In both cases, whether it is a depression in the temperate latitudes or a cyclone in the tropical latitudes, cyclonic rainfall results from the movement of a low pressure system that has air whirling around it.
While the names cyclonic and depression rainfall are used for obvious reasons above i.e. these rains are associated with depressions and cyclones, the name convergence rainfall is sometimes used because this type of rainfall occurs when air masses of different characteristics meet (converges).
The term frontal rainfall is also used because the rainfall occurs along fronts.
The formation of cyclonic rainfall
The formation of Frontal/ Convergence/Cyclonic/Depression rainfall. Image by the BBC.
Two or more winds/currents with different temperatures meet for example in Southern Africa including Zimbabwe the North East Monsoon meets the South East Trade Wind (from the south east of the continent which brings is warm and moist having obtained moisture from the Indian Ocean by blowing accross the Benguala current).
The two air masses (one warm and one cold) do not mix, they form a front.
The colder air mass is heavier than the warmer air mass, therefore the lighter, warmer air rises over the top of the heavier, colder air.
As the warm air is forced to rise it cools. Also, the warm air is in contact with the cold air along the fronts, and this also cools.
Condensation occurs and clouds form.
Rain occurs along the front.
The ITCZ (Inter-tropical Convergence Zone)
A diagram showing the position of the ITCZ during Zimbabwe’s Summer (December/January) and during Zimbabwe’s winter (June/July). Image by IRDC.
Most of the rain that falls in Zimbabwe comes due to the seasonal shift of the ITCZ.
The ITCZ is a zone that is encircles (goes right round) the earth and is roughly parallel to the equator.
It moves north and south following the sun, usually with a lag of 4–6 weeks.
The sun is over the equator on 21 March each year, it reaches the tropic of Cancer (North of the equator) on 21 June, the equator again on 21 September, and the tropic of Capricorn (South of the equator) on 21 December.
The sun’s heat causes a low-pressure regions to develop within the ITCZ.
North and south of the ITCZ are there are high-pressure belts that also encircle the earth.
This forces air masses to “converge” within this zone as they move from the North and South of the ITCZ to meet within this zone.
The ITCZ brings cyclonic rainfall to Zimbabwe during the December-January months.
In fact most of the rain received in Zimbabwe and most of the Southern African countries like Mozambique, Madagascar and Zambia is due to the movement of the ITCZ during our summer.
Sometimes these rains can last for days on end cause minor to major floods especially in low lying areas like Muzarabani and Tokwe-Mukosi.
When the ITCZ moves North during the winter months we experience High pressure conditions with no/very little precipitation and clear skies.
Because of Zimbabwe’s proximity to the equator it means there are very little changes in pressure.
The yearly migrations of the ITCZ bring about the major seasonal changes of winter and summer.
Although other forms of precipitation occur like the Orographic rainfall in the Eastern Highlands of Zimbabwe the ITCZ is the major governor of rain in the country.