Chapter 12 — THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



TOTHL C»LO«ICS 

 TAKEN FKOM 



THE »m 



IOT»L C*(.0»IES 

 »00E0 TO 



I. EVAPORATION COOLS AIR. 

 2. CONDENSATION HEATS. 

 3. CALORIES SHOWN TO NEAREST 

 WHOLE FIGURES 



Figure 12-8. — Thermal history of 1 gram of ice. 



209.12 



During the process of evaporation, heat is 

 absorbed by the water being vaporized. The 

 amount absorbed is approximately 539 calories 

 per gram of water if the water is at a tempera- 

 ture of 100° Co On the other hand, the amount 

 is 597.3 calories, if the evaporation takes place 

 at a water temperature of 0° C. This energy is 

 required to keep the molecules in the vapor 

 state and is called the latent heat of vaporization. 

 Since the water needs to absorb heat in order 

 to vaporize, heat must be supplied or else 

 evaporation cannot take place. The air provides 

 this heat. For this reason, evaporation is said 

 to be a cooling process, because by supplying 

 the heat for vaporization, the temperature of 

 the air is lowered. 



Basically condensation is the opposite of 

 evaporation, in that water vapor undergoes a 

 change in state from gas to liquid. However, 

 a condition of saturation must be fulfilled before 

 condensation can occur; that is, the air must 

 contain all the water vapor it can hold (100 

 percent relative humidity) before any of it can 

 condense from the atmosphere. 



In the process of condensation, the heat that 

 was absorbed in evaporation is released from 



the water vapor into the air and is called the 

 latent heat of condensation. 



SOLID TO GAS 

 AND VICE VERSA 



Sublimation is the change of state from a 

 solid to a vapor or vice versa at the same 

 temperature. In physics and chemistry, subli- 

 mation is usually regarded only as the change 

 of state from solid to vapor, but meteorologists 

 do not make this distinction. The heat of 

 sublimation equals the heat of fusion plus the 

 heat of vaporization for a substance. The 

 calories required for water to sublime are: 

 80 + 597.3 = 677.3, if the vapor has a tem- 

 perature of 0°C. 



The process of vapor passing directly into 

 the solid form without going through the liquid 

 phase is called crystallization. This process, as 

 well as its reverse, is often called sublimation 

 in meteorology. 



The calories liberated by crystallization are 

 the same as those for sublimation. Crystalli- 

 zation frequently takes place in the atmosphere 



273 



