. A thunderstorm develops after updrafts 

 first form cumulus clouds; the cumulus then grow 

 into cumulonimbus; and, finally, the cumulonimbus 

 develop anvil-like tops, or thunderheads. Owing to 

 activity within a cumulonimbus, there are con- 

 centrations of positive and negative charges within 

 the cloud. Thus, there is an attraction of unlike 

 electrical charges, either within the cloud, or be- 

 tween the cloud and objects on the ground. Con- 

 tact of these charges produces violent electrical dis- 

 charges, lightning. The flashes of lightning partly 

 involve the burning of atmospheric oxygen which, in 

 turn, leads to a partial air vacuum. The partial 

 vacuum is almost immediately replaced by an inrush 

 of air. It is the striking of inrushing air that causes 

 thunder. In addition, this entire disturbance is as- 

 sociated with rain. Therefore, a thunderstorm con- 

 sists of lightning, thunder, and frequently rain. 



Squalls, blizzards, and monsoons are terms gen- 

 erally associated with storms. A squall is any sudden 

 and violent burst of wind that is accompanied by 

 rain or snow. A blizzard is a high, cold wind con- 

 taining snow. Monsoons are steady trade winds 

 along the western coast of the Pacific Ocean. The 

 summer wet monsoons are winds from the southwest; 

 the winter dry monsoons are from the northeast. 



CLIMATE AND GEOGRAPHY 



World climate is largely a reflection of conditions 

 over the oceans. Moreover, ocean climate chiefly 

 stems from the physical aspects of the oceans and 



