Atmospheric HumidityAtmospheric Humidity

Global water cycle:Global water cycle: precipitation = evaporationprecipitation = evaporation

Phase changes of water Phase changes of water (latent heat transfer)(latent heat transfer)

HumidityHumidity

Amount of water vapor in atmosphereAmount of water vapor in atmosphere

Expressions of humidity:Expressions of humidity:Specific humiditySpecific humidity

Relative humidityRelative humidity

Dew PointDew Point

Specific humiditySpecific humidity

mass of water vapor per mass of air (g/kg)mass of water vapor per mass of air (g/kg) higher temperature, higher maximum specific higher temperature, higher maximum specific

humidityhumidity

Warm air can “hold more water” than cold airWarm air can “hold more water” than cold air

Latitudinal patternsLatitudinal patterns

Line is maximum (capacity) specific Line is maximum (capacity) specific humidityhumidity

Relative Humidity (RH) :Relative Humidity (RH) :

actualactual amount of moisture in air RELATIVE to the amount of moisture in air RELATIVE to the maximum maximum capacitycapacity amount that the air can hold amount that the air can hold

RHRH (%)(%) = = actual amount moistureactual amount moisture

max. capacity max. capacity x 100x 100

(“saturated air” means that RH = 100%)(“saturated air” means that RH = 100%)

relative humidity varies with temperaturerelative humidity varies with temperature

Warm air has a higher capacity than cold air Warm air has a higher capacity than cold air to hold moistureto hold moisture

During day, relative humidity and temp. have During day, relative humidity and temp. have inverse relationshipinverse relationship

4 am4 am : : T = T = 55ooCC actual = 7g/kgactual = 7g/kg capacity = 7 g/kgcapacity = 7 g/kg RH = 7 / 7 x 100 = RH = 7 / 7 x 100 = 100%100%

10 am10 am : : T = T = 1616ooCC actual= 7 g/kgactual= 7 g/kg capacity = 14 g/kgcapacity = 14 g/kg 7 / 14 x 100 = 7 / 14 x 100 = 50%50%

3 pm3 pm : : T = T = 3232ooCC actual = 7 g/kgactual = 7 g/kg capacity = 35 g/kgcapacity = 35 g/kg 7 / 35 x 100 = 7 / 35 x 100 = 20%20%

Calculating RH from specific humidity:Calculating RH from specific humidity:

a) If temperature = 20 a) If temperature = 20 °°C andC and

actual sp. hum. is 7.5 g/kg, what actual sp. hum. is 7.5 g/kg, what is RH?is RH?

SolutionSolution: : 1. Use graph to find capacity 1. Use graph to find capacity

amount.amount.

2. RH = (7.5 / 15) x 100 = 2. RH = (7.5 / 15) x 100 = 50%50%

b) If temperature is 25 b) If temperature is 25 °°C and C and actual sp. hum. is 5 g/kg, what actual sp. hum. is 5 g/kg, what is RH?is RH?

RH = (5/20) x 100 = 25%RH = (5/20) x 100 = 25%

sling psychrometersling psychrometer

Psychrometric tablesPsychrometric tables

dry bulb and wet bulbdry bulb and wet bulb

wet bulb depressionwet bulb depression = = dry bulb temp - wet bulb tempdry bulb temp - wet bulb temp

If air temperature is 35If air temperature is 35°C and wet bulb °C and wet bulb temperature is 31°C, what is relative temperature is 31°C, what is relative humidity?humidity?

Dry bulb temperature is Dry bulb temperature is 3535 Wet bulb depression = 35-31= Wet bulb depression = 35-31= 44 RH = ?RH = ?

Dew Point Dew Point

temperature at which air becomes saturated temperature at which air becomes saturated during cooling.during cooling.

When air temperature equals dew point, air is When air temperature equals dew point, air is saturated andsaturated and CondensationCondensation (or (or depositiondeposition) will occur) will occur

Examples: glass of ice water on a hot day; window in shower Examples: glass of ice water on a hot day; window in shower roomroom

DewDew formation: formation: condensationcondensation in place in place

FrostFrost formation: formation: depositiondeposition in place in place

RisingRising parcel will parcel will coolcool due to due to expansionexpansion DescendingDescending parcel will parcel will warmwarm due to due to compressioncompression

RATERATE depends on whether parcel is dry or wet (saturated) depends on whether parcel is dry or wet (saturated)* differs from environmental temperature lapse rate* differs from environmental temperature lapse rate