HUMIDITY

Humidity refers to the water in gas form or water vapor in the atmosphere and not to liquid droplets of clouds, fog or rain

Water vapor has nothing to do with wetness, for it is dry like any

other gas and exerts a pressure which is known as the vapor pressure of the air

Air is saturated when its vapor pressure equals that over a smooth water surface, i.e. it cannot hold any more

Although water vapor comprises less than 2 percent of the atmospheric volume, it is the single most important component of the air from the standpoint of weather and climate

The composition of the air by water vapor is highly inconsistent varying from 0 to a maximum of 3 percent in mid latitudes and 4 percent in tropics

This variability from place to place and from time to time is of outstanding importance for several reasons

l. water vapor is the source of all forms of condensation and precipitation. The amount of water vapor in a given mass of air is one indicator of the atmosphere's potential for precipitation

2. As the air's principal absorber of solar energy, water vapor acts

as a heat regulator and has tremendous influence on temperature.

3. The latent heat contained in water vapor is an important energy source both for the circulation of the atmosphere and for the development of many atmospheric disturbances, especially those in the tropics

4. Also because of its latent heat, the amount and vertical distribution of water vapor in the atmosphere indirectly affects the buoyancy of the air and hence its tendency to ascend. This in turn is closely related to the formation of clouds and precipitation

5. Affects the rate of evaporation, a process that greatly influences plant and animal life. By affecting the human body's rate of cooling, evaporation is one determinant of its sensible temp or feeling of heat and cold

6. Unlike other gases comprising the air, water vapor can be changed to liquid and solid forms well within the range of atmospheric temperatures.

MISCELLANEOUS INFORMATION

Water boils at the temperature where the SVP=surrounding air pressure

Why does warm air hold more water vapor? higher molecular speeds means more water can escape from the liquid phase. Temperature of air is a measure of the average kinetic energy of molecules--higher temps correspond to higher average speeds, therefore at high temps condensation is less likely because of most molecules have sufficient speed to remain as vapor.

As air temperature lowers, average speed decreases and fewer molecules have energy to remain in air as vapor

Vapor pressure and boiling

-as water boils bubbles of water vapor rise to the top of the liquid and escape

-for this to occur the saturation vapor pressure exerted by the bubbles must equal the pressure of the atmosphere, otherwise the bubbles would collapse.

-boiling therefore occurs when the saturation vapor pressure of the escaping bubbles is equal to that of the entire atmosphere

-tough to get a hot cup of coffee on the top of Mt. Everest

MEASURES OF HUMIDITY

Vapor Pressure

-simply the pressure exerted by the water vapor in a sample of air.

Concept of Saturation---The kinetic theory of gases indicates that evaporation occurs when molecules in a liquid succeed in overcoming mutual attractive forces and escape from a free surface to for a vapor in the space above. Some molecules strike the surface and are recaptured until, ultimately a state of dynamic equilibrium exists in which the exact number of escaping molecules is balanced by the number of recaptured molecules. At this point the vapor is saturated. For a given temperature there is a definite

SATURATION VAPOR PRESSURE

-pressure wv molecules would exert if the air were saturated with water vapor at a given temperature

-if temperature is elevated this dynamic equilibrium is upset. The higher the temperature the greater the average kinetic energy of molecules and the more readily do water molecules escape the water surface as vapor

-air does not really "hold" water vapor, actually it "coexists" with water vapor

Relative Humidity

- AVP/SVP x 100

Actual Vapor Pressure

-RH x SVP divided by 100

Absolute Humidity

-the ratio of the mass of water vapor to the total volume

of moist air in which it is contained

-217 AVP divided by absolute Temperature T

-expressed in grams/ cm3

Specific humidity and mixing ratio of an air parcel remain constant as long as water vapor not added or removed

Specific Humidity

-ratio of the mass of water vapor in a sample of moist air

to the total mass of the sample (a conservative property)

-as long as the moisture content of a parcel of remains constant the specific humidity will remain unchanged as well

-623 AVP divided by atmospheric pressure -.377AVP

-answer is in grams/kg

- q= MV divided by MV + MA (where MV is mass of water vapor

and MA is mass of dry air

also is related to vapor pressure in following equation

- q =.622 e / P-.378e

where p is pressure and e is partial pressure of vapor in air

Mixing Ratio

-ratio of the mass of water vapor contained in a sample of

moist air to the mass of dry air in a sample ( a conservative

property)

- = MV/MA

or

- 623AVP divided by P-AVP in grams/kg

EVAPORATION

Water evaporates because a fraction of the molecules acquire sufficient energy in their random motions to break intermolecular attraction. Remember speeds of water at a given temperature are an average, some are lower, some are higher

l. strong binding forces at boundary of a liquid (surface tension)

2. escape depends on speed of molecules

-speed increases with temperature

3. Evaporation proceeds as long as vp of air is less than

saturation vapor pressure of water surface

4. Factors Affecting Evaporation

-solar radiation--temperature

-relative humidity

-wind

-moisture supply

On a molecular basis, evaporation is a selective process. Faster

(hotter) molecules leave the water surface, leaving behind those

with less energy. Thus, the average energy of the molecules in

the water becomes less as water temperature is lowered.

The energy carried away by the departing molecules becomes "work" energy in keeping molecules widely separated in the form of vapor.

Thus vapor is a gas, and is called vapor only because it is possible for it it exist in equilibrium with its liquid and solid phases. In the range of atmospheric temperatures, such equilibrium is not possible for the other atmospheric gases.

The heat energy thus carried away is termed latent. It is not

recoverable as heat energy until the vapor condenses or freezes