Chapter 15— METEOROLOGICAL ELEMENTS 



Thunderstorm Detection 



The information of upper air observations 

 and the surface weather charts gives indications 

 of thunderstorm activity. However, since these 

 charts are normally prepared at 6- and 12- 

 hour intervals, it is understandable that certain 

 weather phenomena may form during the periods 

 when observations for maps and upper air 

 soundings are not scheduled. 



Although a synoptic weather map gives def- 

 inite indications of an approaching front or hur- 

 ricane, or of the presence of thunderstorms in 

 a specific area, minute-to-minute tracking of 

 these weather phenomena is not possible. From 

 the above-mentioned sources, the exact time of 

 the occurrence of adverse weather is extremely 

 difficult and, at times, impossible to forecast. 



Radar has provided meteorology with an ad- 

 ditional tool to be used in the collection of at- 

 mospheric data. It has been proved that reflec- 

 tion of radar pulses from precipitable water 

 associated with clouds permits the continuous 

 tracking of the position of such clouds with 

 respect to the location of a station. Radar methods 

 make it possible to forecast the approach of un- 

 favorable weather with greater accuracy and 

 with less difficulty than can be achieved by other 

 methods. 



It is beyond the scope of this training manual 

 to discuss the relation between forecasting and 

 radar in detail; however, one of the basic means 

 of presentation should be mentioned — the PPI 

 (Plan Position Indicator). The PPI scan is fre- 

 quently employed where the tactical conditions 

 require that range and bearing information be 

 obtained concerning objects in or near a hori- 

 zontal plane centered at the site of the radar 

 station. Not only can the proximity of storms 

 be ascertained, but also their speed , area, 

 and development can be judged accurately by 

 an experienced observer. Within the limitations 

 of the radar equipment used with respect to 

 range, precise short-range forecasts vital to 

 the safety of personnel and equipment can be 

 issued. 



Figure 15-14. 



209.62 

 ■Radar echo of a thunderstorm. 



be issued 5 to 6 hours prior to the arrival of 

 a destructive storm traveling at a speed of less 

 than 20 knots. 



The radar echo from a convective thunder- 

 storm of a PPI scope is shown in figure 15-14. 

 The radar was adjusted for a 25-mile range. 

 The concentric lines are 5-mile markers. The 

 bright area at azimuth 190 and the 8-mile range 

 is a thunderstorm. 



The weather map of an area in which convec- 

 tive thunderstorms are prevalent gives no defi- 

 ite indication of the probability of a storm 

 occurring at any given location. All that can be 

 said, following a careful study of the weather 

 map, is that the air in the vicinity of the ship 

 or station is unstable and that thunderstorms 

 will probably occur in the area. The storm 

 picked up on the PPI scope in figure 15-14 

 did not appear on the weather map. 



LIGHTNING 



Destructive phenomena, such as the thunder- 

 storm, can be detected, and their approach to 

 the ship or station can be timed. In this manner, 

 storm warnings can be given sufficiently early 

 so that storm conditions may be set. With a 

 radar range of 80 to 100 miles, a warning can 



Lightning may be defined as a flash of light 

 from a sudden electrical discharge which takes 

 place between clouds or inside a cloud or from 

 high structures on the the ground or from 

 mountains. Four main types of lightning can be 

 distinguished, as follows: 



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