Moisture in the Atmosphere

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Lecture 5

• Moisture in the Atmosphere

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Water in the Atmosphere

• Water comprises 0-4 of the earths atmosphere
• Water is important to the earths energy budget
  because of
• Latent heat release
• Greenhouse gas
• Water has a high heat capacity

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Water (H2O)
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Phase Changes that Release Latent Heat

• Condensation
• Vapor gt Liquid
• Freezing
• Liquid gt Solid (Ice)
• Deposition
• Vapor gt Solid (Ice)

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Phase Changes that Absorb Latent Heat

• Evaporation
• Liquid gt Vapor
• Melting
• Solid (Ice) gt Liquid
• Sublimation
• Solid (Ice) gt Vapor

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Phase Changes

• If you leave a glass of water sitting out for
  many days, what happens to the water level in the
  glass?
• .does it go up?
• ..does it go down?
• ..does it remain constant?

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Phase Changes

• What happens if you cover a glass of water with
  plastic wrap and let it sit out for many days?
• .does the water level go up?
• .does it go down?
• .does it stay the same?

When evaporation balances condensation.. the
air is said to be saturated with water vapor.
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What determines the rate of evaporation?

• Wind
• Temperature of the Water

What determines the rate of condensation?

• Air temperature

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Hydrologic Cycle
Some vapor condenses into clouds, and then falls
as precipitation
Wind transports water vapor to other regions
Water evaporates back into vapor
Precipitation lands on ground and in water ways.
Water evaporates from oceans
Water runs off land into oceans
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Hydrologic Cycle
Precipitation lands on ground and in water ways.
Some vapor condenses into clouds, and then falls
as precipitation
Water evaporates back into vapor
Wind transports water vapor to other regions
Water runs off land into oceans
Water evaporates from oceans
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Hydrologic Cycle (Frozen)
Precipitation lands on Polar Ice Cap.
Some vapor condenses into clouds, and then falls
as precipitation (Snow!)
Ice/Snow sublimates into the air
Wind transports water vapor to other regions
Ice flows off the polar plateaus, onto the ice
shelves, melts into oceans or Icebergs calve and
melt into the oceans.
Water evaporates from oceans
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True or False

• ___Warmer air holds more water vapor than cold
  air.
• ___Humid air weighs more than dry air.

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Water Vapor is a Gas in the Atmosphere, just like
Nitrogen and Oxygen
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Weight of Dry Air

• Oxygen (O2)
• Molecular weight 32
• by volume 21
• 32 x .21 6.72
• Nitrogen (N2)
• Molecular weight 28
• by volume 78
• 28 x .78 21.84
• Dry air
• Molecular weight 21.84 6.72 28.56

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Weight of Moist Air

• Water Vapor (H2O)
• Molecular weight 18
• by volume 4
• 18 x .04 .72
• Dry Air
• Molecular weight 28.56
• by volume 95
• 28.56 x .95 27.132
• Moist Air
• Molecular weight 27.132 .72 27.852

Moist air weighs less!!!!
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How can we measure water vapor in our atmosphere?
Handout
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Measures of Moisture

• Absolute measure
• Mixing Ratio (w)
• Vapor Pressure (e)
• Absolute Humidity
• Specific Humidity
• Dew Point Temperature (Td)

• Relative measure
• Relative Humidity (RH)
• Dew Point Depression (Tdd)
• Capacity/Limit
• Saturation Vapor Pressure (es)
• Saturation Mixing Ratio (ws)
• Temperature (T)

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Vapor Pressure
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Relative Humidity

• The relative humidity can be changed by two
  methods
• Change the airs water vapor content
• Change the temperature
• Relative humidity does NOT give you an idea of
  the actual amount of water vapor in the air.

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Moisture/Temperature During the Day
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Relative Humidity
Mixing Ratio
Vapor Pressure

• RH (w/ws ) x 100
• (e/es) x 100
• Example w 5 g/Kg, ws 13 g/Kg
• RH 38
• Example e 12 mb, es 48mb
• RH 25

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Which has drier air?

• The Sahara Desert
• Air temp 35 oC
• Dew point 5 oC
• Relative humidity 16

The Sahara has more water vapor!

• Antarctica
• Air Temp -2 oC
• Dew Point -2 oC
• Relative humidity 100

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Relative Humidity vs. Specific Humidity
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What is the relative humidity in your home like?

• Do you think it is greater or less than that of a
  desert?
• Where does the air in our homes come from?

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Moisture Variables

• Absolute measures depend only on the moisture
  content
• Relative measures depend on both the water
  content and the temperature

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Relating Variables

• T gt es
• Td gt e

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What does it mean when people say that the
weather in Arizona isnt bad because it is a dry
heat?
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Humidity and People

• Evaporation is a cooling process for human skin.
• The less humid the air is, the easier it is for
  sweat to evaporate.
• The skin can become as cool as the wet bulb
  temperature.(Tw)

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Heat Index

• This index is used by the National Weather
  Service.
• Combines ambient temperature with relative
  humidity.
• Determines an apparent air temperature, or what
  the temperature feels like to the average
  person.
• A comfort index much like Wind Chill

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Why does naturally curly hair get frizzy when
it is humid, and straight hair goes limp?
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Measuring Humidity

• Hygrometers measure humidity.
• Hair Hygrometer
• Electric Hygrometer
• Infrared Hygrometer
• Dew Cell
• Dew Point Hygrometer
• Sling Psychrometer

Blonde Hair 1 Sensor of Humidity