Lab 5:
Atmospheric Moisture
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
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
Measuring Relative Humidity
Dry Bulb
Temp
Saturation
Mixing
Ratio (SMR)
Based on
DB temp;
TABLE 2
Wet Bulb
Temp
Wet-Bulb
Depression
Relative
Humidity
(RH)
Mixing
Ratio (MR)
DB – WB
Based on
DB temp &
WBD;
TABLE 4
SMR * RH
(RH is a %!)
Atmospheric Moisture: Latent Heat
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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
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
• 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 faster…it’s more likely to
evaporate
• Energy NOT used to boil the water is used to change
the state of the water from liquid to gas
• Two ways to measure atmospheric moisture:
1. Vapor pressure –
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Water vapor molecules exert pressure proportional to
their concentration in the atmosphere
Maximum is called saturation vapor pressure
• SVP increases with temperature
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
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
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
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!
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.
Dew Point
1. Find MR of 17
2. Create a STRAIGHT line to
the dew point line
3. Create another STRAIGHT
line down to the
temperature
4. Read the temperature.
A 30° air sample would have
to cool to ~ 22° to reach
saturation (100% humidity)