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The Inter-tropical Convergence Zone, usually referred to as the Doldrums by sailors – or Pot au Noir in French - is a weather zone that is extremely variable in its position from east to west and in its size from north to south. This strip located between 8° and 3° north is characterised by alternating calms and violent squalls, by its cloud cover and its high level of humidity.

 

In French, it is known as the "Black Pot" after jars found in farms at Cape Verde, which are used to store all sorts of waste.  Others say that it there that the slave ships used to throw over the sick to avoid disease spreading. In olden days, sailing ships did not have the potential of today's ocean racers and found it difficult to manoeuvre quickly and especially to beat upwind. They could spend weeks stuck in this area to the point when it was sometimes deemed necessary to throw the ship's animals overboard, as they had become crazy…
The British use the expression the « Horse Latitudes » when talking about the tropics, as it is here that captains threw horses overboard, as they consumed too much drinking water, and supplies were threatened when ships were becalmed here for days on end under a burning sun.  
Often in the Doldrums, scurvy began to decimate the crews.
Due to a lack of fresh produce, this vitamin C deficiency wiped out two thirds of Vasco da Gama's crew on his voyage to India in 1497-1498.  
Synonymous with extreme tiredness, because such effort was required to get out of this area with the crews having to row longboats in order to tow the vessels, the notorious zone of persistent calms gave rise to the expression "to be down in the doldrums."  

The shifting shape of the Doldrums

Like Proteus, the Doldrums have the knack of reappearing in an unexpected shape: tall, thin, round, cone-shaped, pear-shaped, a blob, a jellyfish, a ribbon...  Then, there is the surprise of discovering the contents: a real Pandora's box of fire, water, evil, stodge, stickiness, swampy marshland or nothingness…
Without dragging ourselves into the mental doldrums just contemplating, let us just say that this cloudy strip, officially called the Inter-tropical Convergence Zone, has that mythical ability to change shape and character.  In fact, it is the meteorological phenomenon resulting from the meeting – hence convergence – of the northern and southern trade winds. In the north they blow from the north-east and in the south from the south-east. Because of the proximity to the Equator the sea temperature  (27° to 29° C) and the air temperature (35° to 40° C) rise, which means rapid evaporation and therefore a humidity level close to 100%.  
And water vapour means clouds… The hot air climbs and the higher it gets, the more it cools off and the droplets turn to rain. So the "Black Pot" is indeed really black at times…

The unpredictable Doldrums…

And, as the northern and southern High pressure systems ‘breathe’, expanding and contracting, and the cumulus-shaped clouds move around, merging and reforming all the time, this equatorial area is forever changing.   The Doldrums are constantly metamorphosing and can pop up just where you least expect them.   It is therefore difficult for sailors to predict how far out they will stretch and how severe they will be until they tackle them.

In theory they tend to be thinnest at around 30° west and longer to get through at 20° west. It is therefore important to plan your entry into the Doldrums to use the most favourable wind with a course that is not too far west. If you do venture too far west after the ICTZ you find SE'ly winds, which require upwind sailing and which tend to take the boats off towards the coast of Brazil.  The best path to get through the Dolrdrums would appear to be at around 27° and 28° west.

The Doldrums are in fact more active, when the trade winds are strong, and stretch out further, when the sea temperature is higher: it reaches 28° to 30° off Guinea-Bissau! This heat  leads to rapid evaporation, and therefore the creation of cumulonimbus clouds, which can rise up to more than 12,000 metres in altitude… Under these clouds, it can be very turbulent and vertical currents can reach 15 to 30 m/s with some violent electrical charges; visibility is reduced, the rain is torrential, and the gusts extremely violent. There are sometimes even waterspouts (whirlwinds rising from the sea), thunder and lightning and St. Elmo's Fire (more or less continuous electrical discharges).  It is in the Doldrums that are born the tropical lows, which can become cyclones, as they move westwards…

Down in the Doldrums, down to Hell!

Up in the sky, the Doldrums are not always pretty, especially at night: thunder, lightning and rain bouncing off the deck. A leaden sky, huge temperature changes in the squalls (going from 40° to 25° in the showers), white clouds, grey clouds, jet black clouds, sometimes with a touch of pink or a green flash, as the sun sets… Winds are all over the place, varying all the time in strength and direction, going from 0 to 35 knots in a few seconds as a squall passes over. Even the best sailors admit they feel clueless at times here! You can spot the Doldrums from far away: 100 miles out, sailors can already see the huge clouds above the horizon… Usually, the Doldrums are located between 3° and 8° North from the African side right out to 35° west. But the worst thing with the Doldrums is that you just never know. They stretch out, shrink and expand again without warning: sailors know they are there, when they are there, but they never know when they will get out.  It may take a few hours or a few days...  

Dominic Bourgeois

 

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