What is the difference between true wind and apparent wind?

Apparent wind is the airflow measured on a moving platform, combining the natural wind with the motion of the vehicle or vessel. True wind represents the actual atmospheric wind after the platform’s speed and direction have been taken into account. Applications such as wind-assisted propulsion, sailing and autonomous vessels rely on accurate true wind calculations to optimise performance and operational efficiency.

Why the two diverge

An anemometer measures the air moving past it. On a fixed mast those are the same thing. On anything that moves, the instrument is measuring the vector sum of the atmospheric wind and the platform’s own motion through the air.

A vessel steaming at 15 knots into a 10 knot headwind measures 25 knots. Turn 180 degrees and the same conditions measure 5 knots from astern. Neither reading is wrong, and neither is the wind.

What a true wind calculation needs

Three inputs, and the calculation is only as good as the weakest of them.

• Apparent wind speed and direction, from the anemometer, relative to the platform.
• Heading, so the relative direction can be resolved to a compass bearing. On a moving platform this needs a compass, not a fixed installation offset.
• Platform velocity over ground or through the water, from GPS or the log. Which one you use changes the answer where there is current.

Over ground against through the water is the distinction that catches people out. Using speed over ground gives true wind relative to the ground. Using speed through the water gives wind relative to the water. In a current those are different answers, and which one you want depends on whether you are reporting meteorology or trimming a sail.

Where the compass has to come from

This is the practical part of the specification. A sensor on a moving platform needs to know its own orientation continuously, so a station intended for that job carries a compass.

For a moving platform that also needs extreme wind speed or a formal marine approval, the wind element is a WindObserver job rather than a MaxiMet, with heading supplied by the vessel’s own systems.

Getting it right in the data

• State which one you are logging. A column labelled only “wind speed” on a moving platform is ambiguous, and the ambiguity survives into the analysis.
• Log both where you can. Apparent wind is the measurement, true wind is a derived value, and keeping the raw measurement means a later correction to the heading or speed input can be applied without re-instrumenting.
• Check the sign convention on the heading input before commissioning, not after.