THE THUNDERSTORRI— EVANS AND McEACHRON X91 



As stated, these thunderstorms are caused by the action of the cool 

 dry au' mass on the warm humid air mass. Cool air traveling from the 

 northwest along the rear of a low (low-pressure area) overtakes warm 

 air moving from the south and southwest into the southern sector of 

 the LOW. The surface along which the front of the cool air mass 

 makes contact with the warm air is called the cold-front. It is along 

 this "squall line" or storm line that the thunderstorms are formed. 

 Figure 5,^^ ^^ reproduced from National Research Council Bulletin, no. 

 79, shows horizontal and vertical sections through the cold- and warm- 

 fronts attending a low. In section (a) a view is shown looking down 

 from above the earth upon a low. Referring to that part of the 

 diagram which lies to the left of the dashed vertical line, and disregard- 

 ing the part to the right of this line which will be described under 

 "Warm-front Thunderstorms", the cold front is indicated by the line 

 extending downward and to the left. A vertical section through this 

 cold front is shown in the lower left-hand side of figure 5. It will be 

 noted that the cold air mass has an overhanging front. The overhang 

 frequently may be 4 miles long.^^ It results from retardation of the 

 surface air layers by friction with the ground and consequent over- 

 running of these layers by the faster-moving higher air. Warm air is 

 trapped beneath this overhanging front. In the resulting instability 

 due to cold air above warm, the warm air rises rapidly and forms 

 strong convection systems which often are of sufficient magnitude to 

 produce thunderstorms. As the wedge of cold air advances, it also 

 underruns the warm air and forces it upward, contributing in this way 

 to thunderstorm formation. Thunderstorms formed by these causes 

 are called cold-front thunderstorms. 



OVERRUNNING-COLD-FRONT THUNDERSTORMS 



When, instead of the front of a cool air mass overrunning a warm 

 mass of air near the surface of the ground, it does so at a higher level 

 for a number of miles in front of the surface cold-front, the unstable 

 conditions of cool air above warm air may occur over a much larger 

 area. Thunderstorms which occur because of this condition are 

 called overrunning-cold-front thunderstorms. 



WARM-FRONT THUNDERSTORMS 



It frequently happens that ahead of the warm sector of a low 

 there exists a region of relatively cool air. The front of the advancing 

 warm mass of air is called a warm-front. Its location is shown in 

 figure 5, a, by the line extending downward and to the right. The 

 scale of the diagram causes the warm-front and the cold-front to 



'' Bjerknes, J., and Solberg, H., Life cycle of cyclones and the polar front theory of atmospheric circulation, 

 Qeofysiske Publikationer, vol. 3, no. 1, 1922. 



'« Physics of the earth— III. Meteorology. Nat. Res. Counc. Bull., no. 79, published by Nat. Acad. Sci., 

 Washington, D. C, 1931. 



