In the Doldrums (with the Inter-Tropical Convergence Zone)

Why does it rain? More to the point, why does rain fall here pretty much every day from March to May and then cease from June to September? Why do we have a long rainy season followed by a long dry season followed by a short rainy season followed by a short dry season? It’s hard for me to keep these seasons straight. Spring, summer, winter and fall are what I grew up with and sometimes in East Africa I actually miss winter. But a month (a day?!) of January in Columbus cures me of that.

Seasons here are wet or dry because we live near the equator where winds from the northern hemisphere and winds from the southern hemisphere smash headlong into each other which forces them into the sky for miles where the sun’s heat creates condensation which pours down on us as rain. This convergence of winds from north and south near the equator is called the Inter-Tropical Convergence Zone (ITCZ) and this Zone moves back and forth across the equator region every year over land and sea, all around the earth.

If you hit “play” at this nifty little website, you can see a demonstration of the ITCZ over the continent of Africa: http://people.cas.sc.edu/carbone/modules/mods4car/africa-itcz/index.html.

On land, this weather system produces good rain that fills rivers, helps farmers grow crops, washes the dust out of the sky and makes us all happy. Over oceans, the ITCZ becalms the water and strikes fear in the hearts of those in sailing vessels. Indeed, sailors caught at sea in the becalmed waters of the ITCZ long ago named it “the Doldrums” because, without wind, they were marooned and faced a thirsty death. Part of Samuel Coleridge’s scary poem, “The Rime of the Ancient Mariner” is set in the becalmed Doldrums: “Water, water everywhere, and all the boards did shrink, Water, water everywhere, and not a drop to drink.”

Right now, the ITCZ is moving slowly toward us from the Sahara and when it reaches us, as it usually does by October and November, it brings short rains that are sometimes called “mango rains” because they produce the juicy mangoes of December. When the Zone passes over us and heads on to southern Africa we’ll have a short dry season that goes into February. In southern Africa, the Zone picks up moisture and then sweeps back up over us fully recharged for the big, boisterous rains of the long rainy season in March, April, and May. After that, as the Zone works its way north again, we'll have a long dry season until it returns with the short rains of October and November.

Farmers try hard to get their crops in at the right time for both short and long rains but, despite the nifty little graphic up there, the rains aren’t so neatly predictable. When they come a bit early, or a bit late, or don’t bring the expected amount of rain, the harvest is meager and that can mean a hungry season as people reduce their food consumption in hopes that their remaining food stocks will last until the next harvest.

During the dry season, dust is a big problem partly because the earth is dry but also because many farmers burn off their fields in preparation for planting when the rains come. Burning off is a quick and labor-saving way to clear land when you have only a hoe. It puts a lot of stuff into the atmosphere, which has its drawbacks, but it gives us brilliant sunsets.

Just now, the rain began beating down on our tin roof even though it's still September. If this keeps up it means that, after several months with hardly any rain, the dust will settle, the air will clear, and the sky will turn as brilliant a blue as I have ever seen.