Chapter 14 — AIR MASSES AND FRONTS 



it develops and moves. The warm moist air 

 is lifted by a combination of convergence and 

 instability of the air until it condenses. Upon 

 condensation the latent heat is liberated. How- 

 ever, if the storm passes over a large land- 

 mass, the source of energy is cut off and the 

 storm will eventually dissipate. As the storm 

 moves from southerly latitudes to higher 

 latitudes, the heat source is no longer present 

 and the storm will assume extratropical 

 characteristics. 



The average lifespan of this type storm 

 is about 6 days from the time it forms until 

 it either moves over a land surface or recurves 

 to higher latitudes. Some storms last only a 

 few hours, while some last as long as 2 weeks. 

 The evolution of the average storm from birth 

 to dissipation has been divided into four stages: 



1. Formative or Incipient Stage. This stage 

 starts with the birth of the circulation and ends 

 at the time that hurricane/typhoon intensity 

 is reached. This stage can be slow, requiring 

 days for a weak cyclonic circulation to begin, 

 or in the case of development on a tropical 

 wave, it can be relatively explosive, producing 

 a well-formed eye in as little as 12 hours. In 

 this stage the minimum pressure reached is 

 about 1,000 millibars. A good indication that 

 a system of this type has formed or is forming 

 is the appearance of westerly winds (usually 

 10 knots or more) in low tropical latitudes 

 where easterly winds normally prevail. 



2. Immature or Intensification Stage. This 

 stage lasts from the time the system reaches 

 hurricane-typhoon intensity until the time it 

 reaches its maximum intensity in winds and 

 its lowest central pressure. The lowest central 

 pressure often drops well below 1,000 millibars, 

 and the wind system becomes organized in a 

 tight ring around the eye with a fair degree 

 of symmetry. The cloud and precipitation fields 

 develop into narrow, inward spiraling bands. 

 Usually the radius of the strongest winds are 

 no more than 60 miles around the center. This 

 development may take place gradually or occur 

 in less than 1 day. 



3. Mature Stage. This stage lasts from the 

 time the hurricane/typhoon attains its maximum 

 intensity until it weakens to below this intensity 

 or transforms to an extratropical cyclone. In 

 this stage, the storm may exist for several 

 days at nearly the same level of intensity or 



decrease slowly. The storm grows in size, 

 with strong winds reaching farther and farther 

 from the center. The weather and winds usually 

 extend farther in the semicircle of the storm 

 to the right of its direction of movement. By 

 the time the storm reaches this stage it is 

 usually well advanced toward the north and 

 west, or it has already recurved into the 

 westerlies. The typhoons of the Pacific usually 

 last longer in the mature stage and grow to 

 larger sizes than hurricanes in the Atlantic. 



4. Decaying Stage. TMs stage may be 

 characterized by rapid decay as in the case 

 of many storms which move inland, or after 

 recurvature, the transformation into a middle 

 latitude cyclone. In the former case the storm 

 steadily loses strength and character. In cases 

 of transformation there is frequently a regenera- 

 tion in the middle latitudes which results in 

 maintenance or redevelopment of strong winds 

 and other hurricane/typhoon characteristics. 



There is no set duration for the time a 

 storm may be in any one stage. It is entirely 

 possible that a storm will skip one stage or 

 go through it in such a short time that it is 

 not distinguishable without the available synoptic 

 data. Satellite picture interpretation has improved 

 the possibility of observing the various stages 

 of tropical cyclones. 



Characteristics of Tropical Cyclones 



To make the most efficient analysis of 

 available data in the vicinity of tropical 

 cyclones, the forecaster must be familiar with 

 the normal wind, pressure, temperature, clouds, 

 and weather patterns associated with these 

 storms. No two tropical cyclones are exactly 

 alike. On the contrary, there are very great 

 variations between storms. However, certain 

 general features appear with sufficient frequency 

 to predominate in the mean patterns. These 

 features serve as a valuable guide when 

 reconstructing the picture of an individual 

 tropical cyclone from sparse data. 



Since the meteorological elements are not 

 distributed uniformly throughout all sections of 

 the storm, it is customary to describe the 

 storms in terms of four quadrants or right 

 and left semicircles, separated by the line 

 along which the center of the storm is moving 

 and normal to this line at the cyclone center. 



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