AEROGRAPHER'S MATE 3 & 2 



equal numerically to specific humidity, but it 

 is always slightly greater. The mixing ratio 

 has the same characteristic properties as the 

 specific humidity, in that it is conservative for 

 atmospheric processes involving a change in 

 temperature but is nonconservative for changes 

 involving a gain or loss of water vapor. 



Previously it was learned that air at any 

 given temperature can hold only a certain 

 amount of water vapor before it is saturated. 

 The total amount of vapor which air can hold 

 at any given temperature, by weight relation- 

 ship, is referred to as the saturation mixing 

 ratio. It is useful to note that the following 

 relationship exists between mixing ratio, 

 saturation mixing ratio, and relative humidity: 

 Relative humidity is equal to the mixing ratio 

 divided by the saturation mixing ratio, multi- 

 plied by 100. If any two of the three components 

 in this relationship are known, the third may 

 be determined by simple mathematics. 



Dew Point 



The dewpoint is the temperature to which 

 air must be cooled, at constant pressure and 

 constant water vapor content, in order for 

 saturation to occur. The dewpoint is a con- 

 servative and very useful element. When 

 atmospheric pressure stays constant, the dew- 

 point reflects increases and decreases in 

 moisture in the air, and also shows at a glance, 

 under the same conditions, how much cooling 

 of the air is required to condense moisture 

 from the air. 



clouds, and fogs, while the vapor state of water 

 is in the form of an unseen gas in the air. 

 (See fig. 12-8.) 



Energy is involved in the various changes 

 of state which occur in the atmosphere. This 

 energy is primarily in the form of heat. The 

 heat which is used by the substance in changing 

 its state is referred to as the latent heat and 

 is usually stated in calories. The calorie is 

 a unit of heat energy. It is the amount of heat 

 required to raise the temperature of 1 gram 

 of water 1° C. A closer look at some of the 

 major changes of state of the atmosphere helps 

 to clarify latent heat. 



LIQUID TO SOLID 

 AND VICE VERSA 



Fusion is the change of state from a solid 

 to a liquid at the same temperature. The number 

 of gram calories of heat necessary to change 

 1 gram of a substance from the solid to the 

 liquid state is known as the heat of fusion. 

 To change 1 gram of ice to 1 gram of water 

 at a constant temperature and pressure requires 

 roughly 80 calories of heat; this is called the 

 latent heat of fusion. The opposite of fusion 

 is freezing — a liquid changes into a solid. Since 

 it requires 80 calories to change 1 gram of ice 

 to 1 gram of water, this same amount of heat 

 is released when 1 gram of water is changed 

 to ice. 



LIQUID TO GAS 

 AND VICE VERSA 



CHANGE OF STATE 



A change of state (or change of phase) of 

 a substance describes the change of a sub- 

 stance from a solid to a liquid, liquid to a 

 vapor, vapor to a liquid, liquid to a solid, solid 

 to vapor, or vapor to a solid. In meteorology 

 we are concerned primarily with the change of 

 state of water in the air. Water is present in 

 the atmosphere in any or all of the three states 

 (solid, liquid, and vapor) and changes back 

 and forth from one state to another. The 

 mere presence of water is important, but the 

 change of state of the water in the air is more 

 important because the change of state of water 

 affects the weather directly. The solid state 

 of water is in the form of ice crystals; the 

 liquid state of water is in the form of raindrops, 



Water undergoes the process of evaporation 

 when changing from the liquid to gaseous state. 

 According to the molecular theory of matter, 

 all matter consists of molecules in motion. 

 The molecules in a bottled liquid are restricted 

 in their motion by the walls of the container. 

 However, on a free surface exposed to the 

 atmosphere, the motion of the molecules in the 

 liquid is restricted by the weight of the atmos- 

 phere or, more precisely, by the atmospheric 

 pressure. If the speed of the liquid molecules 

 is sufficiently high, they escape from the 

 surface of the liquid into the atmosphere. As 

 the temperature of the liquid is increased, the 

 speed of the molecules is increased, and the 

 rate at which the molecules escape from the 

 surface also increases. Evaporation takes place 

 only from the free surface of a substance. 



272 



