More than 130 tropical cyclones occur each average year somewhere within the tropical and subtropical parts of the Pacific Ocean. Collectively they affect appreciably much of this vast region and its borders. Although in a single year only a small percentage of the total area is notably damaged by storms, nevertheless over a period of years the damage is widespread. Moreover, tropical cyclones significantly affect a much larger area than is damaged. A careful consideration of the evidence justifies the conclusion that most of the people of this vast region are at least appreciably influenced by tropical cyclones, some of them quite frequently.
Tropical cyclones vary widely in strength and in other respects. The severe storms that occur in the Far East are now generally called typhoons; similar storms are known as hurricanes in the south Pacific and in the eastern Pacific (and also in the Atlantic and Indian oceans ).
Although the destruction caused by severe typhoons and hurricanes often is terrible, tropical cyclones always benefit a much wider belt than they devastate. They do this chiefly by bringing rainfall to large parts of the tropics that otherwise would be too dry. They also bring advantageous weather changes. Relatively few tropical cyclones are notably destructive; the weaker ones usually do little harm and much good. Distant or incipient cyclones set up air currents that induce convectional overturning, giving rise to thunderstorms and other rainfall. Much, ff not most, of the rainfall of the tropical and subtropical belt of the Pacific is induced by cyclonic disturbances, often distant or weak.
Tropical cyclones are whirling storms that vary in intensity from mild disturbances to violent hurricanes. They average about 300 miles across, but many are only a score or twoscore miles wide and some are more than 1000 miles across in the tropics and 1500 after leaving the tropics. They develop chiefly in latitudes 10 to 20° and travel generally roughly westerly in the tropics, pushed by the trade winds. They also commonly move into progressively higher latitudes until they die away or leave the tropics. If they reach mid-latitudes, they generally slowly recurve and are carried eastward by the prevailing westerlies, and then are called lows. Most tropical cyclones travel rather slowly, generally averaging only about 10 miles an hour, but sometimes only a third that fast and sometimes four times that fast. Commonly they gain speed as they progress, except when recurving, when they may move very slowly.
The winds that spiral around the center, however, have velocities commonly ranging from about 15 to more than 75 miles per hour. A few have had measured velocities of about 150 miles an hour. The destructiveness caused by the stronger winds is due partly to their sudden variations in velocity and direction. The zone of greatest wind destruction is usually only 10 to 30 miles wide, that passed over by the "eye of the storm."
Typhoons and hurricanes are often destructive in the Pacific because of the strength of their winds. Upon the land the wind may damage or even demolish houses, break off palm and other trees, shred the leaves of the banana, plantain, and other large-leaved plants, and break off branches of many trees. Fruit-laden trees are likely to have much of their fruit shaken off. The damage done in breaking branches is increased by fungus infections that commonly develop at the wounds. When even twigs of the rubber tree and certain other trees are broken, "bleeding" commonly occurs, seriously injuring the tree. The damage that the wind does is increased by flying objects; for example, the sheet-iron "tin" roofs of houses frequently whirl through the air, sometimes affording veritable swords of disaster. Tornadoes occasionally develop in hurricanes, adding to the destruction caused by the wind. Severe hurricane damage to buildings is so common that illustrations are hardly necessary. Part of the damage done by gales is indirect, especially by rain being driven horizontally into buildings.
Strong winds over the water often produce high and tumultuous waves. There are many records of much damage done to ships and on land by storm waves, some of which attain great heights. For example, numerous low islands have had waves sweep across them, carrying away much of what had been on their surface and moving large blocks of coral. One island in which the highest land was 46 feet above mean sea level was entirely covered over, as was shown by the scouring away of the soil and other objects. There are records that the only human survivors from hurricanes on some low islands were persons who climbed trees and tied themselves thereto.
Most tropical cyclones cause torrential rainfalls. Indeed the amounts of rain occasionally received seem almost incredible to most citizens of the United States, who are accustomed to annual totals of 25 to 50 inches. Tropical cyclones in the Pacific region often cause falls of 10 inches of rain within 24 hours. Occasionally more than 20 inches falls within 24 hours, a considerable share of it within a few hours; for example, 25.5 inches fell at Haiku on Oahu on January 16, 1949. There are a number of records within the Pacific region of 30 inches or more in 24 hours; one record from Queensland, Australia, of 63 inches in 3 days and one from the Philippines is of 88 inches in 4 days. It is obvious that torrential rainfalls of large proportions cause much damage not only to cultivated land, roads, bridges, buildings, and other structures but also by drownings.
Tropical cyclones of varying intensity occur at least occasionally in almost all parts of the Pacific region. The records from various areas are few partly because the regions contain few persons qualified to record them properly. For example, large fractions of the Pacific Ocean have no islands and perhaps are seldom crossed by a ship. However, a considerable amount of evidence is presented in detail elsewhere that such storms occur.
In addition to the storms that have violent winds, tropical cyclones are significant in numerous other ways, sometimes far from their centers. Examples are the rains that they cause, the "tidal waves" set in motion, and the stimulating effects of changes of weather; the effects elsewhere of remote disasters are also often appreciable.