Physical States of Water


  1. Solid state (e.g. ice) occurs at temperature below

      freezing point 00 C or 320 F. (Ice)

  2. Gaseous state (e.g. water vapor) occurs at

      temperatures 1000 C or 2120 F

  3. Liquid state (e.g. sea water) occurs between freezing

      and boiling point temperatures (320 F - 2120 F).


Sources of Moisture in the Atmosphere

        i)    Evaporation from oceans, lakes, rivers & soil

        ii)   Transpiration from plants and other vegetation

        iii)  Perspiration from people and animals. 

        iv)  Sublimation: change from ice to water vapor




Absolute Humidity is the maximum amount of water vapor that a    given column of air can hold. Relative Humidity expressed as a percentage (%) is the amount of water vapor in a column of air compared to the total amount of moisture that the same column of air can hold. When water vapor becomes saturated, it reaches its capacity for holding water so further cooling results in condensation and possibly rainfall.




Condensation is a process by which a gas such as water vapor is   changed into liquid water. When moisture cools and reaches saturation point, the tiny particles of water condenses into larger drops of water.


Forms of Condensation:

Dew: Tiny drops of water formed when condensation of

          water vapor occur at or near the surface of the earth.


Frost: It is a frozen condensation that occurs when air at

            ground level is super cooled below the freezing



Fog:   A mass of tiny drops of water that form when water

           vapor condenses on a nuclei near the earth's surface.


Clouds: A cloud is a mass of tiny drops of water that

             results from condensation which takes place high

             up in the atmosphere.


Adiabatic Cooling: 

 When air rises, it moves from a zone of dense air on the surface to areas of less dense air in the atmosphere. The rising air thus has less weight above it and the lower pressure allows the air to expand and cool down. The decrease in air temperature that result from expansion of rising air is called Adiabatic Cooling.


Adiabatic Warming

 It is the warming of air that results from the compression of the air as it falls from the sky to the earth's surface. When air falls from higher up in the atmosphere it moves into a region of denser air on the earth's surface. The air is compressed in the process resulting in it being warmed. 




Precipitation occurs when tiny droplets of water, ice, or frozen water vapor join together into masses too big to be held above the earth. They then fall to the ground as precipitation.


Forms of Precipitation:


a) Snow: When water vapor is frozen directly into a solid

    without first forming a liquid, it forms tiny ice crystals

    called snow

b) Sleet: is a frozen rain that forms when rain droplets

    encounter a cold air and freezes into ice before falling

    from the sky.

c) Hail: Rounded lumps of ice that falls from the sky.

d) Rain: Consists of droplets of liquid water that falls

     from the sky.


Types of Rainfall:


There are three main causes of rainfall. These are: 


1.  Convectional Rainfall:  The rainfall results when a heated air expands, rises and in the process condenses to form drops of rain. Convectional rainfall is more common in the Humid Tropical Regions that receive much of the sun's energy. It usually occurs in the afternoons after surface air has been heated.


2.  Orographic (Relief) Rainfall:  The rainfall results from uplift of air above some highland. Mountain ranges force air blowing over them to rise, cool and then condense to fall as rain. The side of the mountain that faces the wind and receives the rainfall is the Windward side. The opposite side that receives dry winds is the Leeward side.


3.  Frontal (Cyclonic) rainfall:  This type of rainfall occurs along the zone of contact between a warm and cool air mass. When two large air masses of different temperature meet, the warmer and hence lighter air is lifted above the cooler air. The warm air then rises, cools and condenses to form rain. The boundary that separates the cold air and the warm air is called a Front.