A crosswind is any wind that is blowing perpendicular to a line of travel, or perpendicular to a direction. For example, in aviation, a crosswind is the component of wind which is blowing 90 degrees to the runway, making a landing more difficult than if the wind were blowing straight down the runway. In fact if a crosswind is strong enough it may exceed an aircraft's crosswind limit, and attempting to land under such conditions could cause structural damage to the aircraft's undercarriage.
Crosswinds can also occur when traveling on road, especially on large bridges and highways, which can be dangerous for motorists because of possible lift force created. The safest way for motorists to deal with crosswinds is by reducing their speed to reduce the effect of lift force and steering into the direction of the crosswind.
Crosswind is sometimes abbreviated X/WIND.
When winds are neither perpendicular nor parallel to the line of travel, then the wind is said to have a crosswind component - that is, it can be separated into two components - a crosswind component, and a headwind or tailwind component. A vehicle behaves as though it is directly experiencing a crosswind in the magnitude of the crosswind component only.
The crosswind component is computed by multiplying the wind speed by the sine of the angle between the wind and the direction of travel. For example, a 10-knot wind coming at 45 degrees from either side will have a crosswind component of 10 kts•sin(45°) or approximately 7.07 knots. Similarly, the headwind component is computed in the same manner, using cosine instead of sine. To determine the crosswind component in real world flight, aviators frequently refer to a chart on which the wind speed and angle are plotted, and the crosswind component read from a reference line.
An old pilot's saying: "You may forget the wind, but the wind will never forget you."