AEROGRAPHER'S MATE 3 & 2 



transfer of heat or other properties along the 

 HOR : ZONTAL, Convection is the term reserved 

 for the VERTICAL transport of heat. Hence- 

 forth in this training manual the words "con- 

 vection" and "advection" are used to mean 

 the vertical and horizontal transfer of atmos- 

 pheric properties, respectively. 



Horizontal transfer of heat is accomplished 

 by motion of the air from one latitude and 

 longitude to another. It is of major importance 

 in the exchange of air between polar and 

 equatorial regions. Much more heat is trans- 

 ported from place to place by the process of 

 advection than by any of the other methods of 

 heat transfer. This is easily understood when 

 you think of the fact that the air is almost 

 always in motion at all levels of the atmosphere. 



Transfer of heat by advection is accom- 

 plished not only by the transport of warm air, 

 but also by the transport of water vapor which 

 releases heat when condensation occurs. 



Specific Heat 



The specific heat of a substance shovvs how 

 many calories of heat it takes to raise the 

 temperature of 1 gram of that substance 1° C. 

 Since it takes 1 calorie to raise the temperature 

 of 1 gram of water 1° C, the specific heat of 

 water is 1. The specific heat of a substance 

 plays a tremendous role in meteorology because 

 it is tied directly to temperature changes. For 

 instance, the specific heat of the earth in 

 general is 0.33. This means it takes only 0.33 

 calorie to raise the temperature of 1 gram of 

 earth 1° C. Stated another way, the earth heats 

 and cools three times as fast as water. 



From this the Aerographer's Mate can see 

 that ocean weather is milder and less extreme 

 in temperature than continental weather because 

 it takes 3 times as long (or 3 times as much 

 heat/cooling) for water to both heat and cool. 



The specific heat of various land surfaces 

 is also different, though the difference between 

 one land surface and another is not as great 

 as between land and water. Dry sand or bare 

 rock has the lowest specific heat. Forest areas 

 have the highest specific heat. This difference 

 in specific heat is another cause for differences 

 in temperature for areas of different types of 

 surfaces which are only a few miles apart. 



The specific heat of ice is 0.421 and that 

 of steam is 0.502. These specific heats are 

 reflected in the thermal history of 1 gram of 



ice as shown in figure 12-8. They also point 

 out the tremendous amount of energy involved 

 in the fusion, sublimation, and vaporization 

 processes in the atmosphere. 



Energy cannot be created or destroyed; 

 however, it can be transformed from one type 

 of energy into another. Even the amount that 

 seems to be lost can be accounted for in the 

 form of light, sound, heat, and the like. 



PRESSURE-TEMPERATURE-DENSITY 

 RELATIONSHIP 



The conditions under which gases must be 

 compared, densities determined, and gas con- 

 stants derived are known as the standard 

 conditions for gases. The standard conditions 

 are a pressure of 760 millimeters of mercury 

 (1,013.25 mb) and a temperature of 0° C, 

 sometimes referred to as STP (standard tem- 

 perature and pressure). 



KINETIC THEORY OF GASES 



The kinetic (motion) theory of gases is very 

 helpful in understanding the behavior of gases. 

 Gases, like some other substances, consist of 

 molecules which have no inherent tendency to 

 stay in one place as do the molecules of a solid. 

 Instead, the molecules of gas, since they are 

 smaller than the space between them, move 

 about at random (but in straight lines until 

 they collide with each other or with other 

 obstructions). When gas is enclosed, its pressure 

 depends on the number of times the molecules 

 strike the surrounding walls. The number of 

 blows which the molecules strike per second 

 against the walls remains constant as long as 

 the temperature and the volume remain constant. 



If the volume (the space occupied by the 

 gas) is decreased, the number of blows against 

 the wall is increased, thereby increasing the 

 pressure, the temperature remaining constant. 

 Temperature is a measure of the molecular 

 activity of the gas molecules and a measure 

 of the internal energy of a gas. When the 

 temperature is increased, there is a corre- 

 sponding increase in the speed of the molecules; 

 they strike the walls at a faster rate, thereby 

 increasing the pressure, provided the volume 

 remains constant. 



Therefore, there is a close relationship of 

 volume, pressure, and density of gases. 



268 



