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Temperature, Dew Point, and Relative Humidity - 14220_322
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Temperature, Dew Point, and Relative Humidity, Continued - 14220_324
Temperature, Dew Point, and Relative Humidity, Continued Measuring Dew As  already  mentioned,  the  amount  of  water  vapor  the  atmosphere  can Point and hold  varies  with  the  temperature.  When  the  atmosphere  contains  all  the Relative Humidity water  it  can  hold  for  a  given  temperature,  the  air  is  at  the  saturation point  or  humidity  is  100%.  If  it  contains  50%  of  what  it  could  hold  at that  particular  temperature,  relative  humidity  is  50%.  Relative  humidity and  dew  point  are  determined  through  the  use  of  a  psychrometer. Using the Sling A  psychrometer  is  simply  two  ordinary  thermometers  mounted  together Psychrometer on  a  single  strip  of  material.  The  bulb  of  one  thermometer  is  covered by  a  water-soaked  wick  from  which  the  water  evaporates  rapidly  or slowly,  depending  on  the  amount  of  water  vapor  in  the  surrounding atmosphere. Evaporation  of  water  around  the  wet  thermometer  cools  it.  The  amount of  cooling  depends  on  the  rate  of  evaporation.  The  reading  on  the  wet bulb  is  lower  than  the  reading  on  the  dry  bulb  except  when  the  humidity is  100%,  at  which  time  both  readings  coincide.  The  difference  between the  wet-bulb  and  dry-bulb  readings,  when  applied  to  tables  developed for  that  purpose,  results  in  relative  humidity  and  dew  point  temperature. The  dew  point  is  the  temperature  to  which  air  must  be  cooled  at constant  pressure  and  constant  water  vapor  content  to  reach  saturation (100%  relative  humidity).  When  air  is  cooled  to  its  dew  point temperature,  small  water  droplets  condense  on  objects;  that  is,  dew forms. A  sling  psychrometer  (fig.  10-12)  is  sometimes  used  to  speed  up  the process  of  getting  accurate  wet-bulb  and  dry-bulb  readings.  The  sling psychrometer  can  be  whirled  around  to  rapidly  bring  the  wet  bulb  into contact  with  a  great  volume  of  air.  This  contact  with  air  accelerates  the evaporation  rate.  The  person  using  the  sling  psychrometer  should  face the  wind  and  should  shield  the  instrument  as  much  as  possible  from  the direct  rays  of  the  Sun.  The  whirling  should  be  repeated  until  no  further change  can  be  detected  in  the  wet-bulb  reading. 10-27

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