Humidity

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Humidity

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

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Humidity
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• Definition
• The water vapor content of the air

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

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Evaporation
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Lutgens and Tarbuck The Atmosphere
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Controls of Evaporation
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• Four Controls of Evaporation

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Controls of Evaporation (1)
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• 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

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

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

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

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Vapor Pressure
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• 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

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

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

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Vapor Pressure Water Vapor Capacity
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• 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

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Saturation Vapor Pressureas a Function of
Temperature
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Relates to Figure 4.3
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Vapor Pressure Water Vapor Capacity
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• 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

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Saturation Vapor Pressureas a Function of
Temperature
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Relates to Figure 4.3
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Measurements of Humidity
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• 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)

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Maximum Specific Humidity
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Not in Book
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Measurements of Humidity
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• 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
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Relative Humidity
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• Relative Humidity can change in two ways
• A change in the amount of water vapor
• A change in temperature

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Relative Humidity
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Not in Book
Strahler and Strahler Introducing Physical
Geography
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Daily Cycle of Relative Humidity
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Not in Book
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Dew Point Temperature
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• 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

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

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

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