A tailwind is a wind that blows in the direction of travel of an object, while a headwind blows against the direction of travel. A tailwind increases the object's speed and reduces the time required to reach its destination, while a headwind has the opposite effect. Tailwinds and headwinds are commonly measured in relation to the speed of vehicles — commonly air and watercraft — as well as in running events — particularly sprints.

In aeronautics, a headwind is favorable in takeoffs and landings because an airfoil moving into a headwind is capable of generating greater lift than the same airfoil moving through tranquil air or a tailwind at equal ground speed. As a result, aviators and air traffic controllers commonly choose to take off or land in the direction of a runway that will provide a headwind. In sailing, a headwind may make forward movement difficult, and necessitate tacking into the wind.

## Aeronautics calculations

Pilots calculate the Headwind or Tailwind Component and the Crosswind Component of local wind before takeoff. The direction of wind at a runway is measured using a windsock and the speed by an anemometer, often mounted on the same post. Headwind and Tailwind are opposite interpretations of the wind component which is parallel to the direction of travel,[1] while Crosswind represents the perpendicular component. Determining the ground speed of an aircraft requires the calculation of the head or tailwind.

Assume:

${\displaystyle A={\text{Angle of the wind origin from the direction of travel}}}$

${\displaystyle WS={\text{The measured total wind speed}}}$

${\displaystyle CW={\text{Crosswind}}}$

${\displaystyle TW={\text{Tailwind}}}$

${\displaystyle HW={\text{Headwind}}}$

Then

${\displaystyle CW=\sin(A)\cdot WS}$

${\displaystyle HW=\cos(A)\cdot WS}$

For example, if the wind is at 09015 that means the wind is currently from heading 090 degrees with a speed of 15 Knots and the aircraft is taking-off from runway 24; having heading of 240. The pilot prefers the runway side with less than 90 difference from wind direction, in this case Runway 06; heading 060. Here, ${\displaystyle A=30^{\circ }}$.

${\displaystyle {\text{Crosswind}}=\sin[30^{\circ }]\cdot 15{\mathsf {knots}}\approx 7.5{\mathsf {knots}}}$

${\displaystyle {\text{Headwind}}=\cos[30^{\circ }]\cdot 15{\mathsf {knots}}\approx 13{\mathsf {knots}}}$

The aircraft is said to have 7,5 knots of crosswind and 13 knots of headwind on runway 06, or 13 knots of tailwind on runway 24.

Aircraft usually have maximum tailwind and crosswind components which they cannot exceed. If the wind is at eighty degrees or above it is said to be full-cross. If the wind exceeds 100 degrees it is common practice to takeoff and land from the opposite end of the runway, it has a heading of 060 in the above mentioned example.