Two Speeds, One Aeroplane
Airspeed is measured against the air, and the air is moving. Point the aeroplane into a thirty knot wind doing a hundred knots through the air and you are covering seventy knots of ground. Turn around and the same aeroplane at the same airspeed covers a hundred and thirty.
Nothing about the aeroplane changed. The wing does not know which way the wind is blowing, because from the wing point of view there is no such thing as wind, only air arriving at a certain speed from a certain direction.
Groundspeed is the sum of the two, and it is a vector sum, so wind from the side pushes you sideways as well as slowing you down. A flight planned on a map without an allowance for that arrives somewhere else.
The Leg That Comes Back
The arithmetic of a there and back trip catches people out. Fly ten miles out with a headwind and ten miles home with the same wind behind you and the return does not cancel the outbound. It cannot.
Working a simple case makes it obvious. With no wind, two hundred miles at a hundred knots takes two hours. With a fifty knot wind straight down the outbound leg, the outbound takes eighty minutes and the return takes two hours and forty. The same trip, the same aeroplane, four hours instead of two.
Which is why Cargo is laid out the way it is. The wind is drawn on the minimap and it is worth more than any piloting skill in the game, because a leg chosen against the wind can cost you the entire flight.
Reading It Before You Take Off
A pilot on the ground has several ways to find out what the wind is doing, and they all exist because the answer changes the plan rather than merely informing it.
The windsock is the obvious one. It points downwind, so where the small end is pointing is where the wind is going, which is the opposite of where it is coming from, and this confuses people more often than it should. How far the sock is lifted tells you the strength, and a sock standing straight out is a day worth respecting.
Runways are named for the direction they face, and they face into the prevailing wind. A runway numbered 27 points roughly 270 degrees, which is west, which means the wind at that airfield usually comes from the west. Two ends of the same strip get different numbers, so the same runway is 27 from one direction and 09 from the other.
What a pilot actually calculates before landing is the crosswind component, which is how much of the wind is pushing across the runway rather than down it. A wind thirty degrees off the runway centreline is mostly a headwind and easy. A wind sixty degrees off is mostly a crosswind and not. Every aeroplane has a published figure for the most crosswind it can handle, and exceeding it is not a matter of skill.
In the air, the correction is called crabbing. Rather than pointing the aeroplane down the runway, the pilot points it upwind by the angle that makes the drift cancel out, so the aeroplane travels straight while sitting noticeably sideways. It looks wrong and it is exactly right, and it is the visible proof that the aeroplane flies through the air rather than over the ground.
When the Wind Turns Dangerous
Wind on its own is not a hazard. An aeroplane does not care how fast the air is moving, because from the wing point of view it is always standing still and the air is always arriving at flying speed. What causes trouble is the wind changing, and the closer to the ground that happens, the less room there is to deal with it.
Wind shear is a change in wind over a short distance, and near the ground it can be abrupt enough to matter. An aeroplane on approach flying into a headwind that suddenly drops loses airspeed at the exact moment it is slow and low, and if the drop is large the wing can stop flying. Every commercial airport has equipment watching for this, and pilots are trained to add speed on approach when it is forecast.
A downburst is the extreme version. A thunderstorm throws a column of cold dense air at the ground, and it spreads outwards in every direction from the point of impact. An aeroplane flying into the middle of one meets a headwind, then a downdraft, then a tailwind, in that order, over a few seconds. The headwind makes it climb when it wants to descend, the downdraft pushes it at the ground, and the tailwind takes away the airspeed it needs to do anything about either. This is the mechanism behind some of the worst accidents in aviation history, and it is why aircraft now give thunderstorms a wide berth rather than a tidy one.
Over high ground the same effect appears as mountain wave and rotor. The smooth lift that makes wave soaring possible has a violent underside, and the air downwind of a ridge can contain rotor that has broken gliders apart. Which is the general rule with wind: the useful parts and the dangerous parts are produced by the same terrain, and you do not get one without the other being somewhere nearby.
Where Wind Comes From
Air moves because pressure differs and because the planet is turning. Warm air rises and cold air sinks, which sets up a circulation, and the rotation of the earth deflects that circulation sideways rather than letting it run straight.
Near the ground friction slows the wind and makes it gusty. Higher up it is smoother and usually stronger. That difference is why a balloon pilot choosing an altitude is choosing a direction, and it is the only steering a balloon has.
A weather system passing over changes the wind entirely, in direction as well as strength. Which is the subject of Weather, where the cells are the visible part and the wind pushing them is the part you have to think about.