Wind

Air in motion relative to the surface. Three forces set it, and knowing which one dominates at your altitude tells you what the wind will actually do.

Weather Information — FI.III.C · CA.I.C · PA.I.C

Why there is wind at all

Wind is the atmosphere trying to even out a pressure difference. It is also the mechanism that forms, dissipates and redistributes weather — nothing moves anywhere without it, which is why a forecast is so often really a wind forecast in disguise.

Pressure gradient force

Where pressure differs, air is pushed from high toward low. The steeper the difference over distance, the stronger the push — and this is directly readable off a chart: isobars packed close together mean strong wind, isobars far apart mean light wind. You can estimate wind strength from a surface analysis before reading a single number.

Coriolis

Because the Earth rotates beneath the moving air, wind is deflected — to the right in the Northern Hemisphere, to the left in the Southern. Three properties are worth committing to memory because examiners like them and because they explain real behaviour:

• It acts at a right angle to the wind direction, so it turns the wind without speeding it up or slowing it down.
• It is proportional to wind speed — double the wind and you double the deflection.
• It varies with latitude: zero at the equator, maximum at the poles.

The consequence is the one that matters in the cockpit. Above the friction layer only the pressure gradient and Coriolis are acting, and they balance — so the wind ends up blowing along the isobars rather than across them, not from high to low as intuition insists.

Friction, and why the surface wind is different

Near the ground, the surface drags on the air. Rougher terrain and stronger winds produce more friction, and the effect is insignificant above the lowest few thousand feet.

Slowing the wind weakens Coriolis — which is proportional to speed — so the balance aloft is broken and the surface wind turns back across the isobars toward the low. This is why the wind you get on the ground is both slower and from a different direction than the wind a few thousand feet above it, and why climbing out of the friction layer usually brings a veer to the right and an increase in speed in the Northern Hemisphere.

It is also the reason a wind correction angle computed from a surface observation is wrong for cruise, and why winds aloft are a separate product rather than an extrapolation.

Where you meet it next

• Air masses and fronts — converging winds are how you find a front
• Turbulence and shear — what happens where wind changes fast
• Crosswind component — the wind, resolved onto a runway
• Winds aloft over the US

Sources

The three forces and their behaviour are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28).