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VERTICAL CHARACTERISTICS.— The vertical structure of a mature tropical cy-clone also differs considerably from the extratropical cyclone. The first difference is the tropical cyclone’s warm core. Between roughly 850 mb and 100 mb, the air tempera-ture is generally 5°C to 15°C warmer at levels within the core than at corresponding levels in the surrounding atmosphere. The slope is assumed to be nearly vertical, and cloud distribution and the fact there is no air mass difference tend to support this assump-tion.

Strong convergence occurs in the layer from the surface to roughly 10,000 feet, while strong divergence occurs above 30,000 feet. A net outflow of air is usually very pronounced in the vicinity of the 200-mb level, and it is for this reason that the 200-mb chart is one of the primary analysis tools in the tropics. The outflow at the 200-mb level is manifested by anticyclonic circulation unless the storm is unusually severe. In such cases, cyclonic flow then extends to even the 200-mb level.

Learning Objective: Identify tropical cyclone source regions and seasons of occurrence.

Seasons and Regions of Occurrence 

Tropical cyclones may occur during any month, but they occur most often from May to November in the hemisphere experiencing summer. Their frequency, however, varies from ocean to ocean.

There are eight principal regions of tropical cyclone formation. Five regions are in the Northern Hemisphere, and three in the Southern Hemisphere. The south Atlantic and the eastern South Pacific oceans are entirely free from tropical cyclones. The southwestern North Pacific, on the other hand, has by far the greatest number of tropical cyclones forming in it. A detailed breakdown of formation areas, with main tracks and months of most frequent occurrence, is found in table 9-3-1. Tropical cyclones of hurricane or typhoon intensity are more prevalent in late summer and early fall in both the Northern Hemisphere and Southern Hemisphere. This does not preclude the formation or intensification of tropical cyclones of any intensity during the other seasons.

Learning Objective: Describe the use of Buys Ballot’s Law in estimating the position of tropical cyclones and the problems associated with this method. Estimating the Position of Storm Centers

One of the simplest means of locating a storm center from surface ship reports is to apply the law of Buys Ballot. To apply the law, you need at least two ship reports from different ships or two reports from the same ship at different times. The latter application does not permit calculating the exact distance to the storm center. An example of locating the center from two ship reports in the Northern Hemisphere is shown

Table 9-3-1.—Tropical Cyclone Data

in figure 9-3-9. From Buys Ballot’s Law, with your back to the wind, a cyclone’s position is expected to the left and slightly forward of your position. Obviously, ship B is closer to the storm center, as evidenced by the stronger winds and lower pressure. The center is north or northeast of ship B’s position and west-southwest or west of ship A. The heavy dashed arrows intersect at the storm center near 16°N and 80°W.

The general rule for locating the direction of the center based on wind direction is subject to error when the circulation around the storm is not circular and/or the storm is fast moving. See figure 9-3-10, views A and B. When wind fields are elliptical, as in view A, large errors may result. These errors are related to the indraft of wind (cross-isobar flow) in tropical storms. The angle of incidence varies with distance from the storm; in the outer limits of the wind field, it is about 45°, while near the center, the wind direction is nearly tangent (parallel) to the isobars. In fast-moving tropical cyclones, the cross-isobar flow is small directly ahead of the storm. while to the rear, the cross-isobar angles are greatest. The wind rule as it pertains to fast-moving tropical cyclones is shown in view B. In summary, the wind rule works best in a circular wind around a stationary or slow-moving tropical cyclone.

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