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Humidity

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Water molecules escape from liquid surface to surrounding air. Requires and uptake of energy ... Measurements of Humidity. Specific Humidity ... – PowerPoint PPT presentation

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Title: Humidity


1
Humidity
  • Humidity
  • Controls of Evaporation
  • Vapor Pressure Water Vapor Capacity
  • Measurements of Humidity
  • Precipitable Water Vapor
  • Condensation
  • Chapters 4.1 4.2

2
Humidity
____________________________
  • Definition
  • The water vapor content of the air

3
Evaporation
____________________________
  • Conversion of water to water vapor
  • Water molecules escape from liquid surface to
    surrounding air
  • Requires and uptake of energy
  • A cooling process

4
Evaporation
____________________________
Lutgens and Tarbuck The Atmosphere
5
Controls of Evaporation
____________________________
  • Four Controls of Evaporation

6
Controls of Evaporation (1)
____________________________
  • Availability of Energy
  • Evaporation requires the uptake of energy
  • More net radiation (Q) in low latitudes than in
    high latitudes
  • Warm air and water can have higher rates of
    evaporation

7
Controls of Evaporation (2)
____________________________
  • Availability of Water
  • Oceans
  • Dry Surfaces
  • Moist Surfaces

8
Controls of Evaporation (3)
____________________________
  • The humidity gradient away from the surface
  • Dependent upon the moisture available at the
    surface and in the atmosphere

9
Controls of Evaporation (4)
____________________________
  • The wind speed above the surface
  • Less evaporation with no wind
  • More evaporation with increased wind
  • Example of lake effect snow

10
Vapor Pressure
____________________________
  • Air Pressure
  • The sum of all the pressures exerted by the
    individual gases of the atmosphere
  • Vapor Pressure
  • The share of air pressure contributed by water
    vapor molecules

11
Vapor Pressure Water Vapor Capacity
____________________________
  • Saturation
  • When the movement of water molecules between the
    surface and the air is in equilibrium

12
Vapor Pressure Water Vapor Capacity
____________________________
  • Saturation vapor pressure
  • Air is saturated with water vapor when there are
    enough water vapor molecules to exert the maximum
    water vapor pressure possible

13
Vapor Pressure Water Vapor Capacity
____________________________
  • Saturation Vapor Pressure
  • When this maximum vapor pressure is exceeded,
    some water vapor molecules must become liquid
    (condensation) until equilibrium is reestablished
  • A non-linear function of air temperature

14
Saturation Vapor Pressureas a Function of
Temperature
____________________________
Relates to Figure 4.3
15
Vapor Pressure Water Vapor Capacity
____________________________
  • Water vapor capacity
  • An alternative way to think about the saturation
    vapor pressure
  • The capacity of the air to hold water vapor
  • A non-linear function of temperature

16
Saturation Vapor Pressureas a Function of
Temperature
____________________________
Relates to Figure 4.3
17
Measurements of Humidity
____________________________
  • Specific Humidity
  • The actual quantity of water vapor held by a
    parcel of air
  • The mass of water vapor (g) per unit mass of air
    (kg)

18
Maximum Specific Humidity
____________________________
Not in Book
19
Measurements of Humidity
____________________________
  • Relative Humidity
  • This measure compares the amount of water vapor
    present to the maximum amount that the air can
    hold at a given temperature, expressed as a
    percent

Specific Humidity Water Vapor Capacity
X 100
RH
20
Relative Humidity
____________________________
  • Relative Humidity can change in two ways
  • A change in the amount of water vapor
  • A change in temperature

21
Relative Humidity
____________________________
Not in Book
Strahler and Strahler Introducing Physical
Geography
22
Daily Cycle of Relative Humidity
____________________________
Not in Book
23
Dew Point Temperature
____________________________
  • The temperature at which an air parcel would
    become saturated if it were cooled without a
    change in moisture content
  • Moist air will have a higher dew point
    temperature than dry air

24
Dew Point Temperature
____________________________
Not in Book
25
Precipitable Water Vapor
____________________________
  • Definition
  • The total amount of water vapor available to fall
    as precipitation

26
Precipitable Water Vapor
____________________________
Not in Book
27
Condensation
____________________________
  • Conversion of water vapor to water
  • Results in a release of energy
  • A warming process

28
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