Sunday March 20th 2011 marks the March Equinox when the subsolar point begins to move into the northern hemisphere bringing longer days to northern latitudes for the next 3 months.

On the Kenyan Coast in equatorial latitudes - the equinox also announces the arrival of the Intertropical Convergence Zone (ITCZ), which brings with it some heavy rainfall. The ITCZ generally follows a few weeks later - which is why the first rainy season of the year occurs in April and May.

As described in the previous article - the earth orbits the sun with it's axis tilted at an angle of about 23.4 degrees in relation to the Plane of the Ecliptic. The resulting effect of the sun's changing position in the sky causing the subsolar point to move between the tropics, and across the equator twice a year causing a change in the season's in the northern and southern hemisphere, and more locally in East Africa, the changing of the monsoon winds, and the alternating wet and dry seasons.

This article looks at the Kenya Coast's weather pattern in more detail and outlines a generalised climate model for this region.

The equatorial region's weather patterns are dominated by the Hadley Cell, an atmospheric circulation pattern occurring between the tropics that produce the easterly trade winds.

In the Hadley cell, air rises near the equator, flows out towards the poles at high altitude, descends back to the Earth's surface at the outer edges of the tropics, and completes the cycle by flowing back towards the equator near the earth's surface.

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 equatorial Kenya Coast has a rich and ancient history, thanks in no small part to the magnificent Indian Ocean, with its consistent and relatively predictable monsoon 'tradewinds' that have been an integral part of coastal life for the peoples of this region for many centuries. The pattern of these winds has been understood and harnessed by mariners, traders and explorers from as far off as Arabia, India and China. The word monsoon is said to originate from the Arabic name for these winds - 'muusum'.