The weather can be a sailor’s best friend, or worst enemy. Learning how to predict what kind of mood it is in is a vital skill, and you will need to know about this not just for safe and pleasant sailing, but also if you intend to take a Yachtmaster exam. This article is the first in a series of three about the weather and sailing, with a tutorial of highs and lows, and another on local weather systems, to be published in the coming weeks.
Meteorology is a huge and fascinating subject. If you want to really get into the details, we’d recommend the book Modern Marine Weather, and the associated workbook. In this article, we are going to look at some of the basics of global weather systems.
The coriolis effect
Warm air rises, cold air falls. If the Earth were not spinning, we’d see a constant and predictable pattern of air circulation from the equator to each pole and back again. Because the Earth is spinning, the air is pulled off in different directions. Air moving from the equator to either pole is dragged east, and air coming from one of the poles to the equator is dragged west. This deflection to east or west is called the coriolis effect, and is true whether the air is moving from the equator to a pole, or vice versa.
Top tip: If you are a polar bear, you live in the north and wind is always dragged off to the right. If you are a penguin, you live in the south, where the wind is always dragged off to the left.
Prevailing winds
It is generally possible to predict the direction from which the wind will blow, on a global scale at least. The normal direction of the wind is the prevailing wind, and looks like this:
The equator is at 0 degrees latitude. The area about 5 degrees north and south of the equator (though this does move about) is the intertropical convergence zone, also called the doldrums – where sailors can be stranded for days or weeks with no wind. Running towards the equator, north and south, are the trade winds, which facilitated the spread of commerce in the age of sail. Then come the horse latitudes, generally calm and dry, about 30 degrees latitude, north and south. Why are they called the horse latitudes? Who knows. Perhaps this is where long-distance sailors of old finally had to eat the horses.
For round the world sailors, anything more than 30 degrees north or south can mean a serious challenge and ferocious seas. In these places, we talk of the roaring 40s, the furious 50s and the screaming 60s – terms which relate to the kinds of wind that can be expected.
Two of the cells mentioned in the diagram are more or less closed circulation systems. The band indicated by the Hadley cells, on either side of the equator, is responsible for the trade winds. In the areas covered by the Polar cells, conditions can be expected to be more or less stable. The Ferrel cells are influenced by the Hadley cells and Polar cells, so these are areas in which sudden changes of wind direction might be expected.
Top tip: the wind direction is the direction FROM which it is coming, not TO which it is blowing.
Hurricanes, typhoons and cyclones
What’s the difference between a hurricane, a typhoon and a cyclone? The answer is, nothing. They are different names given to tropical revolving storms in different parts of the world. If it’s in the Atlantic, north-east Pacific, or round Australia, Fiji, Samoa or New Zealand, it’s a hurricane. If it’s in the north-west Pacific, it’s a typhoon. And if it’s in the Indian Ocean, it’s a cyclone.
Hurricane season in the northern hemisphere generally runs from June to November in the Atlantic, with August, September and October being the months of highest activity. Hurricanes may begin earlier in the north-east Pacific, starting in May and being most intense in July, August and September. In the southern hemisphere, hurricanes can be expected around west, north and north-west Australia from November to April, and will be most likely between January and March.
Typhoons (north-west Pacific) can be expected from April to December, and are most likely from July to September.
You might expect a cyclone in the north Indian Ocean from March to December, mostly in May, June, October and November. In the south Indian Ocean, cyclones are normally from November to May, peaking in January, February and March.
How to predict a hurricane, typhoon or cyclone…
Aside from modern weather forecasting systems, observing conditions and keeping a record in your log book can help you predict and avoid a tropical revolving storm. If you’re in the north, put your back to the wind, and the low (that’s the dangerous bit) is to your left, with the wind flowing anti-clockwise around it. Also observe the direction of swell, which is likely to pick up as a storm approaches. Cloud cover will thicken and lower the closer the storm comes. Keep an eye on the barometer, too. If the pressure drops consistently below the tropical (1012mb) or sub-tropical (1020mb), you’re in for trouble. As a rule of thumb, if the wind direction is stable but the speed increases and the pressure falls off quickly, you’re directly ahead of a storm.
… and how to avoid one
Storms are sometimes unavoidable, so what are you going to do if you suspect that one is on the way? In terms of navigation, it’s important to keep a careful eye on the wind. In the northern hemisphere, you want to be in a position where the wind is backing over time – and you definitely don’t want to see it veering constantly. The reverse is true in the southern hemisphere. The idea is that, if you know where the storm is, you can take action to avoid it. You will need to adjust course to ensure that you stay in the “safe” (less dangerous, navigable) semi-circle. At all costs, you need to avoid the dangerous quadrant, because this is where the storm is going to go if it changes course.
If you are caught in a tropical revolving storm…
Sailors have different ideas about what to do in a storm. For example, will you heave to or run on bare poles? Decisions often depend on the type of boat, and the experience and condition of the skipper and crew. Most, though, agree that there are certain things that you can do to improve your chances.
Among these, shorten sail in good time, get the storm jib up before you need it, avoid being hit by huge waves beam on, and, above all, run away (but avoid being trapped on a lee shore). Tom Cunliffe’s book on storm tactics is a must-read in this respect, and if you want to get deeper into the subject, try Adlard Coles’ Heavy Weather Sailing.
On a final note, it’s worth remembering an old saying: there are no atheists on the battlefield or in a storm at sea…