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Lecture 8

GGR206H1 Lecture Notes - Lecture 8: Vapor Pressure, Sensible Heat, Evapotranspiration


Department
Geography and Planning
Course Code
GGR206H1
Professor
chen
Lecture
8

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Evapotranspiration is when water changes from liquid to gas and is thus released to the
atmosphere as water vapor
Evaporation absorbs energy and condensation releases energy
Transfers mass and energy from surface to atmosphere
Represents loss of both mass and energy from surface
Evaporation is main control on precipitation and hydrologic cycle since it releases moisture
from ocean and land reservoirs
Over land surfaces, approximately two thirds of precipitation is returned to the atmosphere as
evapotranspiration
There is vapor pressure at and above an evaporating surface
Vapor pressure depends on relative humidity and temperature
1.
Is energy required to evaporate the amount of water E
Lambda is latent heat of vaporization, E is amount of water lost through
evaporation
Denoted by 
Latent heat is transferred from evaporating body into the air
Evaporation reduces the surface temperature


at 10oC
Latent heat of vaporization:
Latent heat is energy used or liberated in evaporation or condensation
2.
Is the heat energy we can feel/sense
Creates turbulent transfer as warm air rises and cold air descends
Heating of ground surface heats surrounding air, which then rises and brings sensible heat
to upper layers
Depends on the vertical gradients of wind speed and temperature
Sensible heat
3.
Is the ratio of sensible-heat exchange to latent-heat exchange

Bowen ratio
4.
Physical characteristics of evaporation
Evaporation represents net flux of water molecules from a surface to the atmosphere
More energy increases temperature of water
Energy required to break these bonds for evaporation
Positive/negative charges bind water molecules together
Water is a polar molecule
i.
Molecules align along the surface
ii.
Excess kinetic energy is required to overcome barrier
iii.
Energy is required to break surface tension of water
Controls the number of molecules with sufficient energy to break free of the liquid mass
(latent heat of vaporization) because:
Energy input
1.
Surrounding air is dry and has capacity to hold more water vapor
If >0, then evaporation
If <0, then condensation
Evaporation and condensation rate are equal
If =0, then equilibrium
Measures how dry it is
Vapor pressure gradient is difference between saturation vapor pressure and actual vapor
pressure
Vapor pressure gradient
2.
Evaporation will cease once air above water becomes saturated unless there is wind to
move water molecules away from surface layer
Exchange mechanism (wind)
3.
Evaporation requirements
Lecture 8: Evapotranspiration
October 1, 2019
GGR206 Page 1
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