A crosswind is any wind that has a perpendicular component to the line or direction of travel. In aviation, a crosswind is the component of wind that is blowing across the runway making landings and take-offs more difficult than if the wind were blowing straight down the runway. If a crosswind is strong enough it may exceed an aircraft's crosswind limit and an attempt to land under such conditions could cause structural damage to the aircraft's undercarriage. Crosswind is sometimes abbreviated as X/WIND.
Crosswinds can also be a difficulty when traveling on wet or slippery roads (snow, ice, standing water, etc.), especially with gusting conditions and vehicles that have a large side area such as vans and SUV. This can be dangerous for motorists because of the possible lift force created as well as causing the vehicle to change direction of travel. The safest way for motorists to deal with crosswinds is by reducing their speed to reduce the effect of the lift force and to steer into the direction of the crosswind.
When winds are not parallel to or directly with/against the line of travel, 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 knots × sin(45°) or approximately 7.07 knots. 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 is read from a reference line.
Often smaller aircraft are not limited by their ability to land in a crosswind but their ability to taxi safely prior and post-flight.