48 



WEATHER AND CLIMATE 



c. Problem 4 



Where do hurricanes usually form? 

 How do they differ from tornadoes? 



2. You may find the following new words and 

 phrases in this study : 



centrifugal force force which tends to cause parts of a 

 rotating body or system to fly away from the center of 

 rotation. 



rotate to spin. 



spiral curl-shaped or shaped like a coil spring. 



tornado a whirling wind of great violence. 



3. If possible during this study find pictures and 

 written accounts of tornadoes and hurricanes. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What paths do storm areas take across North 

 America? When a low-pressure area originates over 

 the north Pacific its usual path is across the country, 

 touching the states of the United States bordering 

 on Canada. Storms which originate off the California 

 coast nearly always follow a path across the south- 

 west, either veering to the north and passing up the 

 St. Lawrence Valley or passing into the Atlantic over 

 Georgia and Florida. 



Sometimes a storm or low-pressure area over the 

 midwest shows a great difference of pressure from a 

 "high" lying far to the northwest in Canada. When 

 this occurs in fall or winter, conditions are right for 

 cold northerly winds to set in, blowing much chilled 

 air from the north region into the low-pressure area 

 to the south and causing what we commonly speak 

 of as a cold wave. The general eastward movement of 

 the "low" will then spread this cold air as it travels 

 toward the Atlantic seaboard. In this way freezing 

 weather is sometimes carried as far south as Florida, 

 which ordinarily enjoys a warm climate. 



Heat waves in late spring, summer, and early au- 

 tumn are caused by a delay in the movement of low- 

 and high-pressure areas across the country when con- 

 tinued warm winds blow from the south to the front 

 of the stagnated "low." These gather heat from the 

 radiation of the earth as they blow northward. In this 

 way great quantities of hot air are spread over wide 

 areas, and the country blisters in a heat wave. 



What causes thunderstorms? During the warm 

 days of late spring, summer, and early autumn the air 

 just above the earth becomes heated through the 

 early part of the day much more than the air at higher 

 levels. Some of this air begins to rise and filter through 

 the heavier layers above. As the air rises it expands 

 and cools, and if the cooling continues long enough, 

 the dew point will be reached. This will cause the 

 formation of the bottom of a cloud at this level, and 

 as the rising current continues, a very tall cloud 



with a broad base will be formed. The bottom of this 

 cloud will be dark while its top will be a fluffy white. 

 This is the beginning of a thunderstorm. 



Such a local cloud may be moving in any direction 

 but will usually move from west to east with the 

 prevailing winds, at a velocity of twenty to fifty miles 

 an hour. As the storm travels, the higher portions 

 move more rapidly than the lower parts, and hence the 

 first appearance of a storm is a fringe of very high 

 clouds with darker portions below. 



As the storm comes on, the air becomes warm and 

 oppressive. Wind squalls are observed and develop 

 into a breeze which blows directly into the low-pres- 

 sure center of the storm. As the center reaches an ob- 

 server the wind suddenly shifts because of the inrush- 

 ing currents of colder air, and rain begins to fall from 

 the lower part of the cloud. Following the passage of 

 the storm the barometer usually rises and the tem- 

 perature falls. 



Such storms may be local or may travel for dis- 

 tances up to six hundred miles. They travel somewhat 

 more rapidly than the general pressure area of which 

 they are a part. If the rising air currents of a storm 

 are very strong they may carry the condensed water 

 drops to high altitudes. There the temperature may 

 be cool enough to freeze them into pellets which fall 

 as hail. 



What is a tornado? One of the most destructive 

 storms known to man is the tornado, which occurs 

 many times each year during the summer and early 

 fall over the states of the Central West. While the 

 tornado is not such a widespread storm as the thun- 

 derstorm, it is much more violent and sometimes re- 

 sults in loss of life and much damage to property. 

 Figure 79 gives three photographs of an on-coming 

 tornado. 



The most striking point of the tornado is the fun- 

 nel cloud which reaches the earth from a mass of low- 

 lying black clouds above it. The funnel cloud moves 

 along a path usually from southwest to northeast with 

 a velocity ranging from thirty to sixty miles an hour. 

 This cloud is made up of a rapidly rising current of 

 warm air having a swirling motion which may attain 

 a velocity of several hundred miles per hour. This has 

 a tendency greatly to reduce the atmospheric pressure 

 near the funnel cloud and to cause buildings which 

 have normal pressure on the inside to push their walls 

 outward toward the lesser pressure. 



What is a hurricane? In recent years the state of 

 Florida has been visited several times by West In- 

 dian hurricanes which have developed over the Carib- 

 bean Sea and gradually increased in intensity as they 

 were carried by the winds until, after moving along 

 the coast of Florida, they passed over the north At- 

 lantic toward Europe. 



