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