Ch-5, Part-3: Humidity

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HUMIDITY

Humidity is a measure of the amount of Water Vapor in the atmosphere

The types of humidity are formulated to serve different needs from scientific to general public use

  • Absolute humidity
    • Grams (weight) of water vapor per unit volume of air
    • For scientific work
  • Specific humidity
    • Grams (weight) of water vapor per kilogram (weight) of air
    • Used in scientific work
  • Relative humidity
    • Measured as a percent of the maximum amount of water vapor that air could hold at its current temperature
    • The most popular measurement for the general public
  • Water holding capacity of the atmosphere varies with temperature differences  [F 5-11, p 146]  
  • This is measured as Specific Humidity
    • Cold air can hold less water vapor
    • Warm air can hold more water vapor

  • This also means that average moisture capacity of air varies with latitude 

    • High latitudes are cold -- low water holding capacity

    • Low latitudes are warm/hot -- high water holding capacity

Fig 5-11: Maximum Specific Humidity
 

RELATIVE HUMIDITY AND SATURATION

  • Saturation of air with H2O occurs when air can hold no more water vapor (neither liquid nor solid water)

  • Relative humidity is the is the percentage of water vapor in air compared to the maximum it could hold at its current temperature

  • Saturation = 100% relative humidity

  • Hence relative humidity changes when air temperature
    changes [F 5-7, p 144, below]  

    • Warming air gets drier; its relative humidity lowers 

    • Cooling air gets moister; its relative humidity rises

Fig 5-7: Water vapor content, capacity and relative humidity during daytime

 

INSTRUMENTS TO MEASURE RELATIVE HUMIDITY

Fig 5-12b:  Measuring Humidity -- The Sling Psychrometer
  • Wet- and dry-bulb thermometers
  • Swing thermometers in air
  • Evaporation cools wet bulb most
  • The dryer the air, more the cooling
  • Relative humidity result of wet bulb depression and ambient temperature
  • Look up in a pre-calculated table

 

Fig 5-12a:  Measuring Relative Humidity -- The Hair Hygrometer
Hair hygrometer (Above)
  • Hair absorbs or releases moisture as air humidity changes
  • Length of hair changes with dryer or more humid air
  • Changes in hair length transmitted mechanically to pen movement
  • Pen traces graph of relative humidity through the hours of each day
  • Clock mechanism rotates recording drum for two weeks
  • Paper chart does not require person present to observe several times per day
  • Can be placed in remote areas where humans are not always present
  • Paper chart provides a semi-permanent record

 

DIURNAL (DAILY) CYCLE OF RELATIVE HUMIDITY [F 5-9, p 145, below]  

  • Assume that moisture (H20 by weight) in the air does not change for the sake of the "experiment" (example, above)

  • Relative humidity will be lowest when the air is warmest, in the middle to late afternoon

  • Relative humidity will be highest when the air is the coolest, just after dawn

    • Hence dew can form

    • If the temperature is below freezing, frost  may form

Fig 5-9: Typical diurnal cycle of temperature and humidity
 

COMMON MANIFESTATIONS OF THE DEW POINT
   

Fig 5-8: Dew formed on a chilled beverage glass

 

Fig 5-8b:  Cold air at the dewpoint

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