Inter Tropical Convergence Zone (ITCZ)

Definition

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 Intertropical Convergence Zone (ITCZ)

The ITCZ is the band of bright white clouds that cuts across the center of the image above.

Source: Wikicommons/NOAA

Description

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 ocean heats up more slowly than the 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.

Position of the intertropical convergence zone in January (in blue) and in July (in red).

Position of the intertropical convergence zone in January (in blue) and in July (in red). Source: Wikicommons. Author Mats Halldin, 13 Dec 2006. 

Where the trade winds are weak, the ITCZ is characterised by isolated Cumulus (Cu) and Cumulonimbus (Cb) (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.

Tropical Cyclone Formation

As the ITCZ migrates to tropical and subtropical latitudes and even beyond during the respective hemisphere's summer season, increasing Coriolis force makes the formation tropical cyclones within this zone more possible. Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis.

Threats to Flight Safety

Aircraft flying through an active ITCZ (strong trade winds) will probably encounter some or all of the hazards associated with Cb clouds such as icingturbulencelightning, 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 National Aeronautics and Space Administration 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 (Cb).

Even at the altitudes flown by modern jet air transport aircraft, it is likely that minor adjustments to route will be required to avoid either passing through or above significant convective build-ups. Turboprop aircraft are unlikley to be able to climb over such convection activity and weather avoidance actions can take aircraft significantly off track. It is therefore prudent to carry additional contingency fuel forweather avoidance and icing. 

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