Intertropical convergence zone

Intertropical convergence zone ITCZ

Tim Takeoff
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6 minutes

The Earth’s entire atmosphere is defined by a whole series of different weather systems, which are often dynamic and produce our seasons and typical weather phenomena. One of them, the Intertropical Convergence Zone, abbreviated to ITCZ, is a very special and unique system – particularly for aviation.

The ITCZ itself is a belt of low pressure that circles the Earth, generally near the Equator. The word convergence describes the ‘meeting’ of two air masses, a phenomenon that causes convection as both air masses attempt to rise after the “collision”.

Trade winds in both hemispheres

The Earth’s Coriolis force produces a westerly air stream along the Equator: in the northern hemisphere, the winds generally come from the north-east and, in the southern hemisphere, from the south-east. These continuous wind patterns were referred to as the “trade winds” by the early seafarers, who learned that they could rely on them and use them to cross the oceans. The ITCZ forms at the point at which the trade winds from the northern and southern hemispheres converge.

Convergence of air masses

Clouds generally form wherever air is forced to rise contrary to its usual behaviour. This might be caused by a mountain range “standing in the way” (forming cap clouds or wall clouds) or even the convergence of air masses mentioned above. Unlike the opposite process, “divergence” (in which the air masses stream away from each other and generally cause clouds to dissipate), convergence generally leads to the massive accumulation of clouds.

This goes hand in hand with a high level of convective activity, which rapidly generates vigorous thunderstorms. Convection is the term given to the vertical movement of air: warm, less dense air rises, colder, heavier air falls. Convection is most active over continental land masses by day and weakest over the oceans. Water has a higher heat capacity than land, which further intensifies the effect of convergence by thermal processes.

At the zenith of the sun

The precise position of the ITCZ varies with the seasons. The most active region always lags by about 1 to 2 months behind the sun’s relative position above the Earth’s surface and usually correlates to the Earth’s Equator. In July and August, over the Atlantic and Pacific Oceans, the ITCZ is between 5 and 15 degrees north of the Equator. The effects of the belt can be felt up to nearly 30 degrees north of the Equator only over the land masses of Africa and Asia.

By contrast, the ITCZ tends to sit south of the Equator in January, once again here following the sun’s zenith point more closely over land, and extending much further south over South America, South Africa and Australia.

Up to the tropopause and beyond

The effects of the ITCZ on weather depend on the strength of the trade winds: if the winds are weak, there are generally only isolated thunderstorms, whereas stronger trade winds produce large contiguous storm systems. Aircraft crossing the ITCZ sometime refer to literally encountering a “wall”. Long lines of storms obstruct the aircraft’s flight path and force it to make extensive detours. Climbing over the storms is almost never an alternative as the convective cloud height can often reach an altitude of up to 55,000 feet (almost 17 kilometres) extending into the tropopause. Normal commercial aircraft generally fly at a cruising altitude of around 40,000 feet with a normal load.

If an aircraft attempts to cross the ITCZ at an “inopportune” spot, a series of phenomena are likely to adversely affect its flight path and the crew needs to be prepared for icing, turbulence, lightning and strong wind shear within the clouds.

Air France 447 disaster

The most notable example of an aviation accident when crossing the Intertropical Convergence Zone concerns Air France flight 447 . The Airbus A330 flying from Rio de Janeiro to Paris confronted all the aforementioned phenomena during the night of 1 June 2009 in the middle of the ITCZ. The crew experienced the icing-up of various key sensors, with the resulting misinformation ultimately causing the autopilot to disengage. The crew members were inadequately prepared for the situation and were unable to regain control of the aircraft. In the midst of the clouds at night they failed to appreciate their current flight status and take appropriate action. The aircraft crashed into the Southern Atlantic Ocean.

A number of scenarios simulated the precise situation after the wreck of the aircraft and the voice recorder had been found following a two year-long search. The appropriate responses and conclusions are now incorporated in current training scenarios for cockpit crews. It is hoped that teaching about these routine processes and immediate actions will effectively prevent such a disaster in future.

Advancements through technology and training

The use of modern systems, including advanced weather radar and weather forecasting services, now enables safe flight paths to be navigated through the ITCZ in advance. All flight crews are aware of the phenomena that occur in the Convergence Zone and are able to take appropriate precautionary measures. And in the cockpit, pilots should never forget the power of nature and always treat it with sufficient respect.

If you would like to read more about this topic, we would recommend that you read our article on weather radar on board commercial aircraft. 

by Tim Takeoff

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