Primaterra

Air movements

When we are sailing, the wind is a determinating factor. It is created by the movement of air masses, by warm currents moving upwards and cold currents taking their place, and the obstacles in its way.

The wind blows where there are pressure and/or temperature differences, and always goes from high to low pressures, like a tyre deflating. If the Earth wasn’t rotating, the air would move from low pressure to high pressure (anticyclone) in a straight line. But as the planet is rotating, the air is deviated, and its path is altered to the right in the Northern hemisphere, and to the left in the Southern hemisphere. This is called the Coriolis effect. We can then deduce that to the north of the equator, the wind will rotate clockwise around an anticyclone and anticlockwise around a low pressure. This is what Buys-Ballot explains in a nutshell : If you are facing into the wind, the high is to your left and the low to your right. Of course, the situation is opposite in the Southern hemisphere. The lines we can see on Fig. 1 are called isobars. They join points of equal pressure at sea level. The higher the pressure differential (the closer concentric lines are to each other), the stronger the wind will be in that area.

The image on the left should give you a feel for what isobars really look like. It is provided courtesy of Met Eirann, the Irish weather service. This image is updated everyday, at the same time as they update it on their server. You can check back here often to see what the weather is like in Ireland, or you can go directly to Met Eirann for a more detailed weather report.

Winds are linked to the global circulation system of the atmosphere, which is caused by the temperature difference between the equator and the poles. As can be seen from Fig 2, the equatorial regions get more concentrated solar radiations, raising the temperature, while the solar energy in higher latitudes is distributed over a larger area. This creates a convection effect which tends to even out temperatures over the globe and creates several main zones were the winds are fairly constant. These are as shown in Fig. 3 below.

The sailor must develop aptitudes to predict the weather at sea by associating the strength and direction of the wind with other meteorological signs. With a bit of experience, and paying attention to the smoke, the state of the sea, etc, it will become easy to evaluate the wind strength and pronosticate how it might evolve.

The wind and its characteristics

Wind strength

The wind speed is generally measured using an anemometre. However, this measurement is useless if it is not put into context. A instantaneous wind speed measurement, however precise, does not tell us much if we do not know how much sail to carry, the sea state, etc... Generally, the course of action taken is to work by feel. For example, the same wind strength will look much more managable on a smooth sea than if the waves are five metres high. Not only this, but the boat will actually be able to carry more sail on a calm sea, as the strain from the waves will disappear. We also have to note that the actual wind strength felt on the boat will be very different sailing upwind or downwind, as can be seen in our Theory pages.

It is worth saying as well that the power of the wind increases by the square of its speed, therefore a doubling of wind speed will entail a quadrupling of its power.

In 1805, a Royal Navy admiral, Sir Francis Beaufort (1774-1857) devised a wind speed scale based on the effect that a particular wind strength would have on his ship. Take a look at the Beaufort scale

Effects of the wind

Waves

No wind, no waves... The wind is the main cause of waves. Depending on its direction, strength and the time it blows for, it creates waves, which can become very rough. The height of waves thus depends of three factors: The wind speed, the fetch and the duration. The fetch is the distance

on which the wind blows from the same direction. The larger the fetch, the bigger the waves, everything else being equal. It will also make the waves longer. In a sheltered location, or if the wind is blowing from the shore, the waves will be much smaller, but also shorter, which can make them quite steep. In

open sea, the wind creates a swell, very long waves that easily cross oceans. The waves are generally fairly smooth and not dangerous, but can become so if they get close to a sudden reduction in depth, such as a reef, as they will start breaking. Also if the wind changes direction, it will give rise to a second swell, crossing the first one at an angle, which can create dangerous breaking waves. The table that follows gives the wave heights that can be expected with a given wind speed in open sea (infinite fetch).

Wave parameters for a developped sea state and a constant wind strength.

Wind speed (km/h) Medium height (m) Medium length (m) Medium period (s) 20 0.33 10.6 3.2 30 0.88 22.2 4.6 40 1.8 39.7 6.2 50 3.2 61.8 7.7 60 5.1 89.2 9.1 70 7.4 121.4 10.8 80 10.3 158.6 12.4 90 13.9 201.6 13.9

Water accumulation

When strong winds are blowing, the water is pushed downwind and accumulates there. So if the wind is blowing towards the coast, the sea level will go up. This effect can also accentuate high tides and make them come earlier than expected. It should also be noted that low pressures have a similar in that they allow the sea level to rise, the pressure on top of it being lower. This can become dangerous in periods of springs tides, where the sea can go on top of keys or breakwaters.

Wind and current

If the wind is blowing against the tide, the waves will get higher (50 to 100%) and shorter, sometimes breaking. The sea can become dangerous for the boat in very little time, or even just going around a headland. A particular occurence of this was when I was sailing happily down the west coast of ireland, and just when rounding Mizzen head, we got out of its shelter, and the sea immedaitely became fairly high and very confused. We had to reduce sail and speed quite dramatically (one reef and change down one jib) to try and keep the situation bearable for the boat.

Local weather

The moving air is displaced around mountains, valleys islands and capes. depending on the nature of such obstacles, the strength and direction of the wind can be modified. Some winds only blow locally, and have acquired specific names, such as the Mistral in the south of France or the Scirrocco in the Mediterranean.

Probably the easiest local winds to recognise are the thermal breezes. These are caled sea or land breeze, as they blow from the sea or from the land, respectively. They are caused by the temperature difference between the sea and the land at different times of day. The figures below show the basic principle. During a sunny day, the land is warmer than the sea, so the air above it rises, and the air above the sea falls. This creates a current from the land to sea in altitude and a current from the sea to the land at sea level. The opposite happens at night, when the land is colder than the sea. This effect is due to the huge thermal mass of the ocean, which means that it will tend to keep a stable temperature, whereas the land will experience much more sudden changes of temperature.

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