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Conditions of Readiness

The destructive force of tropical cyclones is well documented, and when they bear down on a ship or station, a myriad of questions is asked. Do we get underway or stay in port? Do we evacuate aircraft to other airfields, or are they safe in the hangars? Should emergency shelters be opened and base-wide recall be implemented? These types of questions are asked at all levels of the chain of command in order to make prepara-tions to lessen or minimize damage and personal injury. Plans are implemented and preparations made as far in advance as possible, because of the destructive nature of these cyclones. Conditions of readiness are time based and indicate the time until the destructive winds or other related phenomena will impact a ship or station. They are set for winds of gale force and higher and other destructive phenomena such as thunderstorms and tornadoes. The conditions of readiness are as follows:

1. Gale/Storm/Hurricane (Typhoon) Con-dition IV—Trend indicates a possible threat of destructive winds of force is indicated within 72 hours.

2. Gale/Storm/Hurricane (Typhoon) Con-dition III—Trend indicates a possible threat of destructive winds of force is indicated within 48 hours.

3. Thunderstorm/Tornado/Gale/Storm/ Hurricane (Typhoon) Condition II—Destructive


Figure 9-3-11.—Example of text of tropical cyclone warning.

  winds of force indicated are anticipated within 24 hours or, in the case of thunderstorms or tornadoes, are reported or expected in the general area.

4. Thunderstorm/Tornado/Gale/Storm/ Hurricane (Typhoon) Condition I—Destructive winds of force indicated are anticipated within 12 hours.

TROPICAL EASTERLY JET STREAM

The tropical easterly jet is a persistent feature over extreme southern Asia and northern Africa (between 5° and 20°N latitude) during the Northern Hemisphere summer. It is found in the layer between 200 mb and 100 mb. Since this jet stream is a semipermanent summertime feature and flows east to west, it implies a reversed temperature field across the jet. The cold air is south and warm air north. The deep relatively warm layer of air to the north is the result of heat transferred from the hot Asian land mass directly to the atmosphere. South of the jet, the ocean absorbs and stores much of the heat it receives, thereby creating a relatively cooler atmosphere.  

Upper-level divergence associated with this jet occurs north of the jet axis at its eastern end and south of the axis at the western end. Both areas are directly related to heavy monsoon rains at the surface.

SUMMARY

Tropical meteorology differs considerably from the meteorology of higher latitudes. Seasonal weather changes (especially temperature) are minimal; the tropics are hot throughout the year. Geographically, this region of Earth is predominantly oceanic, and weather observations are therefore sparse. You must make the most of every observation and have an understanding of each element’s representativeness. Temperature and pressure fields are not nearly as important in the tropics as in higher latitudes, because gradients are so weak. The most significant element is the wind. It reflects the changes in pressure, and as long as you take orographic effects into consideration at island and coastal stations, wind reports take on significant importance with regard to the overall analysis. Wind regimes play a major role in tropical meteorology. The trades are the predominant wind of the tropics. The trade wind belt is found between the horse latitudes and the doldrum belt, and fair weather is the norm in this region. The doldrums is an area of convergent wind flow; the trades of each hemisphere come together in this belt and create widespread convective activity and precipitation. The winds in the doldrums are, for the most part, light and variable, much like the winds of the horse latitudes. The horse latitudes are those regions of Earth that correspond to the location of the subtropical high-pressure belts of each hemisphere. The weather in the horse latitudes is generally fair.

Streamline analyses are used in conjunction with surface isobaric and upper-level constant-pressure analyses to adequately portray the tropical atmosphere. Other charts, such as the 24-hour pressure change, weather distribution, and time sections, also play a key role in analyzing the weather in this region. Weather features common in tropical analysis are easterly waves, vortices, asymptotes, shear lines, the Intertropical Convergence Zone, tropical cyclones, and the tropical easterly jet stream.

Of all tropical weather phenomena, the most destructive and feared is the tropical cyclone. There are three classifications assigned to these low-pressure systems. The first is the tropical depression. It is so designated when at least one closed isobar appears at the surface and winds are less than 34 knots. Advisories on the location and expected development of tropical depressions are numbered. When a depression intensifies and produces sustained winds of 34 to 63 knots, it is upgraded to a tropical storm and named. When sustained winds exceed 63 knots, the cyclone is reclassified as a hurricane or typhoon. Since high winds and seas, heavy rain, severe thunderstorms, and tornadoes accompany these storms, we track their progress in order to provide as much advance warning as possible to ships and shore activities.

References

Aerographer’s Mate 1 & C, NAVEDTRA 10362-B1, Naval Education and Training Program Development Center, Pensacola, Fla., 1974.

Byers, General Meteorology, 4th cd., NAVAIR 50-1B-515, McGraw-Hill Book Company, New York, 1974.

Day and Sternes, Climate and Weather, Addison-Wesley Publishing Company, Reading, Mass., 1970.

Riley and Spolton, World Weather and Climate, Cambridge University Press, Cambridge, England, 1974.

The Practical Aspect of Tropical Meteorology, NA 50-1P-537, Atmospheric Analysis Laboratory, Geophysics Research Directorate, Air Force Cambridge Research Center, Bedford, Mass., 1955.

Treawartha and Horn, An Introduction to Climate, 5th cd., McGraw-Hill Book Com-pany, New York, 1980.

Tropical Streamline Analysis, NAVEDTRA 40530, Naval Oceanography Command Facil-ity, NSTL, Bay St. Louis, Miss., 1983.

Tropical Synoptic Models, NAVEDTRA 40620,  

U.S. Air Force, Forecaster’s Guide to Tropical Meteorology, Technical Report 140, Air Weather Service (MAC) USAF, 1971.

Weather Analysis in Tropical Regions, NAVAER 50-1P-534, Chief of Naval Operations, Washington, D.C., 1955.

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