NATS 101 Lecture 9 Atmospheric Moisture

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NATS 101Lecture 9Atmospheric Moisture

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Hydrological Cycle
Ultimate source of all fresh water
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85
50
50
Ahrens Fig. 4.1
85 of water vapor in atmosphere evaporates from
oceans. About 50 of precipitation that falls
over land is runoff, and the other 50 is
transpired/evaporated. Water vapor molecules
reside in atmosphere for one week.
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Humid Air

• Humid air is a mixture of molecules that make up
  dry air (mostly N2 and O2) and lighter water
  vapor (H2O) molecules.
• Each type of molecule contributes a fraction of
  total air pressure, or a partial pressure,
  proportional to the number molecules per unit
  volume. The partial pressure of water vapor is
  termed the vapor pressure.

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Saturation Vapor Pressure

• The partial vapor pressure at which the rate of
  evaporation equals the rate of condensation in a
  closed system is called the saturation vapor
  pressure or SVP. The SVP effectively denotes the
  maximum water vapor that air the can hold.
• SVP depends strongly on temperature.
• Vapor pressure and SVP provide a measure of the
  actual water vapor content and the airs
  potential capacity, respectively.

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Williams p62
saturatedEC
warmedEgtC
cooled EltC
dry EgtC
SVP depends on temperature. As temperature
increases, more molecules are energetic enough to
escape into the air. Concept applies to an ice
surface. SVP over ice is lower because water
molecules are bonded more tightly to ice. For the
temperatures of interest, some water molecules
are energetic enough to escape into atmosphere
and SVPgt0.
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SVP and Temperature

• SVP nearly doubles with a 10oC warming
• SVP and T Graph
• Supercooled water droplets can exist to temps of
  -40oC
• For temps below 0oC, SVP runs 10-30 lower over
  ice

Ahrens Fig. 4.5
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Relative Humidity

• Air with a RH100 is said to be saturated.
• RH depends on air temperature (SVP).
• RH changes by either changing airs water vapor
  content or the airs temperature.

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Relative Humidity
Ahrens Fig. 4.6
The RH for constant water vapor content can
fluctuate greatly during the course of the day
solely from the temperature changes
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Relative Humidity(Ahrens, Appendix B)

• Consider air that is saturated at 0oC
• Temp VP SVP RH
• 0oC 6 mb 6 mb 100
• 10oC 6 mb 12 mb 50
• 20oC 6 mb 23 mb 28
• 30oC 6 mb 42 mb 14
• 40oC 6 mb 68 mb 9

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Other Measures of Humidity
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Dew Point

• DP-temperature to which air must be cooled at
  constant pressure to become saturated. Higher DP
  ? Higher water vapor content.
• DP is a good indicator of the actual water vapor
  content since air pressures vary very little
  along the earths surface.
• DP is plotted on surface weather maps.
• DP depression (Temp-DP) is plotted aloft.

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Arizona Dew Point Ranges

• Vapor Pressure Dew Point
• 24 mb 20oC
• 12 mb 10oC
• 6 mb 0oC
• 3 mb -10oC

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Wet Bulb Temperature

• Wet Bulb Temp -Lowest temp to which air can be
  cooled by evaporation of water into it.
• Warmer than dew point since moisture is being
  added to air which raises dew point.
• Measured with sling psychrometer.

wet bulb
dry thermometer
Ahrens, Fig 4.9
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Wet Bulb Temperature(Ahrens, Appendix D)

• Wet bulb temperature lies about 30 of the way
  from the dew point to the temperature
• Application-Wet bulb temp gives maximum possible
  efficiency for a swamp cooler
• Month MAX Dew P Wet Bulb June 100oF 37oF
  65oF July 100oF 63oF 75oF

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Heat Index
Humidity reduces the rate at which sweat
evaporates. Thus, the cooling rate is lowered.
Tucson Record MAX
July MAX
June MAX
Rocky Pt Summer
Ahrens, Fig 4.8
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Humid Air is Less Dense
Williams, p72
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0.5 lighter
Williams, p72
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Summary Moisture

• Water vapor comes from the evaporation of sea
  water and resides in atmo. for 1 week.
• Air has a saturation level for water vapor
• Saturation level depends on air temperature
• Humid air is less dense than dry air
• Water vapor content can be quantified by
  RH, dew point temp, wet bulb temp

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Assignment for Next Lecture

• Topic - Cloud and Fog Formation
• Reading - Ahrens pages 89-96
• Problems - 4.3, 4.13, 4.14, 4.15