Other indications of development are sea swell and tide observations. Swells associated with a tropical storm will have a frequency less than average and an amplitude greater than average. The normal swell frequency is 8 per minute in the Atlantic and 14 per minute in the Gulf of Mexico. Hurricane winds set up swells with a frequency that can decrease to four per minute. The period of the swell will also be much longer than usual. Swells will approach the observer approximately from the direction in which the storm is located. The swell height is an indication of the storms intensity, especially when the swells have not encountered shallow water before reaching shore. Abnormally high tides along broad coastlines and along shores of partially enclosed water bodies are also a good indication of storm development. The highest tides will normally be found to the right of the storm path, looking downstream.

Meteorological satellites have greatly aided in the detection of tropical disturbances, especially during the early life cycle. It is important for meteorologists, analysts, and forecasters to be able to effectively interpret these pictures to extract their maximum benefit. Through proper interpretation of satellite data, determination of size, movement, extent of coverage, and an approximation of surface wind speed and direction can be made. Satellites have become the most important method of detection of disturbances. Aircraft reconnaissance also plays an important role in tropical cyclone detection throughout the North Atlantic, Caribbean Sea, and the Gulf of Mexico. This has become even more important when used in conjunction with the data from satellites. Areas of suspicious cloud structure can be investigated by aircraft whose crews include trained meteorologists. These reconnaissance flights can provide on-station data, either high- or low-level, that would not be available otherwise.

 Another method of detecting tropical disturbances is through the use of radar. Present radar ranges extend about 300 miles and can scan an area approximately 300,000 square miles.

The Navy and NOAA also maintain several different types of moored METOC buoys that report various meteorological and oceanographic elements.

These stations send out measured data automatically or upon query.

The problems of formation, detection, and location are in reality a single three-in-one problem. One is dependent on the other.

 In the case of satellite pictures, reconnaissance, and radar detection, the location is fairly certain, barring navigational errors. If detection is made through the analysis, the exact location is more difficult to ascertain in the incipient stage of the storm, especially when the analysis is diffuse. The exact location should be decided upon only after the most intensive study of the data. The analyst should be prepared to revise his or her decision in the face of developments which are more conclusive.

Only when easterlies extend vertically to 25,000 feet or more at the latitude of the vortex is intensification possible. This most frequently occurs when the subtropical ridge lies poleward of its normal position for the season. The following characteristics are indications of intensification:

. A migratory anticyclone passes to the north of the storm center.

. A strong net outflow (divergence) is manifested by anticyclonic flow in the upper levels (200 hPa).

. Long waves are slowly progressive. (Applicable to genesis also.)

. Poleward movement of the cyclone is favorable for intensification; equatorward motion is not favorable.

. Intensification occurs only in areas where the sea surface temperature is 79F or greater, with a high moisture content at all levels.

. Other factors being equal, deepening will occur more rapidly in higher than in lower latitudes. (Coriolis force is stronger.)

. When a storm moves overland, the intensity will immediately diminish. The expected amount of decrease in wind speeds can be 30 to 50 percent for storms with winds of 65 knots or more; and 15 to 30 percent for storms with less than 65 knots. If the terrain is rough, there is more decrease in each case than if the terrain is flat.

Once an intense tropical cyclone has formed, there will be further changes in its intensity and in the course of its motion within the Tropics and during recurvature. The forecaster should consider these changes in connection with the predicted path of movement.

Tropical cyclones usually move with a direction and speed that closely approximates the tropospheric current that surrounds them. Logically, therefore, charts of the mean flow of the troposphere should be used as a basis for predicting the movement of tropical cyclones, but lack of observations generally precludes this approach. Generally there is a tendency for tropical cyclones to follow a curved path away from the Equator; however, departures from this type of track are frequent and of great variety,

Tropical cyclones move toward the greatest surface pressure falls and toward the area where the surface pressure falls increase fastest with time. Calculation is necessary for this rule to be used.

Numerous theories have been advanced to explain the cyclone tracks of the past and to predict those of the future. Observational data have never been sufficient to prove or disprove most of them.

Tropical storms move under the influence of both external and internal forces. The external forces are a result of air currents that surround a storm and carry it along. The internal forces appear to produce a tendency for a northward displacement of the storm (which probably is proportional to the intensity of the storm), a westward displacement that decreases as latitude increases, and aperiodic oscillation about a mean track.

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