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PRECIPITATION.— Very heavy rainfall is normal amount and distribution of precipitation cannot be said to exist. Precipitation is generally concentrated in the inner core, where the slope of the barograph trace is the highest. Amounts of 20 inches are not uncommon. Over the open sea, rainfall is considered of operational interest primarily from the standpoint of its effect on ceiling and visibility. Over land, the threat of flooding is of paramount importance and must always be taken into consideration. Oro-graphic effects produce concentrations of rainfall that often result in costly floods. Hurricane- or typhoon-force winds pushing moisture-laden trop-ical air up a steep mountain slope often results . in phenomenal rainfall. A fall of 88 inches was recorded during one storm in the Phillipines. At the other extreme, as little as a trace was recorded at a station in Florida that had winds up to 120 knots during the passage of a hurricane.

STATE OF THE SEA.— The winds of a tropical cyclone produce wind waves which move outward from its center. The size and speed of the waves depends upon the wind velocity and the length of the over-water trajectory of the winds. As the wind waves move away from the center, their height diminishes, their length is reduced, and they become low undulating waves known as swell. The wind waves and swell move outward from the storm center at a rate that nearly always exceeds the forward speed of the storm. Since these waves precede the storm, they are one of the first signs of its approach.

In the open sea, the combination of wind and swell direction gives some indication of a storm’s location, while the period of the swell (that is, the time, in seconds, between the passage of successive wave crests) is helpful in determining a storm’s intensity. The wave period of the swells associated with a tropical cyclone is considerably longer than other swell waves normally observed in the tropics. In fact, in the Caribbean Sea and the Gulf of Mexico, long-period swell waves are not common except in connection with tropical cyclones. In these waters during hurricane season, the appearance of heavy swell waves with a period of 9 to 15 seconds is an indication of the existence of a tropical cyclone of storm intensity in the direction from which the swells are coming. Longer swell wave periods, 12 to 15 seconds, are almost a certain sign of a hurricane. Figure 9-3-8


Figure 9-3-8.—Direction of wind and swell around the - hurricane center. Solid arrows show the direction of the wind; dashed arrows show direction of swell. Large arrow shows direction of movement of the storm.

shows the direction of wind and swell around a hurricane center. One of the most severe effects of hurri-canes and typhoons is the damage to coastal areas caused by large ocean waves. The most severe waves occur where land partially sur-rounds bodies of water, such as the Bay of Bengal and the Gulf of Mexico. Strong sus-tained winds in the right semicircle push water into coastal areas, thereby causing tides to rise by as much as 10 to 15 feet above normal. This effect is referred to as a storm tide. An added and even greater threat is the so-called hurricane wave. This term applies to the marked rise in the level of the sea near the center of intense tropical cyclones. This rise can reach 20 feet or more and seriously affects small islands and continental shorelines. These waves have pro-duced many of the major hurricane disasters of history. The hurricane wave may occur as a series of waves, but it is usually one huge wave. In partially enclosed seas, the hurricane wave may superimpose itself on the storm tide. There is usually little warning of its approach; however, it should be antici-pated near, and to the right of intense tropi-cal cyclone centers.

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