1. Lab 5:
Atmospheric Moisture

2. 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

3. Relative Humidity Swing for ~ 60 seconds Record DB & WB temps
If you have a fraction, change to a whole number
Record DB & WB temps
Individuals or groups
4 locations:
Inside • Outside

4. 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 %!)

5. 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.

6. Heat: average kinetic energy of a given amount of liquid
Condensation = warming process
Evaporation = cooling process

7. 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
Clothes drying; evaporation off a water surface
Ice → water vapor
Solid air freshner; sublimation of snow/ice
Water vapor → water
Released
Dew; condensation on a cold can of pop; contrail
Water vapor → ice
Frost on grass or on a window
Water → ice
Freezing ice cubes

8. 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

9. 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

10. 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

11. 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

13. 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

15. 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!

16. 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.

17. 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)