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## More on Humidity

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### Sling Psychrometer. A sling psychrometer consists of a dry bulb ... When the psychrometer is whirled around, some of the water evaporates from the wick. ... – PowerPoint PPT presentation

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Title: More on Humidity

1
More on Humidity
• Other Humidity Variables and Measurement of
Humidity

2
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.

3
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

4
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

5
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)

6
Specific Humidity (Cont.)
• The equation normally used to compute the
specific humidity is
• qv r / (r1)

7
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.

8
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

9
Ideal Gas Law for Moist Air
• The ideal gas law for moist air can be expressed
as
• p ? RdT1 (r/e)
• 1 r

10
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.

11
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

12
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.

13
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.

14
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.

15
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.

16
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.

17
Sling Psychrometer (Cont.)
• The wet bulb depression is the dry bulb
temperature minus the wet bulb temperature.

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22
Example
• Suppose we measured
• T 25.0C
• Tw 20.0C
• Then
• T Tw 25.0C 20.0C 5.0C

23
Td 18C
24
RH 63
25
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.

26
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.

27
Chilled Mirror Device
Light source (LED)
Photo Resistors
Chilled Mirror
28
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.