Hence the region of maximum energy input wanders seasonally to the north and south of the equator. Climatologically the region with maximum energy input sets up a circulation in the form of so-called Hadley cells where warm air rises where it is heated the most and then transported at elevation to the north and south.
At the location of upward air flow formation of convective clouds yielding precipitation becomes the norm. This zone to which ground winds flow towards the region of maximum energy influx is the ITCZ and because the zone of maximum solar energy influx wanders, the ITCZ flows.
The zone is fairly wide so the regions around the equator sees the effects of tropical precipitation more or less without seasonality, actually two rain periods and two drier periods as the ITCZ moves once to the north and once to the south over the region each year. Away from the equator the resulting period of precipitation becomes more and more marked as one moves to the north and south resulting in strongly seasonal rains.
This movement of the large scale circulation on the earth produces several associated effects and the different monsoons are such phenomena. There are several monsoons on the earth created by weather phenomena typical for the specific region.
The Asian monsoon is perhaps the most known. The specific conditions leading up to this monsoon is found in the high elevation Tibetan plateau. During the northern winter the plateau sees a larger high pressure forming which generates winds flowing south over India and causing a dry climate. During the northern summer large parts are heated generating a low pressure over land which draws in moist air from the sea yielding strong precipitation.
Hence the monsoon follows the same seasonality as is seen in the wander of the ITCZ but the cause for the regional change in seasonal wind and precipitation pattern has a different cause. So to understand the large scale weather systems and their seasonal patterns, you must start by understanding how solar energy drives the atmospheric circulation.
This causes a specific and typical pattern which is locally disrupted by the distribution of land and oceans on the earth. Land masses can be heated more and more quickly than oceans on a seasonal basis, land masses also includes topography that can impede the atmospheric circulation, so it it perhaps obvious that land masses contribute on regional and local scales. Winds blow from areas of high pressure cold air masses towards areas of low pressure warm air masses.
The difference between the ITCZ and the Monsoon Trough is simply the direction of the winds from both hemispheres when they meet. During the northern hemisphere summer, the winds from the northern hemisphere sometimes change direction from NE to SW. When this occurs you have a Monsoon Trough.
Due to the new movement of winds they favor the genesis of vorticity around very low pressures which in turn could form more easily tropical cyclones. Seasons are the result of this.
ITCZ moves toward the hemisphere with most heat, wich are either hemisphere summers. According to me ITCZ is basically the trough where both the north eastern and south western winds converge or meet.
The air cools and rises see image below , causing water vapor to be "squeezed" out as rain, resulting in a band of heavy precipitation around the globe. This reliable circulation feeds the lush rain forests of central Africa, and also defines the limits of the Sahara desert. The ITCZ has been called the doldrums by sailors because there is essentially no horizontal air movement, that is, no wind the air simply rises.
Hadley Circulation. The position of the ITCZ varies predictably throughout the year. Although it remains near the equator, the ITCZ moves farther north or south over land than over the oceans because it is drawn toward areas of the warmest surface temperatures.
It moves toward the Southern Hemisphere from September through February and reverses direction in preparation for Northern Hemisphere Summer. Thus the position and migration of the ITCZ are important in defining the Earth's climate on a global scale. So how does it work in Africa? The ITCZ migrates latitudinally on a seasonal basis. The most important consequence of this shifting is the annual alteration of wet and dry seasons in tropical Africa.
Annual movement of the intertropical convergence zone ITCZ. But these short lived storms can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed one inch per hour. The ITCZ follows the sun in that the position varies seasonally. It moves north in the Northern Hemisphere summer and south in the Northern Hemisphere winter. Therefore, the ITCZ is responsible for the wet and dry seasons in the tropics. The sun crosses the equator twice a year in March and September, and consequently makes for two wet seasons each year.
In December and July, when the sun is at its greatest extent north or south of the equator makes for two dry seasons. Further away from the equator, the two wet seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season.
Because of its location just north of the equator, Nigeria's climate is characterized by the hot and wet conditions associated with the movement of the Inter-Tropical Convergence Zone ITCZ north and south of the equator.
This is easily seen in the normal monthly rainfall for two cities, Kano and Lagos, separated by miles km. When the ITCZ is to the south of the equator, the north-east winds prevail over Nigeria, producing the dry-season conditions.
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