Water in the Atmosphere

EvaporationCondensation

andCloud Formation

Hydrologic Cycle• Hydrologic cycle: circulation of water over earth:

A closed system!• Solid (ice) goes to liquid (oceans/lakes/rivers), liquid to vapor

(gas), vapor to liquid (tiny droplets/clouds) or solid (SOME CLOUDS ARE ICE!), droplets combine to form rain (or snow).

Earth’s Water Resources• How might global warming or cooling alter the image below?

Hydrologic Cycle• Hydrologic system– Water changes of state = Redistribution of energy

Water in the Atmosphere• Water absorbs and reflects energy• Water Budget:• Total quantity of water remains the same. Any

deficit must balance gains• Heat Budget:

• Latent Heat (energy from evaporation)• Energy gained from evaporation is released during

condensation to form clouds• Heat transfer involved with evaporation and

condensation is huge! • The energy stored and transferred in phase changes

provides the power for Earth’s storms (think hurricanes)

• Saturation: air at a certain temperature holds all of the water vapor possible. It has reached capacity.

• Dew Point: the temperature at which the air becomes saturated. As the dew point and the air temperature get closer the humidity increases.

Water in the Atmosphere

• Check page 14 of the ESRT: the average temperature in the troposphere goes from 15 °C at the bottom to -55 °C at the top!

• How much water vapor can the air hold at that temperature?

• Humidity: Amount of water vapor in the air.

• Different ways to measure Humidity:– Specific Humidity– Relative Humidity

(R.H.)

Water in the Atmosphere

• Condensation: occurs when air temperature reaches the dew point. The R.H. becomes 100% (saturated).

• Condensation requires the presence of Condensation Nuclei. These provide a surface for the water vapor to condense on.

• Fog and Clouds form when water vapor condenses and a large number of these droplets form a mass.

Condensation

• Relative sizes of:– Raindrops– cloud droplets– condensation nuclei

Precipitation occurs when droplets become too large to be suspended by wind

Precipitation Processes

Cloud Formation• As you go higher in the earths troposphere,

temperature decreases• Therefore the air capacity for water vapor also

decreases and the R.H. INCREASES! Even as specific humidity decreases!

• When the R.H. reaches 100% and condensation nuclei are present condensation (cloud formation) starts

CLOUD LAB

• Look at the chart on page 1!– What are the two lines?– Dashed lines (air temp) is the same as

Dry Adiabatic Lapse Rate– Solid Lines (Dew Point) is the same as

Wet Adiabatic Lapse Rate

Ex: Dew Pt=-10, Air T: 0

• Orographic Lifting – air is forced upward

due to land barriers (mountains, plateaus)

– Rising air is cooler than sinking air

– Condensation of moisture in the rising air provides the extra heat

Climate Effect of Condensation

Windward climate is cool and humid

Leeward is warm and dry

• Fog is a cloud at the ground surface.

• Types of Fog:– Radiation Fog– Advection Fog– Upslope Fog

Fog