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
• 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
Relative Humidity
Measuring Relative Humidity
Dry Bulb Temp
Saturation Mixing
Ratio (SMR)
Wet Bulb Temp
Wet-Bulb Depressio
n
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 %!)
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.
• Heat: average kinetic energy of a given amount of liquid
• Condensation = warming process
• Evaporation = cooling process
Atmospheric Moisture: Latent Heat
Phase changePhase change EnergyEnergy ExamplesExamples
Ice Ice → (liquid) water→ (liquid) water ConsumedConsumed Ice cubes melting in a glass; Ice cubes melting in a glass; melting of falling snowflakemelting of falling snowflake
Water Water → water vapor→ water vapor ConsumedConsumed Clothes drying; evaporation Clothes drying; evaporation off a water surfaceoff a water surface
Ice Ice → water vapor→ water vapor ConsumedConsumed Solid air freshner; Solid air freshner; sublimation of snow/icesublimation of snow/ice
Water vapor Water vapor → water→ water ReleasedReleased Dew; condensation on a cold Dew; condensation on a cold can of pop; contrailcan of pop; contrail
Water vapor Water vapor → ice→ ice ReleasedReleased Frost on grass or on a Frost on grass or on a windowwindow
Water Water → ice→ ice ReleasedReleased Freezing ice cubesFreezing 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 –
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.
1. Find MR of 172. Create a STRAIGHT line to
the dew point line3. 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)
Dew Point