THE THUNDP:RST0RM— EVANS AND McEACHRON Jgg 



Another important condition must be satisfied before a heat 

 thunderstorm can occur. This is the establishment, in the region 

 where the storm originates, of a dry adiabatic or superadiabatic 

 temperature gradient from the earth up to the cloud level. That is, 

 a condition must be established in which the temperature of the air 

 decreases with increase in height above the earth at the same or a 

 greater rate than unsaturated air, warmed at the surface, cools as 

 it rises and expands adiabatically.^^ This requires time, for nor- 

 mally the rate of decrease of air temperature with height is only 

 about half the required dry adiabatic temperature gradient. Fre- 

 quently a day or two of hot weather seems to be necessary before 

 such a gradient can be estabHshed by progressive convection and 

 mixing of the air. 



When an adiabatic gradient is established, air heated at the sur- 

 face can rise to the condensation level. There its moisture will 

 begin to condense and in doing so will give off its latent heat of 

 condensation. This heat warms the rising column of air. The 

 resulting temperature difference between the rising column of air 

 and the surrounding air increases the upward air movement and 

 helps to produce the violent convection necessary to electrify clouds. 



A heat thunderstorm will thus form if (1) a temperature gradient 

 equal to or greater than the dry adiabatic has been established be- 

 tween the ground and the condensation level, (2) the general hori- 

 zontal air movements are mild enough and other conditions are 

 favorable to the strong local heating of the surface air, and (3) if 

 this air contains sufficient moisture. 



MOUNTAIN THUNDERSTORMS 



Mountain thunderstorms are closely related to heat thunderstorms. 

 The slopes of the mountain are heated by the sun's rays. They 

 reradiate this heat energy at a longer wave length which can more 

 readily be absorbed by the atmosphere than the sun's direct shorter- 

 wave-length radiation. The air near the mountain slopes is thus 

 heated to a temperature above that of the surrounding air. This 

 relatively warm column of air can be compared to the warm air in a 

 huge chimney tipped at a considerable angle from the perpendicular. 

 In a chimney the difference in weight of the column of warm air 

 within the chimney compared with a column of equal area and height 

 outside of the chimney suppUes the necessary pressure to force air up 

 the chimney. Similarly, the greater weight of a column of relatively 

 cool air in the free atmosphere at a moderate distance from a moun- 

 tain, compared with the weight of a column of the warm air near the 

 surface of the mountain, results in a pressure difference forcing the 



•♦ By adiabatic expansion is meant expansion during which no heat is added to or subtracted from the 

 expanding system. This condition is nearly realized when air expands while rising rapidly. 



