sources of moisture probably exist--the summer-circulation pattern that pulls moisture 

 northward from the Gulf of Mexico and the weak cold fronts that move inland from the 

 Pacific Ocean. These weak fronts are often of the underrunning type and trigger frontal 

 showers and thunderstorms. The arrival of these cold fronts is independent of time. 

 Therefore, the curve for the Davis County area does not peak markedly in the early 

 afternoon. However, thermal convection is a common occurrence here, and this mechanism 

 increases the occurrence of afternoon and evening thunderstorms. Weak summer cold fronts 

 often pass to the north of the Great Basin area. 



Storm Occurrence by Storm Duration 



Unlike the storm-occurrence-by-hour data, these data show only minor differences 

 between study areas, but rather marked differences between elevations. Figures 7 and 8 

 are mass curves of accumulated percent of total storms by storm duration for Zones 1 

 and 4 of the Davis County area, respectively. The curves for each zone of the Davis 

 County area are typical of those for the Great Basin area. Figures 7 and 8 are the two 

 extremes of the Davis County area. 



As elevation increases from 4,350 feet in Zone 1 to 8,760 feet in Zone 4, the storms 

 become notably shorter. Thus, in Zone 1, 25 percent of the storms last less than 70 

 minutes, 50 percent less than 135 minutes, and 75 percent less than 255 minutes. Thir- 

 teen percent of the storms exceed 6 hours in length. In Zone 4, 25 percent last less 

 than 55 minutes, 50 percent less than 100 minutes, and 75 percent less than 155 minutes. 

 Only 4 percent exceed 6 hours in length. 



O 60 120 180 240 300 360 



DURATION, MINUTES 



Figure 7. — Mass curve of aooumulated percent of total storms by storm duration for 

 ■precipitation Zone 1 of the Davis County Experimental Watershed. 



18 



