The ITCZ
The Intertropical Convergence Zone, or ITCZ, is a belt of low pressure that encircles the globe near the equator where the trade winds of both hemispheres converge.
When the ITCZ lies near the equator it is called the ‘Near Equatorial Trough’. It is known amongst sailors as the ‘doldrums’ due to its erratic weather patterns. The ITCZ is where converging air masses meet near the thermal equator. Like the thermal equator, ITCZ movement is a function of seasonal heating that is much greater over the land than over the sea.
The Thermal Equator is the position of the maximum thermal temperature around the earth’s surface.
It should not be confused with the global equator, which is the greatest parallel of latitude around the earth’s surface.
The position of the thermal equator moves according to the sun’s heating, which varies widely with season. In the summer season, the thermal equator moves toward the pole (in both northern and southern hemispheres) due to the greater heating experienced at higher latitudes.
The ITCZ It is formed by vertical motion, largely appearing as convective activity of thunderstorms driven by solar heating. This area is subject to intense rainfall and cumulonimbus clouds with tops reaching, and sometimes exceeding, an altitude of 55,000 ft.
The location of the ITCZ varies seasonally. Since oceans heat up more slowly than land masses, the ITCZ tends to move further north and south over land areas than over water. Over South America and Southern Africa, ITCZ movement is large, especially in the Sumer season, whereas over the Atlantic Ocean, its movement is small. In other words, it is stable. The effect of the ITCZ determines the weather pattern over a significant portion of the globe.
Therefore, the ITCZ is responsible for the wet and dry seasons in the tropics. During the months of July and August it lies well north of the geographical equator, as it runs over Africa, Asia and Central America, before moving south into South America, central Africa and Australia by January and February.
Where the trade winds are weak, the ITCZ is characterised by isolated cumulus and CB cells.
Where they are stronger, solid lines of active CB cells embedded with other cloud types develop as a result of instability at higher levels.
An active ITCZ presents hazards to pilots, such as;
- Icing
- Turbulence
- Lightning
- Wind shear
Twice yearly, during March and September, the sun crosses the equator and consequently makes for the two wet seasons. In December and July, when the sun is at its greatest extent to the north and south of the equator, makes for the two dry seasons.
At higher latitudes the climate becomes more monsoonal, with one wet and dry season.
In the northern hemisphere the wet season occurs from May to July and in the Southern Hemisphere, from November to February.
