More than any other continent, Africa is swept by the back-and-forth migration of meteorology's equator. Africans watch the globe-straddling intertropical convergence zone (ITCZ) as intently as a day trader watches stocks. As the ITCZ sloshes north and south over its annual cycle, it brings much of Africa its only period of annual rainfall. The dry-season/wet-season rhythm flavours almost every aspect of life across the heart of the continent. In Botswana, the word for rain, pula, is also the nation's motto, the name of its currency and an everyday greeting.
The rainfall contrasts across this continent are stunning. Parts of the Sahara see less than 5mm/0.2in of rain a year – barely enough to cover the bottom of a glass – while Reunion Island, off Madagascar, holds the world record for the most rain observed in a 24-hour period, a mind-boggling 1825mm/72 inches. In general, the wet and dry seasons and locations are fairly well-defined across Africa. Thus, it's possible for a visitor to target, say, the lushness after weeks of rain have ended, or the tail end of a dry period, when more places may be accessible than after the rains begin.
Nowhere is the contrast between dry and wet more acute than through the middle of West Africa. The parched Sahara and the lush Guinea coast run side by side along an east-to-west band more than 1600km/1000 miles long. This stretch includes the semi-arid, drought-prone Sahel, whose name derives from an Arabic term for border. During the northern spring, the ITCZ slowly migrates into the Sahel and toward the Sahara. The northernmost penetration of the ITCZ varies from year to year, and the timing and length of its visit can make the difference between life or death for herders and farmers who work this unforgiving land. From July through September, dramatic bands of thunderstorms sweep west along the ITCZ in waves that sometimes form Atlantic hurricanes. By August, the ITCZ is typically so far inland that the Guinea coast sees a relative lull in rainfall before the “little rains” return. The ITCZ's push south is faster than its movement north, thus making the little rains briefer than the main wet season.
Further north is a kingdom of sand, sun and heat. The Sahara reigns as the world's largest desert, but the air isn't completely dry. Even in summer, there's more moisture per volume of air than you might find in the middle of a Siberian snowstorm. However, the Sahara's temperatures are so scorchingly high that the relative humidities drop toward the single digits, and what paltry rain does manage to develop often evaporates as virga – “ghost rain” – before it reaches the ground. Thanks to the low humidity, Sahara nights can be surprisingly cool, especially in December and January. Winter is also the season for the harmattan, a northeast wind that dominates the Sahel.
On the narrow south end of Africa, near 30°S, is a much smaller counterpart to the Sahara, the Kalahari Desert. Between these dry zones is the dampest part of mainland Africa, the forested highlands and valleys of the Congo Basin. The Great Rift Valley separates the Congo's year-round rain from the often tortuous wets and drys of eastern Africa. The average rainfall from Tanzania to Ethiopia is sufficient for farming, but this obscures a marked year-to-year variability – due in part to El Nino and La Nina – that leaves the area vulnerable to wrenching drought.
Both the north and south ends of Africa extend into the mid-latitudes, where you'll find some of the continent's most varied weather. The Atlas Mountains accentuate the divide between the Sahara and the much cooler Mediterranean climate of northern Morocco and Algeria. Fierce spring sandstorms blow off the Sahara, and the tail ends of winter storms crossing Europe and Asia can bring blustery coolness and a touch of rain to the coast. At the continent's other end, the Drakensbergs help produce a smorgasbord of micro-climates across South Africa.