I. Water, The Hydrologic Cycle, and Atmospheric Moisture

A. Earth's Oceans

• Earth is referred to as the "water planet" because 71% of its surface is covered by global ocean

• The continents and oceans are not evenly divided between the Northern and Southern Hemispheres

• In the N. Hemisphere 61% of the surface is water, and 39% is land -- "Land Hemisphere"
• In the S. Hemisphere 81% of the surface is water, and 19% is land -- "Water Hemisphere"

Distribution of Land and Water in the N. and S. Hemispheres

B. Composition of Seawater

• 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"

Relative Proportions of Water and Dissolved Salts in Seawater

• 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

C. Distribution of Earth's Water

• 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

• 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

Distribution of Earth's Waters

Distribution of Earth’s Waters (PPT)

D. The Hydrologic Cycle

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

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

• 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

  The Hydrologic Cycle

E. Changes of State/Phase

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

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

(1) Phases:

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

Water (liquid phase):

• it assumes the shape of its container

• the water molecules are more loosely bonded

Water Vapor (gas phase):

• water vapor is invisible and compressible

• each water molecule moves independently

Phases of Water

(2) 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

Latent Heat Energy Involved in the Phase Changes of Water

Latent Heat and Phase Changes

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

Saturation Vapor Pressure for Different Temperatures

• 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

(2) Saturation Vapor Pressure (e_s)- describes how much water vapor is required to make the air saturated at a given temperature

• 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

(3) Relative Humidity (RH) - is a ratio of the actual amount of water vapor in the air compared to the maximum amount of water vapor required for saturation:

Relative Humidity  =  [H₂O Vapor Content/ H2O Vapor Capacity] multiplied by 100

e.g.,     RH = e/e_s 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 (e_s increases) then the RH will decrease; if the air temperature decreases (e_s decreases) then the RH will increase

Figure Showing Typical Variation in Temperature and Relative Humidity (as temperature increases, RH decreases)

(4) Dew Point Temperature (T_d) - 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

• High T_d's > high water vapor content

• Low T_d'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? Answer: 25^oC = Td

• when T = Td; or e = es, RH = 100 %

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

• Answer: the molecular weight of an average molecule of dry air is greater than that of a water molecule

A Sling Psychrometer (an instrument used to determine RH and Dew Point Temperature)

Psychrometric Table (for Determining RH)

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