2. Water Vapor
• Source of all condensation and
precipitation
• Most important gas in the atmosphere for
understanding atmospheric processes
• Zero to 4% by volume
3. Water Changes State
• Always requires heat
• Heat is either absorbed or released
• Water must pass through the atmosphere
in the form of water vapor
• If all the water vapor was in a global layer
it would only be 2 mm deep
4.
5. Solid to Liquid
• Melting
– Heat is transferred to ice water
– Temperature of the water remains a constant 0
degrees C until all ice has melted
– Heat breaks apart crystal structure of ice forming
liquid water
6.
7. Liquid to Gas
• Evaporation
– Changing a liquid into a gas
– Requires 2500 joules of energy to convert 1
gram of liquid water to water vapor
8. Liquid to Gas
• “Evaporation as a cooling process”
– Energy is needed to evaporate water
– Cooling effect after getting out of the pool
– Energy needed comes right from your skin
• Condensation
– Water vapor changes to liquid
– Generates clouds and fog
9.
10. Solid to Gas
• Sublimation
– Conversion of solid directly to a gas
– No liquid state
– Dry ice (frozen carbon dioxide)
– Generates “smoke”
• Deposition
– Conversion of vapor directly to a solid
– Frost on cold objects, grass, windows
11. Humidity
• Humidity – the amount of water vapor in the air
• Saturation
– Balance between liquid water and water vapor
– Closed jar: half water, half air
– Water begins to evaporate increases pressure in
the air
– Pressure is caused by more water vapor molecules
moving into the air
– Pressure in air continues to increase
– Forces more water molecules to return to liquid
– When a balance occurs, air is said to be saturated
13. Relative Humidity
• Ratio of air’s actual water vapor content
compared to the amount of water vapor
the air can hold at the time
• How much water vapor can the air hold?
– Depends on temperature and pressure
• Indicates how near the air is to saturation
14.
15.
16. Relative Humidity
• How can it be changed?
– 1 – adding or removing water vapor
• Occurs naturally
• Oceans and smaller bodies of water
– 2 – varies with temperature
• Lowering air temperatures causes INCREASE
• Raising air temperatures causes DECREASE
17. Dew Point
• The temperature at which one parcel of air
would need to be cooled in order to reach
saturation
• If the air was cooled further it would
condense
• This would cause dew,
fog, clouds
18. How do you measure humidity?
• Hygrometer – used to measure relative
humidity
• Psychrometer
– Two thermometers side by side
– One is dry bulb, the other wet bulb
– Larger the difference on the reading the lower
the relative humidity
– If air is saturated the thermometers will have
the same reading
