The Intertropical Convergence Zone, ITCZ, (also known as The Equatorial trough) is a zone of low atmospheric pressure and ascending air which circles the Earth at, or very close to the
equator. It is characterized by the strong upward motion of air and heavy
precipitation.
The rising air currents are due to the global wind convergence and convection of thermal heating, the Earth is heated unequally over the surface. The circulation of the atmosphere transfers warm air from low latitudes towards high latitudes, and cold air from high latitudes towards low latitudes. This movement keeps the low latitude regions (where there is a net gain of energy throughout the year) from continually heating up, and the high latitudes from continually cooling down due to a net loss in energy. The heat gained by the Earth is transferred into the air via radiation and sensible heat transfer (which is the warm Earths surface directly heating the atmosphere that is in contact with it. This warmed air rises, producing convection currents). Adding to this heat is condensing water vapour. This heated air can now rise into the wet tropical atmosphere, resulting in a broad area of low pressure around the equator. As the air rises it cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms.
At the equator, the northeast trade winds of the northern hemisphere meet with the southeast trade winds of the southern hemisphere, and this creates the Intertropical Convergence Zone. These converging winds create a zone of calm weak winds with no wind direction, called the ‘doldrums’ (between about 5 degrees north and south latitude). It was a region where ships were often stranded for days on the calm sea, waiting for the wind to pick up. This area was called the "horse latitudes" by the Spanish sailors because they would throw their horses into the sea to conserve water.
Variation in the location of the ITCZ drastically affects rainfall in many countries close to the equator and results in wet and dry seasons for the tropics rather then cold and warm seasons of higher latitudes. The ITCZ shifts north and south seasonally with the Sun.
Pictures:(really, really cool and very big! The ITCZ is the ring of white cloud)
http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/itcz_goes11_lrg.jpg
This one shows how the trade winds meet:
http://www.usatoday.com/weather/witcz.htm
http://ww2010.atmos.uiuc.edu/guides/mtr/hurr/gifs/mvmt1.gif
Sources:
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/circulation/global_scale_circulation.html
http://iri.ldeo.columbia.edu/~bgordon/ITCZ.html