AEROGRAPHER'S MATE 3 & 2 



FRONTAL. — Frontal thunderstorms are most 

 commonly associated with the warm and cold 

 types of fronts. 



The warm-front thunderstorm is caused when 

 warm, moist, unstable air is forced aloft over 

 a colder, denser shelf of retreating air. Warm- 

 front thunderstorms are generally scattered; they 

 are difficult to identify because they are ob- 

 scured by other clouds. 



The cold-front thunderstorm is caused by 

 the forward motion of a wedge of cold air into 

 a body of warm, moist, unstable air. Cold- 

 front storms are normally positioned aloft along 

 the frontal surface in what appears to be a con- 

 tinuous line. 



Under special atmospheric conditions, a line 

 of thunderstorms develops ahead of a cold front. 

 This line of thunderstorms is known as a pre- 

 frontal squall line. Its distance ahead of the 

 front ranges from 50 to 300 miles. Prefrontal 

 thunderstorms are usually intense and appear 

 very menacing. Bases of the clouds are very 

 low. Tornadoes sometimes occur when this type 

 of activity is present. 



AIR MASS. — Air-mass thunderstorms are 

 subdivided into several types. In this discussion, 

 however, only two basic types are mentioned — 

 the convective thunderstorm and the orographic 

 thunderstorm. 



CONVECTIVE.— Convective thunderstorms 

 may occur over land or water almost anywhere 

 in the world. Their formation is caused by solar 

 heating of various areas of the land or sea, 

 which, in turn, provides heat to the air in trans- 

 it. The land type of convective thunderstorms 

 normally forms during the afternoon hours after 

 the earth has gained maximum heating from the 

 sun. If the circulation is such that cool, moist, 

 convectively unstable air is passing over this 

 land area, heating from below causes convective 

 currents and results in towering cumulus or 

 thunderstorm activity. Dissipation usually be- 

 gins during the early evening hours. 



Storms that occur over bodies of water form 

 in the same manner, but at different hours. Sea 

 storms usually form during the evening after 

 the sun has set. They dissipate during the late 

 morning. An example that combines both types 

 of convective thunderstorms is the situation that 

 exists in Florida. Circulation around the Ber- 

 muda high transports moist air over the land 



surface of Florida during the entire day. The 

 Bermuda high causes air to flow from the east 

 over Florida. Thunderstorms off the east 

 Florida coast at night are caused by warm 

 air advection from the east wind and the warm 

 axis of the Gulf Stream, aided by nocturnal 

 cooling of air above sea level, setting up an 

 unstable lapse rate. During the hours of sun- 

 light, the land surface is considerably warmer 

 than the air; consequently, the air is subjected 

 to heating from below. Convective currents re- 

 sult, and the common afternoon thunderstorm is 

 observed. After sundown, the earth loses its 

 heat. Dissipation occurs, and the apparent 

 movement of the storms to sea takes place. 

 As the circulation causes air to flow over the 

 peninsula at night, the air is cooled by the 

 land surface. As this same air moves out over 

 the warm water, it is heated from below, and 

 cumulus activity occurs. Water, not being sub- 

 ject to such rapid temperature changes as land, 

 retains much of the heat it has gained during 

 the day. When the sun rises, the air over the 

 sea surface becomes warmer than the surface, 

 thereby destroying the balance necessary to 

 keep a storm active, and dissipation occurs. 

 As a general rule, convective thunderstorms 

 are scattered and easily recognized. They are 

 relatively high, and visibility is generally good 

 in the surrounding area. 



OROGRAPHIC. — Orographic thunderstorms 

 form in mountainous regions, particularly ad- 

 jacent to individual peaks. A good example of 

 this type of storm occurs in the northern Rocky 

 Mountain region. When the circulation of the air 

 is from the west, moist air from the Pacific 

 Ocean is transported to the mountains, where it 

 is forced aloft by the upslope of the terrain. 

 If the air is also conditionally unstable, this 

 upslope motion causes thunderstorm activity on 

 the windward side of the mountains. This ac- 

 tivity may form a long, unbroken line of storms 

 similar to a cold front. The storms persist 

 as long as the circulation causes an upslope 

 motion. 



From the windward side of the mountains, 

 identification of orographic storms may some- 

 times be difficult because the storms are ob- 

 scured by other clouds. From the lee side, 

 identification is positive; the outlines of each 

 storm are plainly visible. Orographic storms, 

 almost without exception, enshroud mountain 

 peaks or hills. 



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