A. General Water Concepts

• water is the most common compound on the Earth's surface

• water is in a steady-state equilibrium in the Earth's hydrosphere

• sea level changes do occur - "eustasy"

• "glacio-eustatic" factors are partially responsible for these sea level changes

• water (vapor & liquid) on the Earth's surface & in the atmosphere resulted from the "outgassing" of water from & below the Earth's crust - over the last 4 billion years

• water occurs in two forms - fresh & saline

• water is an excellent solvent (it dissolves materials well)

• seawater is a solution of dissolved solids; the concentration of these dissolved solids is referred to as the "salinity"

• most common elements in seawater: chlorine (Cl), sodium (Na), magnesium (Mg), sulfur (S), calcium (Ca), potassium (K), bromine (Br)

• average salinity: 350/00 (parts per thousand)

• "brine" - > 350/00; "brackish" < 350/000

• 97.22% of all water is held in the oceans
• the remaining 2.78% is classified as fresh water

• 77.14% of all fresh water is held in ice sheets & glaciers

B. The Hydrologic Cycle

• the circulation of water in the Earth's atmosphere, on the Earth's Surface, & near the Earth's surface

• involves the transformation of water molecules between the three states of matter

C. Changes of State/Phase

• when water changes from one state (phase) to another, heat energy must added/released to/from the water.

• this energy is necessary to affect the hydrogen bonds between the water molecules

Ice (solid phase):

• contracts as it cools - to 4 degrees C; below this temp it expands

• water molecules are bonded (hydrogen bonds) to one another in a firmly fixed structure

• it assumes the shape of its container

• the water molecules are more loosely bonded

• water vapor is invisible and compressible

• each water molecule moves independently

Latent Heat :

Defn: the heat energy required to change a substance from one "state" to another (i.o.w., to break the hydrogen bonds)

Solid to Liquid: 80 cal/gm (latent heat of fusion) needed to break the hydrogen bonds of the solid

...............no change in temp........

Liquid to Solid: 80 cal/gm (latent heat of fusion)

are released

Liquid to Gas: 540 cal/gm (latent heat of vaporization) needed to break the hydrogen bonds of the liquid.....no change in temp.....

Gas to Liquid: 540 cal/gm (latent heat of condensation) are released

• latent heat is an important source of atmospheric energy > the water vapor changes into liquid and ice cloud particles at high altitudes & releases tremendous amounts of energy

D. Atmospheric Moisture

• the water vapor content of the air is primarily a function of temperature: warm air can hold more water vapor than cold air

• air is "saturated" if it is holding all the water vapor that it can hold at a given temperature

• there are a variety of ways to express the amount of water vapor in the air:

(1) (Actual) Vapor Pressure (e) - partial pressure due to water vapor in the air

Ptot = PN2 + PO2 + ...... + PH2O

• high vapor pressure indicates lots of water vapor in the air; low vapor pressure indicates comparatively smaller amounts of water vapor in the air

• saturation vapor pressure is the pressure that the water molecules would exert if the air was saturated

• saturation vapor pressure depends primarily on air temperature; the higher the temperature, the more water vapor required to saturate the air

RH = H2O vapor content X 100

H2O vapor capacity

RH = e/es x 100

RH does not indicate the actual amount of water vapor in the air; it, instead, indicates how close the air is to saturation !!

if the amount of moisture stays constant, but the air temperature increases (es increases) then the RH will decrease; if the air temperature decreases (es decreases) then the RH will increase

(4) Dew Point (Temperature) (Td) - the temperature to which air must be cooled (at constant air pressure and moisture content) in order to reach saturation

it is a measure of the "absolute" (actual) water vapor content of the air

Hi Td's > high water vapor content

Lo Td's > low water vapor content

e.g., if we have a volume of air at 25oC & RH=100%; we then raise the temp to 30oC - it's no longer saturated (RH < 100%); what temp would we have to cool the volume of air to in order for saturation to occur? 25oC = Td

• when T = Td; or e = es, RH = 100 %

• moist air is less dense than dry air (at the same pressure and temperature); why?