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ATMOSPHERIC MOISTURE
Relative Humidity

Aerographers Mate, Module 05-Basic Meteorology
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The  quantity  of  water  vapor  needed  to  produce saturation  does  not  depend  on  the  pressure  of  other atmospheric  gases.  At  a  given  temperature,  the  same amount of water vapor saturates a given volume of air. This is true whether it be on the ground at a pressure of 1000 mb or at an altitude of 17,000 ft (5,100 meters) with  only  500  mb  pressure,  if  the  temperature  is  the same.  Since  density  decreases  with  altitude,  a  given volume  of  air  contains  less  mass  (grams)  at  5,100 meters than at the surface. 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. Condensation Condensation occurs if moisture is added to the air after it is saturated, or if cooling of the air reduces the temperature  below  the  saturation  point.  As  shown  in figure 1-11, the most frequent cause of condensation is cooling  of  the  air  from  the  following  results:  (a)  air moves over a colder surface, (b) air is lifted (cooled by expansion), or (c) air near the ground is cooled at night as a result of radiation cooling. Pressure (Dalton’s Law) The English physicist, John Dalton, formulated the laws relative to the pressure of a mixture of gases. One of  the  laws  states  that  the  partial  pressures  of  two  or more  mixed  gases  (or  vapors)  are  the  same  as  if  each filled the space alone. The other law states that the total pressure is the sum of all the partial pressures of gases and vapors present in an enclosure. 1-19 SATURATED CONDENSING HEAT COOL 60  F o 80  F o 60  F o AGf0110 Figure 1-10.—Saturation of air depends on its temperature. WARM COLDER COLDER AIR MOVES IN OVER COLDER SURFACE. COOLED BY EXPANSION. RADIATION COOLING LIFTING A B C AGf0111 Figure 1-11.—Causes of condensation.







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