More on Humidity

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More on Humidity

• Other Humidity Variables and Measurement of
  Humidity

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Moist Air and Dry Air

• As we defined it previously, dry air refers to
  the normal mixture of gases observed in the
  Earths atmosphere, except for the fact that it
  contains no water vapor.
• Moist air refers to the combination of dry air
  plus water vapor.

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Mixing Ratio

• The mixing ratio (r) is defined as the mass of
  the water vapor divided by the mass of the dry
  air.
• r mv/md
• where
• mv is the mass of water vapor
• md is the mass of dry air

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Mixing Ratio (Cont.)

• The equation normally used to compute the mixing
  ratio is
• r e e/(p e)
• where
• e Mv / Md 0.622
• e is vapor pressure
• p is the total pressure

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Specific Humidity

• The specific humidity (qv) is defined as the mass
  of the water vapor divided by the sum of the mass
  of water vapor plus the mass of dry air.
• qv mv / (mv md)

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Specific Humidity (Cont.)

• The equation normally used to compute the
  specific humidity is
• qv r / (r1)

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Virtual Temperature

• The virtual temperature (Tv) is the temperature
  that would occur in a sample of dry air if it had
  the same pressure and specific volume as a sample
  of moist air.
• The virtual temperature is an adjusted
  temperature necessary to use Rd in the ideal gas
  law when dealing with moist air.

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Virtual Temperature (Cont.)

• Tv T 1 (r/e)
• 1 r
• where
• Tv is the virtual temperature in Kelvins
• T is the temperature of moist air in Kelvins
• r is the mixing ratio
• e 0.622

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Ideal Gas Law for Moist Air

• The ideal gas law for moist air can be expressed
  as
• p ? RdT1 (r/e)
• 1 r

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Dew Point Temperature

• The dew point temperature (Td) is defined as the
  temperature at which saturation would occur if
  the air was cooled at a constant pressure.

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Dew Point Temperature (Cont.)

• Td (1/273.16K)-(Rv/Lv)ln(e/610.7 Pa)-1
• where
• Td is the dew point temperature in Kelvins
• e is the vapor pressure in Pascals
• Rv is the gas constant for water vapor
• Lv is the latent heat of vaporization

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Wet Bulb Temperature

• The wet bulb temperature (Tw) is defined as the
  temperature at which saturation occurs when water
  is evaporated into the air until it becomes
  saturated.

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Measurement of Humidity

• Different types of instrument measure different
  humidity variables.
• The most commonly measure variables are wet bulb
  temperature, dew point temperature, and relative
  humidity.
• Once one of the humidity variables has been
  measured, then the other variables can be
  computed from the equations.

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Sling Psychrometer

• A sling psychrometer consists of a dry bulb
  thermometer, a wet bulb thermometer and a handle
  for whirling the thermometers around in a circle.

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Sling Psychrometer (Cont.)

• The cotton wick on the wet bulb thermometer is
  moistened (not soaked) with distilled water.
• When the psychrometer is whirled around, some of
  the water evaporates from the wick.
• The water absorbs energy when it evaporates and
  the wet bulb temperature decreases.

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Sling Psychrometer (Cont.)

• The meteorologist whirls the psychrometer around
  for 30-60 seconds.
• The wet bulb temperature and the dry bulb
  temperature are recorded.
• The wet bulb and dry bulb temperatures can be
  used to determine the dew point temperature and
  relative humidity from tables.

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Sling Psychrometer (Cont.)

• The wet bulb depression is the dry bulb
  temperature minus the wet bulb temperature.

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Example

• Suppose we measured
• T 25.0C
• Tw 20.0C
• Then
• T Tw 25.0C 20.0C 5.0C

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Td 18C
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RH 63
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ASOS Humidity Measurement

• The Automated Surface Observing System (ASOS) is
  the automated set of instruments installed at
  National Weather Service Offices and many other
  official observing sites in the U.S.

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Hygrothermometer (cont.)

• Dew point temperature is measured with a chilled
  mirror device. When a beam of light is
  transmitted onto the mirror and a sensor measures
  the reflected light. The mirror is cooled and
  droplets or frost forms at the dew point
  temperature. The sensor measures a change in the
  reflected light and the dew point temperature is
  recorded.

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Chilled Mirror Device
Light source (LED)
Photo Resistors
Chilled Mirror
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Chilled Mirror Device (Cont.)
Light source (LED)
Photo Resistors
The droplets of water reflect a different amount
of light and the photo sensor detects this
change.
When the temperature of the mirror reaches the
dew point temperature drops of water (dew) start
to condense on the mirror.
Chilled Mirror
The temperature of the mirror at this instant is
recorded as the dew point temperature.