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Why the Monsoon Winds Change

on Tuesday, 26 October 2010. Posted in Climate

Article 2/3: Why the East African Monsoon Winds change at the same time each year

Why the Monsoon Winds Change

The regularity and predictability of the East African monsoon winds have been the driving force behind the shaping of the East African Coast's human geography for centuries. So what explains why this regular wind pattern repeats itself in such a similar fashion around the same times of year, every year.

As described in the previous article - the period of change for the east african monsoon winds is in March and September, by no coincidence - these times of year are also the timings of the equinox - equal day and equal night over the equator.

As the earth orbits the sun along the Plane of the Ecliptic (it's orbital path) it is not angled vertically, but rather tilted at an angle of approximately 23.4 degrees, such as a spinning top angled over slightly to one side. The earth constantly remains at this angle throughout it's annual orbit. It is this tilted axis that causes the sun's position to change in the sky and hence lead to seasonal changes throughout the year.
The closest point of the earth to the sun is called the 'subsolar point', and this point moves between the tropics north and south over the equator two times each year. The northern most limit of this point is the Tropic of Cancer, the subsolar point reaches this line of latitude - 23.4 degrees north on 21/22 June, the Summer Solstice - mid summer in the northern hemisphere and the most hours of sunlight.

The subsolar point then begins moving south and reaches the equator three months later on the 22/23 September - the Autumnal Equinox.

Three months later on 21/22 December, the subsolar point has reached it's southern most limit on the Tropic of Capricorn at 23.4 degrees south, known as the Winter Solstice - mid winter and the least amount of sunlight hours in the northern hemisphere.

The subsolar point then begins to move north again back towards the equator, which it reaches three months later on 21/22 March - the Vernal Equinox.

So the reason for the regularity of the East African Monsoon winds is the same reason as the Summer and Winter seasons can easily be predicted and have shaped human behaviour in those regions where the seasons are more extreme further north and south of the tropics.

The equatorial regions do not experience Summer and Winter - but rather alternating wet and dry seasons. The next article will look into why the winds should change on the Kenyan Coast as the subsolar point passes through, and aim to delve into further detail regarding the East African Monsoon winds and the weather patterns in this region.

Summer Solstice - June 21/22

Autumnal Equinox - 22/23 Sept

Winter Solstice - Dec 21/22

Vernal Equinox - March 21/22

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