Atmospheric Moisture (Text Pg 84-92) Learning Outcomes Describe and apply different measures of water vapor in the air ES 304

Figure 4.2 and Figure 4.4 ES 304

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?) ES 304

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 ES 304

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 + 210 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 ES 304

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 ES 304

Saturation (Maximum Limit) Vapor Pressure Increases with Temp: Figure 4.5

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) ES 304

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  ES 304

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 ES 304

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 ES 304

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% ES 304

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 ES 304

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) ES 304

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 ES 304