AEROGRAPHER'S MATE 3 & 2 



following formula is an expression of the 

 equation: 



P = pRT 



P — pressure in millibars 



p — density 



R — gas constant 



T — temperature (absolute) 



From this relationship, we can draw the 

 following conclusions: 



1. A change in pressure, density (mass or 

 volume), or temperature requires a change in 

 one or both of the others. 



2. An increase in atmospheric pressure 

 results in an increase in atmospheric density. 

 Conversely, a decrease in pressure results 

 in a decrease of density. (Temperature remaining 

 constant.) 



3. With an increase in temperature, pressure 

 and/or density must change. In the free atmos- 

 phere, the temperature-increase frequently 

 results in expansion of the air to such an extent 

 that the decrease in density outweighs the 

 temperature increase, and the pressure actually 

 decreases. Likewise, the temperature increase 

 allows an increase in moisture, which in turn 

 decreases density (mass of moist air is less 

 than that of dry air); couple this with expansion, 

 and almost invariably, the final result is a 

 decrease in pressure. 



HUMIDITY 



As indicated earlier, weather conditions 

 depend greatly upon the amount of water in 

 the air. The water may be in any of three 

 forms — gas, liquid, or solid. As a gas, it is 

 called water vapor, which is invisible. Solid 

 or liquid water is visible as precipitation or 

 as clouds. 



Humidity is a comprehensive concept; there- 

 fore, there are available many different 

 definitions and many different manners of 

 expressing humidity. 



Most of the weather that interferes with the 

 operation of aircraft is directly associated with 

 water in some form. 



WATER VAPOR CHARACTERISTICS 



Water vapor is a universal constituent of the 

 atmosphere. Any given volume of atmosphere 

 at a given temperature can contain only a 

 certain maximum quantity of water vapor. The 

 maximum amount (by volume) of water vapor 

 that the air can hold is about 4 percent. If 

 more and more water vapor is injected into a 

 given container of dry air kept at a constant 

 temperature, a point is reached when the water 

 vapor condenses, or becomes liquid, as fog 

 within the container or as dew on its walls. 

 As more and more water vapor is added, more 

 of it condenses; but the total amount of vapor 

 in the container remains unchanged, though the 

 amount of liquid water in the form of fog or 

 dew increases. The volume of air in the con- 

 tainer is then said to be saturated with water 

 vapor. 



The quantity of water vapor needed to pro- 

 duce saturation does not depend on the pressure 

 of other atmospheric gases; consequently, at a 

 given temperature, the same amount of water 

 vapor will saturate a given volume of air 

 whether it be on the ground at a pressure of 

 1000 mb or at 17,000-ft altitude with only 500 

 mb pressure, if the temperature is the same. 

 Since density decreases with altitude, a given 

 volume of air would contain less mass (grams) 

 at 17,000 ft than at the surface; therefore, in 

 a saturated volume, there would be more water 

 vapor per gram of air at this altitude than at 

 the surface. 



Temperature 



Although the quantity of water vapor in a 

 saturated volume of atmosphere is independent 

 of the air pressure, IT DOES DEPEND ON THE 

 TEMPERATURE, The higher the temperature, 

 the greater the tendency for liquid water to turn 

 into vapor. At a higher temperature, therefore, 

 more vapor must be injected into a given volume 

 before the saturated state is reached and dew 

 or fog forms. On the other hand, cooling a 

 saturated volume of air forces some of the 

 vapor to condense and the quantity of vapor 

 in the volume to diminish. 



Pressure (Dalton's Law) 



The laws relative to the pressure of a mixture 

 of gases were formulated by the English 



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