🌬️ Apparent Wind & Wind Chill
The forecast describes a weather station standing still on land. You are not. Work out the wind your own boat speed creates, what it does to the temperature you actually feel, and dress for that instead.
The forecast is not the deck
The wind you feel is the vector sum of the true wind and your own motion through it. Sail towards the wind and your speed is added to it; sail away and it is taken off. Beating out at six knots into a fifteen-knot forecast puts roughly twenty-one knots across the deck. Turn round and run home at the same speed in the same breeze and it is about nine.
Same day. Same forecast. More than twice the wind on one leg as the other — which is why the crew who were perfectly comfortable running downwind are cold and wet an hour later, and why the layer that came off should not have.
Then the temperature. Below ten degrees the standard wind chill index describes how much faster moving air takes heat off exposed skin, and at twenty knots the difference is several degrees rather than a rounding error. Above ten degrees the index is not defined at all, and this page says so rather than inventing a number — wind still cools you up there, but by evaporation, which is a different thing and not what the formula measures.
One more that no index covers: spray. Water carries heat away from the body about twenty-five times faster than still air, so on a wet day the waterproof layer matters more than the warm one. A soaked fleece is barely a fleece.
❓ Frequently Asked Questions
Why am I always colder on the boat than the forecast said?
Two reasons, and they compound. The first is apparent wind: the breeze you feel is the vector sum of the true wind and your own motion through it, so sailing upwind at six knots into a fifteen-knot forecast puts about twenty-one knots across the deck. The second is that wind removes heat far faster than still air of the same temperature. Put them together and a mild-sounding day at eight degrees can feel like three or four, which is the difference between a fleece and a proper jacket.
What is apparent wind, exactly?
It is the wind an instrument on your boat measures, as opposed to the wind a weather station on shore measures. Because you are moving, you add your own motion to the air's — the same reason a still day feels breezy from a bicycle. Sailing towards the wind adds your speed to it; sailing away subtracts it. The apparent wind also comes from further forward than the true wind does, which is why spray reaches you on a reach and rarely on a run, and it is why the same fifteen knots is a completely different afternoon depending on which way you are pointing.
Does downwind really feel that much gentler?
Yes, and it is the most common way people get caught out. Running downwind at six knots in fifteen knots of true wind leaves about nine knots across the deck — barely more than half of what you felt beating out. The boat feels calm, the noise drops, and everyone takes a layer off. The wind has not changed at all, and the moment you turn round to go home you are back in twenty-one knots, now with a cold crew. Planning the return leg is the useful habit here.
When does the wind chill figure stop applying?
The standard index is only defined at or below 10°C and above about 4.8 km/h of wind, and outside that range it still produces numbers — wrong ones that look plausible. So above 10°C this tool shows the air temperature and says that no wind chill figure applies. That does not mean wind stops mattering: above ten degrees it cools you mainly by evaporating moisture from skin and damp clothing rather than by conduction, which is exactly why a windproof layer is worth carrying on a warm, breezy day.
What difference does spray make?
More than the numbers suggest, because it is a different mechanism. Wind chill describes dry skin losing heat to moving air. Water carries heat away from the body roughly twenty-five times faster than still air does, so wet clothing is not a slightly worse version of dry clothing — it is a different situation. This is why the order of priorities on a cold, wet day is waterproof first and warm second: an excellent insulating layer that is soaked is doing very little, and the shell that keeps it dry is doing most of the work.
Are these figures exact?
The apparent wind is, within the accuracy of your inputs — it is a vector sum and nothing more. The wind chill is a standard index rather than a measurement of you: it assumes an average adult, walking pace, dry skin, and a face exposed to the wind, and real people vary with fitness, food, hydration and how long they have been out. Treat the felt temperature as a much better guide than the air temperature, which is what it is, rather than as a promise.