1. Atmospheric Humidity

2. Global water cycle: precipitation = evaporation

3. Phase changes of water (latent heat transfer)

4. Humidity Amount of water vapor in atmosphere Expressions of humidity:
Specific humidity
Relative humidity
Dew Point

5. Specific humidity 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

6. Line is maximum (capacity) specific humidity

7. Relative Humidity (RH) :
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%)

8. 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

9. 4 am : 10 am : 3 pm : 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%

11. Calculating RH from specific humidity:
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%

12. sling psychrometer Psychrometric tables dry bulb and wet bulb
wet bulb depression =
dry bulb temp - wet bulb temp

14. 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 = ?

16. Dew Point 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

18. Dew formation: condensation in place
Frost formation: deposition in place

20. 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