136 Our Surroundings 



within 12 or 24 hours with wind shifting to the northwest by 

 way of southwest and west. When the wind sets in from points 

 between east and northeast, and the barometer falls steadily, a 

 storm is approaching from the south or southwest and its 

 center will pass near, or to south or east of the observer within 

 12 to 24 hours with the wind shifting to the northwest by way of 

 north. The rapidity of the storm's approach and its intensity will 

 be indicated by the rate and amount of the fall in the barometer." 



Hurricanes. Certain types of storms of an exceedingly vio- 

 lent nature develop over the ocean in the region of the belt of 

 calms in the tropical latitudes. They blow at a rate of from 80 to 

 over 100 miles an hour. Although the effects of their passage 

 may be felt over a belt 1000 miles wide, violent winds are usually 

 limited to a belt of about 250 miles in width. They are accom- 

 panied by heavy rainfall and are often very destructive to ship- 

 ping and other interests, sometimes laying waste cities near the 

 ocean. Such storms in the Atlantic Ocean are known as hurri- 

 canes and in the Pacific Ocean as typhoons. In their progress 

 they take a curved path toward the northwest, gradually chang- 

 ing to a northeast direction as they enter the temperate zone. 



Thunderstorms. A thunderstorm is any storm accompanied 

 by thunder. Thunderstorms commonly come after a spell of hot 

 weather. The hot, moist air, being lighter than the surrounding 

 colder air, rises, and the colder air takes its place. As the air 

 rises, the moisture in it condenses and forms clouds which become 

 charged with electricity, which passes from cloud to cloud or 

 from the clouds to the earth in the form of lightning. Thunder 

 is caused by the vibration that occurs in the air as the electricity 

 passes through it. In the passage of lightning through the air, 

 much of its energy is transformed into heat and often when it 

 strikes an object, such as a building, fire results. Many serious 

 forest fires are caused by lightning striking dead timber. 



Thunderstorms may be either general or local. They are 

 usually accompanied by heavy rainfall and sometimes by hail. 



Tornadoes and Waterspouts. Tornadoes are quite similar 

 to hurricanes, except that they cover a much smaller area and 

 are of shorter duration. A tornado has the same whirling motion 



