Atmospheric Moisture

I. Evaporation & Transpiration (water changes from liquid state into gaseous state.)

It causes an apparent heat loss (latent heat), so drop of temperature of the surrounding is resulted.  The process occurs while the vapour pressure difference exists between different places.


Factors affecting evapotranspiration:

a.   High insolation input (latitudes): It raises up the surface temperature because more energy will be received.  And also high temperature enables the air to hold more moisture.

b.  High relative humidity: More moisture exists, thus the air is near the point of saturation.

c.   The humidity of the wind: Hot dry wind (onshore wind) can increase (decrease) evaporation.

d.  Dense vegetation: increase transpiration rate.


Global pattern of evapotranspiration: greater evaporation in mid-latitude and tropics, and greater in southern hemisphere

Factors affecting the global pattern are insolation input, moisture supply and wind pattern.

²     In high latitudes, there is little available energy for evaporation.

²     In middle and lower latitudes, the rates are high over the oceans in view of the unlimited availability of water.

²     The land maximum occurs more or loss at the equators because of the relatively high solar radiation receipts and large transpiration losses from the luxuriant vegetation of this region.

²     Secondary maximum over land mid-latitudes is related to the strong prevailing westerlies.

²     Minimum rate in the doldrums belt as a result of lower wind speeds and the proximity of the vapour pressure in the air to its saturation value.


II. Condensation (water vapour changes into water droplets.)

3 conditions for condensation

1.  Saturated air

2.  Air cools down to the dew point temperature

a.  contact cooling

b. advection cooling

c.  radiation cooling

3.  Condensation nuclei (Particles of salt, dust, smoke, ions will attract water molecules.  Condensation takes place easily in the industrial cities and coastal regions.)


Forms of condensation

1.  High level condensation: clouds

2.  Low level condensation: fog


III. Precipitation

1.  Uplift mechanisms for cooling

a.  convection uplift

b. orographic uplift

c.  frontal uplift

d. adiabactic cooling (unstable / conditional unstable condition)



2.  Droplets enlargement

a.  Bergeron-Findeison theory (ice crystal method) in temperate region

b. Collisions theory in the tropics: the air temperature does not reach 0 in the upper atmosphere.  Water droplets are enlarged by coalescence.)


Factors unfavourable to precipitation:

1.  Temperature inversion (adabatically warming) DALR & DALR < ELR (absolute stability): cold air masses invade into the warm surface.

2.  Radiation cooling at night on the mountain slopes.

3.  Continentality: the interior part of a continent has limited moisture.

4.  Relief: high mountain cuts off the moisture supply


Forms of precipitation:

1.  Major forms: rain, snow, hail

2.  Minor forms: dew, frost


Types of Precipitation:

1.  Convection type

2.  Orographic type

3.  Frontal type


Global pattern of precipitation

The precipitation decreases with increasing the latitude.  (Factors affecting the precipitation are moisture content in atmosphere, continentality, offshore wind, and pressure belts.  These factors can explain the spatial distribution of arid areas.)

Seasonal variation: shifting of the planetary wind belts (ITCZ and polar front).

Local variation: typhoons, strong local heating, sea breeze, and leeward side in mountain area.


IV. Moisture Balance

      Transfer = Evaporation - Precipitation

The transfer happens in where vapour pressure difference and radiation imbalances between different places.  More moisture is transferred in southern hemisphere.


a.    Vertical transfer: surface to atmosphere happens through evapotranspiration, condensation and precipitation.

b.    Horizontal transfer:

Tropics have the moisture deficit in the atmosphere (E<ppt).

Desert has the moisture surplus in the atmosphere (E>ppt).

Moisture is transferred ploeward and equatorward from 30°.


By Lau Chi-wai