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Crosswind: how much of the wind is really from the side?

The ATIS gives the wind as 250 degrees at 15 knots and you are landing on runway 21. Is that a lot? In a few minutes you will split any wind into its headwind and crosswind parts: in your head with the clock rule, and exactly with a little trigonometry.

  • Student pilot
  • EASA 032 · Performance
  • EASA 061 · Navigation
  • 6 min read
  • 17 October 2026

The problem

ATIS

RUNWAY IN USE 21 · WIND 250 DEGREES 15 KNOTS

Runway 21 points roughly 210° magnetic. The wind blows from 250°, so it comes from 40° to the right of the centreline. Part of it blows down the runway into your face and slows you over the ground, which helps on take-off and landing. The rest pushes you sideways off the centreline: that is the crosswind component, the part you have to correct for and the one your aircraft has a maximum demonstrated value for in its flight manual.

So the question is simple: of those 15 knots, how many are on the nose and how many are from the side?

The wind casts two shadows

Draw the wind as an arrow 15 knots long, coming from 250°. Project it onto the runway centreline and you get the headwind. Project it onto the line across the runway and you get the crosswind. The two make a rectangle whose diagonal is the wind itself.

Runway 21, wind 250/15. The blue arrow is the wind, the one along the runway the headwind (≈ 11 kt) and the magenta one the crosswind from the right (≈ 10 kt).

The clock rule, in your head

There is no time for sines in the cockpit, so pilots use a trick: read the angle between wind and runway, in degrees, as minutes on a clock face. However much of the hour has gone, that much of the wind is crosswind.

60° → all 15° → ¼ 30° → ½ 45° → ¾ 40 min = ⅔
40° reads as 40 minutes, two thirds of the hour. Two thirds of 15 knots is 10 knots of crosswind.
Wind / runway angleOn the clockCrosswind share
0°0 minnone
15°a quarter hour¼
30°half an hour½
45°three quarters¾
60° or morethe full hourall of it

For our example: 40° → 40 minutes → ⅔ → ⅔ × 15 ≈ 10 kt of crosswind from the right.

The headwind works the same way with the complementary angle: 90° − 40° = 50° → 50 minutes → about ⅚ → roughly 12 kt on the nose.

Why it works

The clock rule is a straight-line stand-in for the sine curve. Between 0° and 60° it is off by a knot or two at most for everyday winds, and almost always on the safe side: it shows a little more crosswind than there really is.

The exact calculation

If α is the angle between wind direction and runway direction and V is the wind speed:

  • crosswind = V × sin α
  • headwind = V × cos α

For 250/15 on runway 21: 15 × sin 40° = 9.6 kt crosswind and 15 × cos 40° = 11.5 kt headwind. The clock rule gave 10 and 12, very close.

AngleClock ruleExact sin α
10°0.170.17
20°0.330.34
30°0.500.50
40°0.670.64
45°0.750.71
50°0.830.77
60°1.000.87
90°1.001.00

The same calculation as pseudocode. The only care needed is to bring the angle between −180° and +180°, so the signs tell you which side the wind is on and whether it is a head- or tailwind.

Pseudocodewind components
function components(runway, wind_dir, wind_speed):
    alpha = wind_dir - runway
    while alpha > 180:  alpha = alpha - 360
    while alpha < -180: alpha = alpha + 360

    head  = wind_speed * cos(alpha)   # negative = tailwind
    cross = wind_speed * sin(alpha)   # positive = from the right
    return head, cross

# runway 21 (210°), wind 250/15
components(210, 250, 15)   # → head 11.5 kt · cross +9.6 kt (from the right)

Try it yourself

Change the runway and the wind. The diagram turns so you see the runway the way you see it from the cockpit on final.

Common traps

Gusts

If the ATIS says 250/15G25, work the crosswind with the gust: 25 × 0.64 ≈ 16 kt. That is already above the 15 knots demonstrated in a Cessna 172 flight manual. The demonstrated value is not a structural limit, but it is what the test pilot actually flew, and your personal limits may well be lower.

Magnetic or true

Runways are numbered by magnetic direction, and the ATIS and tower winds are magnetic too, so you can compare them directly. METAR and TAF winds, however, are given from true north. Where variation is a few degrees the difference is small, but in some parts of the world it reaches tens of degrees.

Past 90°: tailwind

Once the angle between wind and runway passes 90°, the cosine turns negative and the "headwind" becomes a tailwind. Even a few knots of tailwind visibly lengthen the take-off and landing distances. That is usually the moment to ask for the other runway.

For performance calculations the PPL syllabus asks for a safety margin: count only 50% of the headwind component and 150% of the tailwind component. A reported headwind can drop just as you start the roll, and a tailwind costs you more than its number suggests. If your flight manual chart already builds these factors in, do not apply them twice.

The runway number is not the exact heading

Runway 21 may really be 207° or 214°. For the clock rule, the runway number with a zero added is good enough. For an exact answer, use the direction published in the AIP or on the aerodrome chart.

Check yourself

1. Runway 09, wind 120/20. How much crosswind?

The angle is 30° from the right → half an hour → 10 kt from the right. Headwind is about 17 kt (17.3 exactly).

2. Runway 27, wind 220/12. Which side, and how much?

220 − 270 = −50°, so from the left. On the clock, 50 minutes ≈ ⅚ → about 10 kt. Exactly: 9.2 kt from the left and 7.7 kt headwind. The clock rule was slightly pessimistic, as expected.

3. Runway 34, wind 190/10. What do you do?

190 − 340 = −150°. The angle is past 90°, so you have about 9 kt of tailwind (10 × cos 150° = −8.7) and 5 kt of crosswind from the left. Runway 16 would have the same wind on the nose: ask for it.

Practise on the navigation computer

On the Digital tab, pick "Wind components", enter the runway and the wind, then watch the solution replay step by step on the wind side, just like on a real CRP-5.

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FlightDesk Aero articles are training material. For real flights, use the aircraft's flight manual, current published data and your instructor's guidance.