WEATHER BRIEFING

CHRIS TIBBS ON NAVIGATING THROUGH THE DOLDRUMS

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Doldrums': a state of stagnation or depression, or an equatorial region of the Atlantic Ocean with calms, sudden storms, and light unpredictable winds. Or so my dictionary puts it.

For sailors, the transition from the northern to southern hemispheres in the Atlantic can be a trying and often frustrating time. We leave the steady tradewinds of the northern hemisphere with our main aim to minimize the time taken before we progress into the South Atlantic trade wind belt - this is passing through the doldrums, or the 'Pot-au-Noir', as the French put it.

In meteorology, we now use the more scientific term Inter Tropical Convergence Zone (ITCZ) where we can meet conditions we associate with the doldrums; calms, squalls, and thunderstorms. The ITCZ can be thought of as the thermal equator of the world that separates the northern and southern weather systems.

WHY THEY'RE TRICKY

The term ITCZ describes why we get the conditions we know as the doldrums. The tradewinds of the northern hemisphere, driven by the Azores High, are east- to northeasterlies, tending to be more easterly as we get further south towards the ITCZ. Meanwhile the tradewinds of the southern hemisphere mirror these, being driven by the South Atlantic High (often referred to as the St Helena High) and are east- to south-easterlies. Where they come together is the ITCZ.

Where we get convergence in airstreams we get an ascent of air: as the air rises clouds form and as this region is very warm and the air humid, when the air is forced upwards large cumulonimbus clouds form giving squalls and thunderstorms. In addition, where we get ascent of air, we get a lowering of the surface pressure so between the winds of the two hemispheres we get a trough of lower pressure.