Lab 5:

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Lab 5

• Atmospheric Moisture

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Relative Humidity

• Sling Psychrometers measures Relative Humidity
• Dry bulb temp
• Web bulb temp

DB WB Wet bulb depression
DB vs. WB Big difference dry air Small
difference moist air
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Relative Humidity

• Swing for 60 seconds
• If you have a fraction, change to a whole number
• Record DB WB temps
• Individuals or groups
• 4 locations
• Inside Outside

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Measuring Relative Humidity
Dry Bulb Temp Saturation Mixing Ratio (SMR) Wet Bulb Temp Wet-Bulb Depression Relative Humidity (RH) Mixing Ratio (MR)
Based on DB temp TABLE 2 DB WB Based on DB temp WBD TABLE 4 SMR RH (RH is a !)
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Atmospheric Moisture Latent Heat

• Water comes in three phases
• Solid
• Liquid
• Vapor
• Unique to this atmospheric component
• Latent heat transfer process of water changing
  phase
• Energy is consumed or released
• Sensible heat heat we can feel measure
• Latent heat energy in the form of heat
• Sensible heat potential energy in the form of
  thermal energy or heat
• Latent heat amount of energy in the form of heat
  released or absorbed by a chemical substance
  during a change of state.
• Kelvin is the most reliable measurement of
  temperature because zero kelvin represents the
  temperature at which all molecular motion stops.

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• Heat average kinetic energy of a given amount of
  liquid
• Condensation warming process
• Evaporation cooling process

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Atmospheric Moisture Latent Heat
Phase change Energy Examples
Ice ? (liquid) water Consumed Ice cubes melting in a glass melting of falling snowflake
Water ? water vapor Consumed Clothes drying evaporation off a water surface
Ice ? water vapor Consumed Solid air freshner sublimation of snow/ice
Water vapor ? water Released Dew condensation on a cold can of pop contrail
Water vapor ? ice Released Frost on grass or on a window
Water ? ice Released Freezing ice cubes
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• Amount of water vapor in the air depends on
  amount of energy available to change liquid ?
  gas.
• As temperature increases, the liquid water
  molecules start moving fasterits more likely to
  evaporate
• Energy NOT used to boil the water is used to
  change the state of the water from liquid to gas

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• Two ways to measure atmospheric moisture
• Vapor pressure
• Water vapor molecules exert pressure proportional
  to their concentration in the atmosphere
• Maximum is called saturation vapor pressure
• SVP increases with temperature

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• 2. Mixing Ratio
• Mass of water vapor in the air
• Ratio of water vapor mass to the mass of dry air
• Units of grams of water vapor per kilogram of dry
  air
• saturation mixing ratio air is saturated
• Depends on temperature (? with temperature)
• Represents max weight of water vapor/kilogram of
  dry air

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• Relative Humidity measures how close the air
  sample is to saturation
• It is a ratio of actual water vapor in the air to
  the saturation level at a given temperature.
• The greater the difference between air
  temperature the dew point, the lower the RH
  (dry air)
• RH is temperature dependant

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Winter?

• Warming up already dry air will DECREASE Relative
  Humidity!
• Does winter indicate air that would be dry or
  moist? In the winter, air is generally rather
  dry. Cold air has less capacity to hold water
  vapor than warm air.
• Heat in your home becomes dry

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Summer?

• In the summer, air is generally moist. Warm air
  can hold greater amounts of water vapor than cold
  air.
• Cooling already moist air will INCREASE Relative
  Humidity!

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Dew Point

• Dew point temperature to which air must be
  cooled to reach saturation (RH 100)
• Generally seen as dew in the morning on the
  grass.

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Dew Point

• Find MR of 17
• Create a STRAIGHT line to the dew point line
• Create another STRAIGHT line down to the
  temperature
• Read the temperature.

A 30 air sample would have to cool to 22 to
reach saturation (100 humidity)