AEROGRAPHER'S MATE 3 & 2 



Actually, in nature there are no clearly defined 

 lines of demarcations between these areas. The 

 general features of hurricanes/typhoons given 

 in the following section apply mainly to the 

 mature stage. 



1. Surface Winds, The surface winds blow 

 inward in a counterclockwise direction toward 

 the center with those in the left rear quadrant 

 having the greatest angle of inflow (in the 

 Northern Hemisphere). The diameter of the area 

 affected by the hurricane/typhoon force winds 

 may be in excess of 100 miles in large storms 

 or as small as 25 to 35 miles. Gale 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 sub- 

 tropical anticyclone which is most frequently 

 to the right of the line of movement of the storm 

 in the Northern Hemisphere. No accurate 

 measurement of the peak wind speeds of large 

 mature storms has been made with any reliable 

 degree of accuracy. Speeds of 140 knots have 

 been recorded and it seems reasonable to 

 assume that speeds of 200 knots may be attained 

 at altitudes several hundred feet above the 

 surface, 



2. Surface Pressure, Since the central sur- 

 face pressure of the mature storm is well 

 below average, sea level isobars furnish an 

 excellent tool for analysis of these storms. 

 Isobars may be nearly symmetrical or they 

 may assume an elliptical shape. The pressure 

 gradient to the right of the storm's motion is 

 strongest due to the forward motion of the 

 storm against the existing pressure gradient. 

 Various other deformations may occur, such as 

 the extension of a trough southward from the 

 storm. Barometric tendencies are not a 

 particularly good indication of the movement 

 of the storm outside of its sphere of influence. 

 Usually the pressure falls with extreme 

 rapidity in the 3 hours before the arrival of 

 the storm and rises at an equal rate after 

 passage. Central pressures of 950 to 960 

 millibars are not uncommon. 



3. Surface Temperature. In contrast to 

 extratropical cyclones the tropical cyclone may 

 show no, or very little, surface temperature 

 reductions toward the center of the storm, 

 indicating that the horizontal adlabatic cooling 

 due to the pressure reduction is largely offset 

 by the addition of the heat through the release 



of heat to the atmosphere through the con- 

 densation processes. Upper air temperatures, 

 it has been found, are actually higher by 5° C 

 or more. 



4. Clouds. The cloud patterns of tropical 

 cyclones are also at variance with those of 

 the extratropical cyclones. In mature cyclones 

 almost all the cloud forms are found to be 

 present, to be sure, but by and large the most 

 significant clouds of these storms are the heavy 

 cumulus and cumulonimbus which spiral inward 

 toward the edge of the eye of the storm, 

 becoming generally more massive and closely 

 spaced as they approach the eye. These spiral 

 bands, especially the leading ones, are also 

 referred to as BARS. Cirrus and cirrostratus 

 occupy by far the largest portion of the sky 

 over the tropical cyclone area. In fact, cirrus, 

 becoming more dense, changing to cirrostratus 

 and lowering somewhat, are more often than 

 not the first indicators of the approach of a 

 distant storm or the development of one in the 

 near vicinity. The original appearance of the 

 sky is very similar to that of an approaching 

 warm front. 



5. The Eye. The eye of the storm (fig. 

 14-27) is one of the oldest phenomena known 

 in meteorology. Precipitation ceases abruptly 

 at the boundary of a well-developed eye; the 

 sky partly clears; and the wind subsides to less 

 than 15 knots, and at times there is a dead 

 calm. The sun or the stars become visible. 

 In mature storms, the diameter of the eye 

 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. 

 Radar observations indicate that the eye is 

 constantly undergoing transformation and does 

 not stay in a steady state. 



6. Precipitation. Very heavy rainfall is 

 generally associated with mature cyclones. 

 However, the methods of measurement are 

 subject to such large errors during high winds 

 that representative figures on the normal 

 amount and distribution of precipitation cannot 

 be said to exist. Precipitation is generally 

 concentrated in the inner core where the slope 

 of the barograph trace is the highest. Amounts 

 of 20 inches are not uncommon. Over the open 

 sea, rainfall is considered of operational 

 interest primarily from the standpoint of its 

 effect on ceiling and visibility. Over land, 



342 



