Crosswind Arithmetic: Splitting the Wind
Wind is rarely straight up the slope. It arrives at an angle, and the angle matters as much as the speed — because what a launch cares about is not the total wind but how that wind splits into the component coming at you and the component trying to push you sideways.
Splitting the wind
It is standard vector trigonometry. Headwind is the total times the cosine of the angle off; crosswind is the total times the sine. The wind window calculator does it and compares both against general guidance bands:
| Wind | Angle off | Headwind | Crosswind |
|---|---|---|---|
| 15 km/h | 0° (straight in) | 15.00 | 0.00 |
| 15 km/h | 30° | 12.99 | 7.50 |
| 20 km/h | 20° | 18.79 | 6.84 |
| 25 km/h | 45° | 17.68 | 17.68 |
| 10 km/h | 60° | 5.00 | 8.66 |
Two things worth noticing. At 45° the wind is split exactly evenly — head and cross components are identical. And at 60° the crosswind is already larger than the headwind, so a modest 10 km/h wind well off the axis is producing more sideways than into-slope.
Why the crosswind limit bites first
Guidance bands treat total wind and crosswind separately, and the crosswind figure is the tighter of the two. A 25 km/h wind at 45° produces 17.68 km/h of crosswind, which exceeds the crosswind guidance even for an advanced pilot — while the 25 km/h total is well inside their total limit.
That is the common shape of a marginal day: the windsock looks acceptable, the number that actually matters is the one nobody quoted, and the constraint is the angle rather than the strength.
These bands are conservative and general
The calculator's limits are general recreational guidance, not a certification standard and not site rules. A 15 km/h wind straight into the slope already sits outside the beginner band, which will strike some readers as very cautious — and being outside a conservative band is information, not a verdict.
What matters far more than any published number is the site. Local wind behaviour, rotor from terrain upwind, thermal cycles, gust factor and the shape of the launch itself all change what a given reading means, and none of them appear in a speed and an angle.
What a number cannot decide
This is worth being blunt about. A calculator that says "within guidance" has compared two numbers against two other numbers. It has not seen the sky, the sock, the cycle, or the pilot.
Launch decisions are made on the hill by people who know the site, and by pilots trained to read conditions in person — which is a skill taught through supervised practice, not through arithmetic. The value of splitting the wind into components is that it makes the crosswind visible before you are standing in it, and that it gives you a consistent way to describe a day afterwards.
Use it to build judgement on the ground. Do not use it as a go signal, and treat "within guidance" as the absence of one particular objection rather than the presence of a reason.
The angle is the variable people underestimate
Wind speed gets quoted; the angle usually does not. Yet the same 15 km/h produces zero crosswind straight in, 7.50 at 30°, and 12.99 at 60° — and at 60° the crosswind has become the larger of the two components.
That matters because the components do different jobs. Headwind is what gives you airspeed over the ground and lets a wing come up and stay up. Crosswind is what tries to turn the wing and walk you along the slope, and it is the one that turns a marginal launch into a bad one.
A useful mental anchor: at 45° the split is exactly even, so anything further off than that is producing more sideways than into-slope regardless of the total.
Gusts are not in the number
A steady reading and a gusty one can average identically. The gust factor — the spread between lull and peak — is frequently what decides whether a day is flyable, and no single speed captures it.
The same applies to direction. A wind that averages 20° off while swinging between straight-in and 45° is a different proposition from one holding steady at 20°, and the components computed from an average describe neither extreme.
Which is the general shape of the caution on this page: components are a way of understanding a reading, and a reading is a summary of something that is not summarised well.
Using it to build judgement
The productive habit is to compute the split for days you were on the hill anyway, and compare it with what the day actually felt like. Over a season that calibrates your sense of what a given reading means at your site — which is the thing that eventually lets you read conditions without arithmetic, and it is built by pairing numbers with experience under supervision rather than by either alone.