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Inter Tropical Convergence Zone (ITCZ)
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Inter Tropical Convergence Zone
The Inter Tropical Convergence Zone, or ITCZ, is a belt of low pressure which circles the Earth generally near the equator where the trade winds of the Northern and Southern Hemispheres come together. It is characterised by convective activity which generates often vigorous thunderstorms over large areas. It is most active over continental land masses by day and relatively less active over the oceans.
The position of the ITCZ varies with the seasons, and lags behind the sun's relative position above the Earth's surface by about 1 to 2 months, and correlates generally to the thermal equator. Since water has a higher heat capacity than land, the ITCZ propagates poleward more prominently over land than over water, and over the Northern Hemisphere than over the Southern Hemisphere. In July and August, over the Atlantic and Pacific, the ITCZ is between 5 and 15 degrees north of the Equator, but further north over the land masses of Africa and Asia. In eastern Asia, the ITCZ may propagate up to 30 degrees north of the Equator. In January, over the Atlantic, the ITCZ generally sits no further south than the Equator, but extends much further south over South America, Southern Africa, and Australia. Over land, the ITCZ tends to follow the sun's zenith point.
Where the trade winds are weak, the ITCZ is characterised by isolated Cumulus (Cu) and Cumulonimbus (Cb) cells. However, where the trade winds are stronger, the ITCZ can spawn a solid line of active Cb cells embedded with other cloud types developing as a result of instability at higher levels. Cb tops can reach and sometimes exceed an altitude of 55,000 feet, and the ITCZ can be as wide as 300 nautical miles in places presenting a formidable obstacle to aircraft transit.
Aircraft flying through an active ITCZ (strong trade winds) will probably encounter some or all the hazards associated with Cb clouds such as icing, turbulence, lightning, and wind shear. However, it is in this zone that the most severe effects may often be encountered. In particular, it is within the ITCZ that convective breakthroughs of the tropopause often occur, with the majority occurring over land, especially in the second half of each day. Convective penetration of the tropopause is less common over oceanic areas where the phenomenon is more likely to occur in the early hours of each day, generating more isolated cells. Research sponsored by NASA has shown that 1% of tropical deep convective activity exceeds 46,000 ft altitude, with a small proportion of this reaching much greater heights. For further information on the potential hazards of transit through or near Cb cloud, see the article Cumulonimbus.