RAINFALL

Rainfall is defined as the release of excess condensed water vapour in the atmosphere into the earth. Rainfall is the most important form of precipitation. On maps, lines used to join places having the same average annual rainfall are called Isoyets.

Other forms of precipitation include:

1. Snow: Snow is formed when water vapour condenses in the atmosphere at temperature below freezing point, resulting in the formation of ice crystals.
2. Hail: This is formed from condensed water droplets at great vertical height and usually associated with thunderstorms. Ice crystals are also formed.
3. Dew: Dew is formed when condensation takes place on the ground, buildings or on top of leaves. The temperature at which condensation occurs is known as Dew point.
4. Fog: This is also formed when condensation takes place near the ground over the sea, resulting in tiny particles of dirt and dust in the air. Fog usually causes poor visibility, especially in the morning.
5. Frost: This form results from the direct condensation of water droplets into ice crystals when the temperature is below freezing point.
6. Mist: Mist, just like fog is formed when condensation takes place near the ground where it occurs as tiny water droplets that are not as thick as fog.

 

TYPES OF RAINFALL

There are three main types of rainfall. These are:

1. Convectional rainfall: This occurs commonly in the tropics and temperate regions during summer. When the earth’s surface is heated by the sun (insolation), evaporation occurs and heated moisture-laden air rises in convection currents upward, towards the atmosphere. As the current proceeds further into the atmosphere, cooling begins to take place. When the current moves further, condensation occurs and the water molecules fall back to the earth as rain.

Convectional rainfall is always torrential or heavy, accompanied by lightning and thunder. The cloud associated with thunder-storm and convectional rainfall is called Cumulonimbus.

 

2. Orographic or relief rainfall: In orographic rainfall, moist air is forced to rise above a mountain/relief barrier, usually on the windward slopes of mountains. Rising air expands and becomes cooler and relative humidity rises and air becomes saturated. Water vapour condenses; cloud is formed with rainfall on the windward slope and descending air on the leeward side.

 

Orographic rainfall is associated with mountainous regions with even distribution of rainfall over a highland. Windward area has rainfall while leeward side has little or no rainfall. It is also characterized by ascending and descending wind. The rain only forms or takes place on the windward side of the mountain while the other side Leeward side experiences little or no rainfall at all. So it remains dry to form rain shadow.

 

3. Cyclonic or Frontal rainfall: Cyclonic rainfall is caused by the meeting of two air masses, which are different in terms of origin and characteristics. Warm, light, moisture-laden air rises above cold and dense air. Ascent of the warm air over cold air lowers the temperature of the warm air, cooling takes place and condensation occurs. Cloud formation and precipitation follow.

Cyclonic rainfall is characterized by fronts and weather system in mid-latitudes. Rainfall is convectional type in cold front and it lasts for greater duration than convectional rainfall. It covers extensive area and moves from the West to the East in the northern hemisphere and vice-versa in the south.

 

Rainfall including other forms of precipitation is measured by an instrument called Rain-gauge.