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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