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.
Standardised Observations
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.
Surface Winds
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) makes 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.
