Winds

Meteorology · Chapter 6 · DGCA CPL and ATPL

Why the air moves, why it never blows straight from high to low, how it changes near the ground and with height, and the local winds that pilots meet.

Remember

Wind is named for where it comes from; heard winds (ATIS) are magnetic, read winds (METAR) true. Veer = clockwise, back = anticlockwise.
Coriolis = 2Ωρ V sin φ: zero at the equator, maximum at the poles, right in the NH. Buys Ballot: back to the wind, low on the left (NH).
Geostrophic: P = f, along straight isobars, above the friction layer, not near the equator. Gradient wind: slower round a low (P + C = f), faster round a high (P = C + f).
Friction layer about 1 km: surface wind backs and slows, about 15° and 2/3 over the sea, 30° and 1/3-1/2 over land. Going up, it veers and strengthens; at night the surface wind backs and weakens.
Gust: seconds. Squall: +16 kt to 22 kt or more for a minute or more. Gale: 34 kt or more.
Sea breeze (day) stronger than land breeze (night); katabatic (night, down) stronger than anabatic (day, up). Föhn and Chinook: warm, dry, down the lee.

What exactly does '120° at 15 kt' tell you?

What 'wind' means

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Lines of equal pressure On a surface chart, isobars join places of equal pressure, drawn every 2 or 4 hPa. Close together: a steep pressure gradient and strong wind. Far apart: a weak gradient and light wind.
Lows anticlockwise, highs clockwise In the northern hemisphere the wind blows anticlockwise round a low (a cyclone) and clockwise round a high (an anticyclone). In the southern hemisphere, the other way round.
Named for where it comes from '120° at 15 kt' is a wind blowing from 120°, at 15 knots. A southerly blows from the south towards the north. The teacher's rule of thumb: a wind you hear (ATIS, ATC) is in degrees magnetic; a wind you read (METAR, TAF, charts) is in degrees true.
Vane and anemometer A wind vane gives the direction and a cup anemometer the speed, mounted 10 m above open ground. The wind is averaged over 10 minutes for reports and over 2 minutes for take-off and landing. Upper winds are measured by tracking balloons: pilot balloons by theodolite, RAWIN by radar.
Knots 1 kt = 1 nautical mile per hour. Roughly 1 m/s ≈ 2 kt ≈ 4 km/h; 1 km/h = 0.54 kt.

Memory hook: Wind is named for where it comes FROM. Heard (ATIS): magnetic. Read (METAR): true.

Takeaway: Wind is horizontal air movement, given as the direction it blows from and its speed in knots. Isobars close together mean a steep pressure gradient and strong wind. Measured at 10 m, averaged over 10 minutes (2 minutes for take-off and landing).

How is the wind measured and reported?

Measuring the wind

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Direction: where it comes from A wind is named after the direction it comes FROM. A southerly blows from the south towards the north. Direction is given on 16 compass points (N, NNE, NE and so on) or in degrees, in steps of 10°. The degrees are measured from true north.
Speed in knots Wind speed is given in knots. A knot is one nautical mile per hour. To convert: 1 km/h = 0.278 m/s = 0.540 kt. As a rough guide, 1 m/s ≈ 2 kt ≈ 4 km/h. So "120° at 15 kt" means a 15-knot wind blowing from 120°, the east-south-east.
The instruments A wind vane shows the direction and an anemometer (spinning cups) measures the speed. Both stand 10 m above open ground, away from buildings and trees. For winds higher up, a hydrogen balloon (a pilot balloon) is followed with a theodolite, a kind of telescope. With RAWIN, radar tracks the balloon instead.
Averaging The wind never stays steady, so one reading would mislead. It is averaged instead. Weather reports give the average over 10 minutes. For take-off and landing the average is over the last 2 minutes, so it is closer to what you will meet on the runway.
Reporting in fives, and the cross wind Speeds are reported to the nearest 5 kt: 0-2 kt is calm, 3-7 kt is 05, 8-12 kt is 10, and so on up to 48-52 kt, which is 50. Runways are built along the usual wind. A wind at 90° to the runway is a cross wind. It pushes the aircraft sideways and swings it, light aircraft most. Each type has a maximum cross wind.

Memory hook: From, true, knots, 10 m, 10 min (2 min for runways).

Takeaway: Wind direction is where the wind blows FROM, in degrees from true north (or on 16 compass points). Speed is in knots. The vane and anemometer stand 10 m up, in the open. Reports use a 10-minute average; take-off and landing use a 2-minute average.