1. Atmospheric Moisture (Text Pg 84-92)
Learning Outcomes Describe and apply different measures of water vapor in the air
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2. Figure 4.2 and Figure 4.4
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3. Moisture in the Atmosphere
three phases (gas, liquid, solid)
gas phase: amount of water vapor in the air varies
evaporation and condensation occurring simultaneously
rate of evap > condensation: net evap
rate of condensation > evap: net condensation
rate of evap = condensation: equilibrium (Saturation)
cooling promotes condensation (why?)
heating promotes evaporation (why?)
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4. water vapour in air varies: there is an upper limit
Moisture in Air
water vapour in air varies: there is an upper limit
Saturation is the upper limit (Equilibrium where evaporation = condensation)
mainly temperature dependent (higher temp, the upper limit or saturation point is higher)
saturation conditions
40 ºC, 50 g/kg of air
20 ºC, 15 g/kg of air:
0 ºC, 4 g/kg of air:
-25 ºC, 0.5g/kg of air
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5. Vapour Pressure: Indicates amount of water vapour in the air
Air Pressure, P, at sea level about 1013 mb (1013 hPa)
P = sum of partial pressures of individual gases
PN2 + PO2 + PH2O +……
= 780 mb mb + 10 mb* +……
= 1013 mb
Vapour Pressure (PH2O) *: partial pressure of water vapour
a measure of:
number of water vapour molecules in the air (i.e. how much water vapour is in the air)
* PH2O can vary: <1% to 4% by volume
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6. Saturation Vapor Pressure
Saturation: the maximum possible amount of vapour in the air (upper limit) at the existing T and P
Saturation Vapour Pressure (the Vapour Pressure at Saturation): the pressure (mb) water vapour molecules exert if the air were saturated at a given T and P
depends mainly on temperature:
saturation vapour pressure is higher at higher T
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7. Saturation (Maximum Limit) Vapor Pressure Increases with Temp: Figure 4.5

8. Terms: Relative Humidity (RH):
RH = actual water vapour content_____
max possible water vapour at a given T
RH = water vapour pressure x 100
saturation vapour pressure
RH: a relative measure:
Actual amount of water vapour in the air, relative to the maximum possible amount (upper limit) for a given temp.
RH does not indicate the amount of water vapour in air.
RH does indicate how close the air is to saturation
RH = 50% (air contains ½ of water vapour required to saturate it)
RH = 100% (saturated)
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9. RH Can Change: How? RH = actual water vapour pressure) x 100
saturation vapour pressure
change water vapour content, example: if temp constant
water vapour added then RH,
if removed then RH 
change the temp (see Figure 4.5) changes the saturation vapour pressure, example: if no change in actual water vapour content
temp , sat vapour pressure , then RH 
temp , sat vapour pressure , then RH 
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10. Figure 4. 7 RH varies with Air Temp Figure 4
Figure 4.7 RH varies with Air Temp Figure 4.6 below assumes no water vapor added or removed increase in air Temperature Lowers RH Decrease in air temperature raises RH
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11. Terms: Dew Point Temperature
unsaturated air can be saturated by adding water vapour, or
by cooling the air (saturation vapour pressure  , thus RH  to 100%)
Dew Point Temp: the temp to which air would have to be cooled (with no change in air pressure or water vapour content) for saturation to occur
indicates air’s water vapour content: RULE
higher Dew Pt temp, greater amount of water vapour in the air
lower Dew Pt temp, lower amount of water vapour in the air
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12. Which has more water vapor? Figure 4.9a, b
Polar Air
Desert Air
Air temp = -2 C, Dew Pt = -2 C
RH = 100%
Air temp = 35 C, Dew Pt = 10 C
RH = 21%
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13. Other Water Vapour Addition or Removal Processes
Add: in addition to evaporation:
evapo-transpiration,
sublimation
Remove:
condensation:
precipitation,
impaction (cloud/ fog moves over objects, droplets left),
deposition
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14. Summary
Vapour Pressure and Dew Point: indicate water vapour content in air
RH indicates how close the air is to saturation (%)
RH changes when: air’s water vapour content changes, or when the air temperature changes, or both
For a given amount of water vapour, cooling the air raises the RH, warming the air lowers it.
dry air (low Dew Pt, low vapour pressure) can have high relative humidity (example: cold polar air)
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15. Determining RH psychrometer obtain wet bulb and dry bulb temperature
determine wet bulb depression (dry bulb – wet bulb temp)
Appendix D, Table D.1 determine Dew Point temperature
Appendix D, Table D.2 determine Relative Humidity
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