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Hpw wind is measured at sea; some warnings for the sailor when estimating the wind.


Over the years I have met and talked to sailors trying to use hand held anemometers. They sometimes seem confused by the instructions with such instruments referring to the need to correct the measured wind to a wind at 10 metres up. This confusion is compounded because they know that the gradient wind - the wind given using geostrophic scales on weather maps - refers to the wind above the friction level. That is at about 1000 metres above sea level.

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Standardised Observations

Why do we need the Beaufort Scale ? First, let me make some points about wind and its measurement/observation.

The World Met Organisation has standardised all measurement techniques (actually, this began back in 1854!). For wind it has been agreed that measurements for "official" purposes should refer to a height of 10 metres above the ground or be at a level that compares with the 10 metres norm. For, example in the case of measuring in a city centre or a forest, an anemometer at a height of 10 metres above ground would only tell you about the wind at that spot among the trees or buildings. It would not be a value that could be compared with a measurement from a nearby airfield or a ship just off the coast. A measurement from a rooftop site has to be evaluated over a period of time and the readings corrected

The whole object is so that meteorologists can readily compare observations from whatever source. For the same reasons surface pressures are all corrected to sea level, temperatures are measured in screens at a standard height above ground and so on. Another standardisation for wind measurement is the averaging time. As we all know, wind is very variable. There are short period gusts lasting a few seconds. There are longer period variations of several minutes. Then there are the longer period changes due to changing weather patterns and diurnal variation. For weather forecasting purposes, wind speeds refer to a 10 minute mean.

Gradient and Surface Winds

Weather forecasters, and their numerical models, usually derive what is known as the gradient wind. That is the wind produced by the spacing and orientation of the surface isobars but not affected by surface friction and the effects of trees, buildings, cliffs and other obstructions. Effectively, this wind refers to a height of about 1000 metres up. Forecasters and the models can estimate the actual surface wind speed and direction in broad general terms.

In principle, they can do this for individual streets and valleys over wide areas, but the amount of data produced and computing power needed would be of astronomic magnitudes.When producing forecasts for use at sea the winds given refer to the best estimate of the surface level wind as it affects the user. The fact that isobars indicate wind speed and direction at 1000 metres is irrelevant. The forecaster (or the mathematical models) make allowance for the effects of surface friction effects.

How do you Measure the Wind at Sea

The two usual methods on a yacht are a masthead anemometer or a handheld one. If you can mount one, then a mast head instrument is better in that you can watch the dial readings and form a mental average of the wind over a longer period than with the hand held. Remember that wind is constantly varying. Although gusts are important, especially to small yachts and dinghies, it is the average wind that the cruising yacht will sail by and it is the average wind that will be related to the sea state. A meaningful average would have to be over about 10 minutes. How strong is your arm? How far can you get the handheld anemometer away from the yacht so that you are measuring the wind undisturbed by the yacht?

The masthead anemometer is, less affected by the presence of the yacht itself. How long is your arm? Whichever method you use, remember that the reading of wind, masthead or handheld has to be taken in a manner to minimise the effects of the yacht itself and its movement. Is it necessary to correct for the height of the anemometer? Over the sea, bearing in mind its smoothness relative to a typical land surface, I doubt whether it is necessary to correct a wind measurement at 2 or 3 metres or so above the water to the standard 10 metre height. Any correction would depend upon how rough the sea is and the temperature structure of the air. These will be complicated by the movement of your vessel including its rolling and pitching. There must be differences between a good handheld reading and the masthead but I doubt if these are sufficiently significant, or quantifiable, in practical terms to worry about.

Should we Measure or Estimate the Wind at Sea?

It is very difficult to measure the wind from a large ship. This is not just because of the movement of the ship. Obviously, that can be allowed for, easily, using a triangle of velocities. The problem on board ship is the effect of the superstructure. The same is true for a small yacht. The traditional way in which an officer-of-the-watch observes the wind is by observing the sea state. I guess that he also notes the motion of the ship. This may sound anachronistic - back to Beaufort - type of thinking. However, it does work. Trials conducted under WMO guidance some few years ago came, basically, to the same conclusion. There are definite advantages to this. For example, the sea state results form the average wind and not the gusts or other short period variations. Using sea state ensures that an average, representative wind is reported. Wind measurement off a very large, ocean going ship would be difficult to compare with that from a small coaster.

Using sea state produces values that are reasonably consistent from one ship to the next. Statistical analysis has verified that the data have a high degree of consistency over very many years. That is why, in my various writings, I always recommend that yachtsmen use the same technique. In other words, learn to recognize the wind force from the sea state and your own yacht handling. The Met Office used to (and probably still does) sell very good photographs of sea state at various forces. On our boat, we use that method while keeping a weather eye on the masthead indicator of relative wind. In my mind's eye I have the approximate correction factors to allow for speed and direction. You can easily set up a spreadsheet to evaluate these (use the cosine rule). If you have a VMG indicator as well as an anemometer, then to get the true wind speed, add the relative wind to the VMG. When going upwind with an indicated force 6 over the dexk the true wind may only be a high force 4. Going down wind at a nice 7 knots and a wind over the deck of 16 knots you will have a true wind of force 6. Thaqt is windy for me to drop the spainnaker!