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Characteristics of Hurricanes or Typhoons

To make the most efficient analysis of available data in the vicinity of hurricanes or typhoons, you must be familiar with the normal wind, pressure, temperature, clouds, and weather patterns associated with these storms. No two hurricanes or typhoons are exactly alike. On the contrary, there are very great variations between storms. However, certain general features appear with sufficient frequency to permit mean pattern classifications. These features serve as a valuable guide in reconstructing the picture of an individual cyclone from sparse data.

Since meteorological elements are not dis-tributed uniformly throughout all sections of these storms, it is customary to describe the storms in terms of right and left semicircles or four quadrants. The division into semicircles is along a line extending through the center of the cyclone and in the direction toward which the storm is moving. The general features of hurricanes or typhoons given in the following section apply mainly to the mature stage.

SURFACE WINDS.— The surface winds blow inward in a counterclockwise direction toward the center. The winds in the left-rear quadrant have the greatest angle of inflow (in the Northern Hemisphere). The diameter of the area affected by hurricane or typhoon force winds may be in excess of 100 miles in large storms or as small as 25 to 35 miles. Gale-force winds sometimes cover an area 500 to 800 miles or more. The maximum extent of strong winds is usually in the direction of the major subtropical high-pressure center, which is most frequently found to the right of the storm’s path in the Northern Hemisphere. Surface wind speeds of 140 knots have been successfully recorded, but accurate measurements of peak wind speeds in large mature storms have not been possible with any reliable degree of accuracy.

SURFACE PRESSURE.— The sea-level isobars are an excellent tool with which to analyze these storms. The isobars take on a nearly symmetrical or elliptical shape, although deformations in the isobaric pattern are not uncommon. For instance, the tightest isobaric spacing (strongest pressure gradient) is found to the right of a storm’s line of movement, and a trough often extends southward from these storms. The central pressures of mature storms are well below average. Central pressures of 890 to 930 millibars are not uncommon. 

SURFACE TEMPERATURE.— In contrast to extratropical cyclones, the tropical cyclone may show no cooling, or very little, toward the storm center. This indicates that the horizontal adiabatic cooling caused by lower pressures is largely offset by the heat added through the condensation process. Upper-air temperatures have been found warmer by 5 ‘C or more. 

CLOUDS.— The cloud patterns of tropical cyclones also differ from those of extratropical cyclones. In mature tropical cyclones, almost all the cloud forms are present, but by and large the most significant clouds are the heavy cumulus and cumulonimbus which spiral inward toward the outer edge of the eye. These spiral bands,

Figure 9-3-7.—Typical cloud distribution associated with a tropical cyclone.

especially the leading ones, are also referred to as BARS. Cirrus and Cirrostratus occupy the largest portion of the sky over these storms. In fact, cirrus, becoming more dense, then changing to cirrostratus and lowering somewhat is more often than not a mariner’s first indication of an approaching distant storm or the development of one in the near vicinity. The appearance of the sky is very similar to that of an approaching warm front. A typical cloud distribution chart for a tropical cyclone is found in figure 9-3-7.

THE EYE.— The eye of a storm is one of the oddest phenomena known in meteorology. Pre-cipitation ceases abruptly at the boundary of a well-developed eye; the sky partly clears; the sun or stars become visible; the wind subsides to less than 15 knots, and at times there is a dead calm. In mature storms, the eye’s diameter averages about 15 miles, but it may attain 40 miles in large typhoons. The eye is not always circular; sometimes it becomes elongated and even diffuse with a double structure appearance. The eye is constantly undergoing transformation and does not stay in a steady state.

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