Are a mass of tiny droplets or ice particles formed when water vapour condenses.

Three Cloud Forms

  1. Cirroform

-Thin and wispy clouds composed of ice crystals.

  1. stratiform

-Appear as greyish sheets covering most of the sky and are rarely broken into units.

  1. Cumuliform

-Are massive rounded with a flat base and limited horizontal extent and billow upwards to great heights.


Basic Cloud Types

  1. Stratus Clouds

-Are found in layers, are flat in shape and resemble fog.

  1. Nimbus Clouds

-are dark at the base and sometimes white at the sides and cause rain and thunderstorms.

  1. Cirro-cumulus

-Are white clouds consisting of white ice crystals.

  1. Nimbostratus

-A rain cloud which is dark grey and spreads over the sky in low uniform layers.

  1. Cumulus Clouds

-Clouds with a flat horizontal base, massive, rounded and less horizontal extent.

  1. Alto cumulus

-High clouds composed of ice crystals which indicate fair weather.

World distribution of Pressure Zones and the Planetary wind System/World Prevailing Winds


The Equatorial Low pressure Zone (ITCZ-low)

  • Found between 23 ½ ◦N and 23 ◦N
  • Experiences high temperatures.
  • A zone of low pressure and doldrums (light and intermediate winds).
  • Zone where S.E and N.E Trade Winds converge.
  • Associated with convectional rain and thunderstorms.
  • Migrates to the N and with the apparent movement of the overhead sun.

The Sub-tropical High Pressure Zone

  • Found within 30◦N and 30◦S.
  • A zone of high pressure.
  • A region of calm descending air.
  • Source of Trade Winds and Westeries.
  • Zone of divergence of T. Winds and Westeries.

The Temperate Low Pressure Zone

  • Found within 60◦N and 60◦S.
  • A low pressure zone.
  • Zone of convergence of westeries and polar easteries.

The Polar High Pressure Zone

  • Found over the poles 90◦N and 90◦S.
  • A high pressure Zone.
  • Zone of descending calm air of low temperature.
  • Source of polar easteries.


The Worlds Prevailing Winds

These are the major winds blowing over the earth frequently and consistently and which influence the world weather.

  1. Trade Winds
  • Blow from sub-tropical high pressure zone and blow to the equatorial low pressure belt.
  1. Westeries
  • Originate from sub-tropical high pressure zone and blow to the temperate low pressure belt.
  1. The Polar Easteries
  • Originate from polar high pressure zone and blow to temperate low pressure zone.


Monsoon Winds

  • Seasonal winds which reverse in the direction of flow.
  • They blow towards the land during summer (onshore) and from the land during winter (off shore).
  • Bring heavy rains when onshore which can cause severe flooding.
  • Well developed in the Indian Sun-continent, china, Japan and S.E Asia.


See also






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