What True Wind Is (vs. Apparent Wind)
True Wind represents the actual speed and direction of the wind relative to the fixed surface of the earth. In contrast, Apparent Wind is the wind you feel on the deck of a moving ship.
When a vessel moves forward, it creates its own "headwind." The anemometer on the ship measures the vector sum of this created wind and the actual True Wind. This combined vector is the Apparent Wind Speed (AWS) and Apparent Wind Angle (AWA). To find the true weather conditions, mariners must subtract the vessel's Speed Over Ground (SOG) vector from the apparent wind vector.
How the Calculation Works
To compute the True Wind Speed (TWS) and True Wind Direction (TWD), we solve the Wind Triangle using the Law of Cosines. The formula requires the vessel's Heading, Speed Over Ground (SOG), Apparent Wind Angle (AWA), and Apparent Wind Speed (AWS).
The general formula for True Wind Speed is:TWS = √(SOG² + AWS² − 2 × SOG × AWS × cos(AWA))
Once TWS is known, the True Wind Angle (TWA) is derived, and then applied to the ship's Heading to determine the true compass bearing (degrees true). Unlike some simpler tools, this calculator outputs the true compass bearing rather than just an angle relative to the bow, giving you a value you can directly log.
Worked Example
Let's say your ship is heading North (Heading = 000°) with a Speed Over Ground (SOG) of 10 knots. Your anemometer reads an Apparent Wind Speed (AWS) of 15 knots, coming from 45° on the Starboard bow (AWA).
- Calculate TWS: √(10² + 15² − 2 × 10 × 15 × cos(45°)) ≈ 10.6 knots.
- Calculate TWA: Using the cosine rule backwards, the internal angle is approximately 93.3°.
- Calculate TWD: Since the wind is on the starboard side, TWD = Heading − TWA + 180° = 000 − 93.3 + 180 = 086.7° True.
The true wind is blowing from roughly East-Southeast (086.7°) at 10.6 knots, which corresponds to Force 3 (Gentle Breeze) on the Beaufort Scale.
When Mariners Use This
- Weather Routing: Cross-checking anemometer readings against weather forecasts to verify if the ship is encountering the expected weather patterns.
- Helicopter Deck Operations: Aviation operations require precise true wind parameters to establish safe approach sectors.
- Anchor Watch: Monitoring wind shifts to anticipate if the vessel might drag anchor.
- Voyage Log Entries: Standardizing deck logbook entries to record true environmental conditions in standard units like knots, m/s, or mph.
FAQ
What's the difference between true and apparent wind?
True wind is the actual movement of air relative to the fixed surface of the earth. Apparent wind is the wind experienced on a moving vessel, which combines the true wind and the wind created by the vessel's own forward motion.
Why does my anemometer reading not match the forecast?
An anemometer on a ship measures Apparent Wind Speed (AWS) and Apparent Wind Angle (AWA). Forecasts provide True Wind. You must use a True Wind Calculator to convert your apparent readings using your Speed Over Ground (SOG) and Heading to compare it with the forecast.
Does this calculation work without a GPS?
Yes, if you use your ship's log speed (Speed Through Water) instead of Speed Over Ground (SOG). However, this will give you the wind relative to the water, rather than the true wind relative to the earth's surface.
Does current or tide affect the result?
This calculator assumes Course Over Ground (COG) equals Heading. If there is a strong current or tidal set, the true wind computed using SOG and Heading will slightly differ from the absolute true wind relative to the seabed.