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CHAPTER V. WEATHER 

 5000. GENERAL 



5001. Weather elements of an observation comprise thunderstorms, 

 lightning, waterspouts, squalls, and precipitation in any form. Fog, haze, 

 smoke, and dust are classified as obstructions to vision. Rainbows, halos, 

 coronas, and auroras are recorded as atmospheric phenomena. 



5002. Clouds are not described here, since they are considered sepa- 

 rately in Chapter VIII, and cloud code charts are illustrated in the back of 

 WBForm 615-5. 



5100. THUNDERSTORMS AND LIGHTNING 



5110. GENERAL. A thunderstorm is regarded as occurring at the ship 

 when thunder is heard. Lightning occurs as an electrical discharge within a 

 cloud, from cloud to cloud, or from cloud to sea. Distant lightning is any 

 lightning that occurs so far from the observer that the resulting thunder can- 

 not be heard. It may be observed as streaks or sheets. 



5120. INTENSITY OF THUNDERSTORM. Classification of a thunder- 

 storm as slight, moderate, or heavy is based upon the appearance of the 

 storm from the point of observation. The thunderstorm may be classified as 

 slight throughout its history as viewed from this point, or it may be classi- 

 fied during its passage by the ship as slight, moderate, heavy; and, as it 

 recedes, moderate and slight. Each classification is described below. 



5121. Slight Thunderstorm. Lightning occurs within the cloud and rain- 

 fall is light or moderate. Small hail may also occur. Thunder is not loud, 

 and lightning occurs at intervals of a minute or more. The surface wind 

 speed, at the beginning of or during the storm, does not exceed 26 knots, 

 and any sudden increase in speed is of short duration. 



5122. Moderate Thunderstorm. Loud peals of thunder occur at brief 

 intervals, and frequent flashes of lightning occur from cloud to surface, as 

 well as from cloud to cloud; rain is moderate or heavy, and small hail is 

 light or moderate. An onrush of wind may precede the storm, with a speed 

 as high as 35 knots. Extensive masses of dark clouds showing visible indi- 

 cations of turbulence and rapid horizontal motion are usually observable. 



5123. Heavy Thunderstorm. Sharp and pronounced thunder and light- 

 ning occur almost continuously. Heavy rain occurs, sometimes accompanied 

 by hail. The wind preceding and accompanying the storm may reach a speed 

 in excess of 35 knots. A rapid drop in temperature occurs, sometimes as 

 much as 10°C. in 5 minutes. 



