True and apparent wind: the difference explained simply
as of 07/10/2026 · 5 min read
In short: Apparent wind is the wind on board, i.e. the true wind plus the wind from the boat's own speed: stronger than the true wind upwind, weaker downwind.
True wind is the wind measured by an observer who is not moving. Apparent wind is the wind you feel on board a moving boat: the sum of the true wind and the wind from the boat's own speed. Upwind it is stronger and comes from further ahead, downwind it is weaker. You trim the sails to the apparent wind.
What is true wind?
True wind is the movement of the air relative to the earth's surface, as measured by an anemometer ashore or on a boat at anchor. This is the wind given in weather forecasts, marine forecasts and wind maps.
It has a direction it blows from (TWD, True Wind Direction, as a compass direction) and a strength (TWS, True Wind Speed). Its angle to your boat's course is called TWA (True Wind Angle). Strictly speaking, onboard instruments calculate the true wind relative to the water, because they take the speed through the water from the log. In a current it therefore differs slightly from the wind over ground that the forecast refers to.
What is boat-speed wind?
Boat-speed wind is the wind your own movement creates: as strong as your speed and coming from dead ahead. If you motor at 6 kn in a flat calm, you feel 6 kn of wind from ahead.
How does apparent wind arise?
Apparent wind is the vector sum of the true wind and the boat-speed wind. Picture it as a triangle: one side is the true wind, the second is the boat-speed wind from ahead, and the third closes the triangle and is the apparent wind.
Three rules follow from this. The closer your course to the wind, the more the two add up and the stronger the apparent wind. The broader your course, the more the boat-speed wind takes away and the weaker the apparent wind. And except when running dead downwind, the apparent wind always comes from further ahead than the true wind. The example uses 15 kn of true wind and assumes the same 6 kn of boat speed on every course:
| Point of sail | True wind angle (TWA) | Apparent wind angle (AWA) | Apparent wind speed (AWS) |
|---|---|---|---|
| Close-hauled | 45 degrees | 33 degrees | 19.7 kn |
| Close reach | 60 degrees | 44 degrees | 18.7 kn |
| Beam reach | 90 degrees | 68 degrees | 16.2 kn |
| Broad reach | 135 degrees | 113 degrees | 11.6 kn |
| Running | 180 degrees | 180 degrees | 9.0 kn |
Why is the apparent wind stronger upwind?
Because the boat-speed wind then comes from almost the same side as the true wind and adds to it. In the example, close-hauled you feel almost 20 kn although only 15 kn is blowing.
Downwind it is the other way round: the boat-speed wind is subtracted, and on board 15 kn feels like 9 kn. That is the classic trap on long downwind legs. The boat runs smoothly, the cockpit stays dry, and only when you luff up, for example to take down sail or off the harbour entrance, is there suddenly more than twice as much wind. So reef according to the true wind, not according to how it feels; see When to reef?.
What do AWA, AWS, TWA and TWS show on the instrument?
AWA and AWS are the angle and strength of the apparent wind, measured by the wind transducer at the masthead. TWA, TWS and TWD are the true wind, which the system calculates from the apparent wind, the speed through the water and the heading.
- AWA (Apparent Wind Angle): angle of the apparent wind to the bow, 0 to 180 degrees to starboard or port.
- AWS (Apparent Wind Speed): strength of the apparent wind.
- TWA (True Wind Angle): angle of the true wind to the bow.
- TWS (True Wind Speed): strength of the true wind.
- TWD (True Wind Direction): direction the true wind is blowing from, as a compass bearing.
The calculated true wind is only as good as the log and compass. If TWS jumps after every tack or TWD turns with the bow, the log is usually miscalibrated. Wind indicators and telltales always show the apparent wind.
Why do weather forecasts give the true wind?
Because the forecast has to apply to every boat, regardless of course and speed. It gives the true wind over ground, usually as the mean wind at 10 m height, plus the gusts.
When you compare the forecast with your instruments, use TWS and TWD, not the apparent wind. The wind transducer is rarely at exactly 10 m height, and higher up it blows a little harder. For planning, the true wind is what counts: together with your course it gives your point of sail.
How does Sailseer help with true and apparent wind?
Sailseer plans with the true wind from the forecast and shows, for each leg, the true wind angle, the point of sail and the apparent wind calculated from them; boat speed comes from a polar diagram. Under way, Sailseer reads your boat's data under Sail > Boat instruments. If the boat only sends the apparent wind, Sailseer calculates the true wind from AWA, AWS, speed and course, passes it to the tacking aid as a two-minute average and shows the forecast at the boat's position alongside for comparison. To the map, more on beating to windward. Not for navigation: official charts and weather reports take precedence.
Frequently asked questions
What is the difference between true wind and apparent wind?
True wind blows independently of the boat; apparent wind is the wind you feel on board: true wind plus boat-speed wind. Only when the boat is not moving are the two the same.
Does the wind indicator show true or apparent wind?
Apparent wind. Masthead wind indicators, Windex and telltales turn with the wind the moving boat feels. Only the wind instruments show the true wind, once they calculate it from speed and course.
Is the wind in the weather forecast true or apparent wind?
True wind over ground, usually as the mean wind at 10 m height with the gusts on top. On board, compare it with TWS and TWD, not with the apparent wind.
Why does it feel like less wind downwind?
Because the boat-speed wind is subtracted from the true wind. With 15 kn of true wind and 6 kn of boat speed you only feel 9 kn when running. When you luff up, the full wind is back immediately.
What do AWA and TWA mean?
AWA (Apparent Wind Angle) is the angle of the apparent wind to the bow, TWA (True Wind Angle) that of the true wind. Upwind AWA is smaller than TWA; dead downwind both are 180 degrees.
Which wind do I trim the sails to?
To the apparent wind, because that is the only wind the sails feel. The telltales on the luff show you whether the sail is set correctly to the apparent wind.