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Atmospheric Humidity

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Atmospheric Humidity

- Amount of water vapor in atmosphere
- Expressions of humidity:
- Specific humidity
- Relative humidity
- Dew Point

- mass of water vapor per mass of air (g/kg)
- higher temperature, higher maximum specific humidity
- Warm air can “hold more water” than cold air
- Latitudinal patterns

- higher temperature, higher maximum specific humidity

- actual amount of moisture in air RELATIVE to the maximum capacity amount that the air can hold
- RH(%) = actual amount moisture
max. capacity x 100

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

- relative humidity varies with temperature
- Warm air has a higher capacity than cold air to hold moisture
- During day, relative humidity and temp. have inverse relationship

- 4 am :
- T = 5oC
- actual = 7g/kg
- capacity = 7 g/kg
- RH = 7 / 7 x 100 = 100%

- 10 am :
- T = 16oC
- actual= 7 g/kg
- capacity = 14 g/kg
- 7 / 14 x 100 = 50%

- 3 pm :
- T = 32oC
- actual = 7 g/kg
- capacity = 35 g/kg
- 7 / 35 x 100 = 20%

a) If temperature = 20 °C and

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

Solution:

1. Use graph to find capacity amount.

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

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

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

- Psychrometric tables
- dry bulb and wet bulb
- wet bulb depression =
dry bulb temp - wet bulb temp

- If air temperature is 35°C and wet bulb temperature is 31°C, what is relative humidity?
- Dry bulb temperature is 35
- Wet bulb depression = 35-31= 4
- RH = ?

- temperature at which air becomes saturated during cooling.
- When air temperature equals dew point, air is saturated and
- Condensation (or deposition) will occur
- Examples: glass of ice water on a hot day; window in shower room

- Condensation (or deposition) will occur

- Dew formation: condensation in place
- Frost formation: deposition in place

Rising parcel will cool due to expansion

Descending parcel will warm due to compression

RATE depends on whether parcel is dry or wet (saturated)

* differs from environmental temperature lapse rate