Relative Humidity

					                   Relative Humidity
   Is how much moisture a given mass of air is
  holding compared to how much water that air
  is capable of holding expressed as a percent.




What % full?

What would
happen to that %
if the same
water were in a
smaller glass?
     How to measure humidity -
                              Sling Psychrometer
• Remember - evaporation
  cools things (sweat,
  misters in outdoor cafes,
  etc.)
• Evaporation goes faster
  in dry air.
• The drier the air - the
  greater the difference
  between wet and dry-
  bulb temperatures.
       Hair Hygrometer -
• Hair changes length depending on
  humidity - “bad hair days”!
    Cold is air’s kryptonite



• Warm air can hold more moisture.
• Cold air holds less moisture.
• So, cooling air reduces the air’s ability to
  hold moisture, and if that air was holding a
  lot of moisture, it may drop moisture
  (condensation).
• What happens to RH if you warm the air?
               Dew point -
• The temperature that you have to cool air to in
  order to make the air drop its moisture is the dew
  point.
• RH = 100% at DP.
• Is dew point high or low on a dry day?
 If you cool air to its dew point,
     condensation happens.
• If you breathe on cold glass, moisture from your
  breath ____________ on the glass.
• If you cool air up in the sky to its dew point,
  water vapor condenses into ___________.
• If you cool air near ground level to its dew point,
  ______ forms.
• On cool mornings, _____ forms on cool grass.
• If the dew point is below freezing (32f or 0c)
  and the air gets that cold , _______ would form
  on grass instead of dew.
Why do clouds have flat bottoms?

				
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posted:8/19/2012
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