Global Water Cycle and Global Water Cycle and Atmospheric MoistureAtmospheric Moisture

Video: Weather – Wet Video: Weather – Wet

The global water cycleThe global water cycle

Water is unique on earth because it can exist in all 3 phases, and the phase changes can release latent heat to drive weather/climate

Flow of water vapor in the atmosphereFlow of water vapor in the atmosphere

Paintings of water vaporPaintings of water vapor

Corot: Ville D'avray (1865-70)

Flow of >1000 rivers on the seven continentsFlow of >1000 rivers on the seven continents

Amazon river Yangtze riverMississippi river

Paintings of riversPaintings of rivers

Jin Nong (金农 ): Moon river (1687-1764)

Flow of ocean currentsFlow of ocean currents

Snow/Ice cover provide a reservoir Snow/Ice cover provide a reservoir

A significant fraction of the human body is A significant fraction of the human body is water (~75%)water (~75%)

Every 16 days nearly 100% of the water in a human body is exchanged.

The remaining: fat, protein, carbonhydrate, other solids

So the water we drink may come from …So the water we drink may come from …

Therefore we need to protect the environment because any pollution we put into the environment may someday come back into our bodies

Video: Weather – Wet Video: Weather – Wet

Phase changes of Water (HPhase changes of Water (H22O ) O )

• An H2O molecule

• 3 states (gas, liquid, solid) depending on how the molecules are connected together

• Can change from any state to any other state. Latent heat is consumed or released in a phase change

e.g. Evaporation -> liberation of water molecules, requires energy

• Humidity: amount of water vapor in air

• Humidity expressed in a number of ways Indices

1. Vapor Pressure (e): the partial pressure exerted by water the partial pressure exerted by water vapor. vapor.

2. Mixing Ratio (r): mass of water vapor (g) relative only to mass of dry air (kg) (in g/kg).

3. Relative Humidity (RH): the amount of water vapor in the air relative to the possible maximum.

4. Dew point temperature: temperature at which saturation occurs in air (generally colder than Tair, equals to Tair when saturated)

Indices of Water Vapor Content

RH=e/esx100

Saturation and CondensationSaturation and Condensation

• Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates.

• Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation.

• There is an equilibrium between evaporation and condensation during saturation. Upon saturation, evaporation rate equals condensation rate.

• Same thing happens between water vapor and underlying ice surface: sublimation/deposition.

Dependence of saturation on temperatureDependence of saturation on temperature

• Saturation vapor pressure – maximum amount of water vapor that the atmosphere can hold at a given temperature

• Saturation vapor pressure increases with temperature, i.e., warmer air can hold more water vapor.

• Saturation vapor pressure for ice is smaller than that for water. Therefore, when ice crystals and water drops co-exist, water vapor tends to condense onto ice rather than onto liquid water.

• Diabatic processes – add/remove heat– Conduction (e.g. movement of air mass over a

cold surface): dew, frost, advection fog– Radiation (e.g. cooling of boundary layer air by

longwave radiation): radiation fog

• Adiabatic processes - no addition/removal of heat– Cooling of air parcel when it rises (because air

parcel expands when it rises, like a balloon): upslope fog, clouds

1st Law of Thermodynamics expanding air cools, compressed warms (like a manual hand air pump).

Methods to achieve saturation and condensation:Methods to achieve saturation and condensation:Cooling down the temperatureCooling down the temperature

Different types of fog found throughout the U.S.

Global water cycle Water Vapor Basics (names of different phase changes,

latent heat) Humidity indices Saturation vapor pressure increases non-linearly with

temperature Saturation vapor pressure for ice is smaller than that for

water Two methods of achieving saturation and condensation

(diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (advection, radiation, upslope), clouds.

Summary

Works citedWorks cited

• http://capita.wustl.edu/capita/datasets/modis/globfused/glob3d.html

• http://www.ssec.wisc.edu/sos/wvsst/wvsst.html • http://www.arm.gov/news/facility/post/1025• http://apollo.lsc.vsc.edu/classes/met130/notes/

chapter4/es_temp.html • http://www.meted.ucar.edu/oceans/currents/print.htm • http://www.smscs.com/photo/

animated_waterfall_wallpapers_for_mobile.html#.Uk1zteAUioU

• http://beutifullworld.blogspot.com/p/blog-page.html