HUMIDITY AND DEW POINT

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HUMIDITY AND DEW POINT
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Humidity

• amount of water vapor in the air

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Specific Humidity

• ACTUAL amount of water vapor in the air at a
  given time and place
• Units - g of water vapor / Kg of air

• Saturated Air
• At a steady temperature the amount of water vapor
  is at its peak (maximum)
• Rate of condensation Rate of evaporation
• Dependent on air temperature
• Higher the air temp. - greater the amount of
  water vapor in the air
• Water vapor in air doubles every 10C

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Dew Point

• The temperature at which air is cooled to become
  saturated and condensation begins
• More water vapor in the air
• Less the air has to cool
• Higher the Dew Point

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Relative Humidity

• Comparison of the specific humidity and the
  maximum amount of water vapor that the air can
  hold at a given temperature and pressure
• UNIT -

• Relative humidity specific humidity X 100
• maximum capacity
• 100 Rel. Hum. saturated air
• 0 Rel. Hum. dry air

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Psychrometer

• Instrument to measure humidity
• Dry bulb measures air temp.
• Wet bulb has a water soak wick around the bulb
  water evaporates cools the air

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Evaporation is a cooling process
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Psychrometer

• Dry air - greater evaporation
• - greater cooling
• - greater the difference in the 2
  temperatures
• High humidity no evaporation (saturated air)
• - temperatures of the 2 bulbs will be
  the same or very close

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Evaporation question

• Why do you feel so hot and sticky on hot, humid
  days in the summertime?

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Answer

• High humidity means air has a lot of water vapor
  in it. Your body sweats but the water has no
  where to go so it stays on your body and you feel
  sticky and arent cooled by the evaporation of
  the sweat.

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