Crosswind refers to any wind that blows across the direction of travel. It is contrasted with tailwinds, which blow in the direction of travel, and headwinds, which blow against the direction of travel. Quickly calculating crosswind speeds is essential for airplane pilots during takeoff and landing. This calculation is based on two factors: the wind's actual speed and the angle at which it intersects with the direction of travel.
Determine the wind's speed using either a wind meter, which will provide a scientifically accurate measurement, or the Beaufort wind scale. On most boats and planes, the wind meter is installed into the dashboard with other navigational instruments (such as the knot meter and depth meter) and displays the speed of the wind with a needle that moves along a numbered dial, the same way that a speedometer displays the speed of a moving car. The Beaufort wind scale estimates wind speed based on the wind's effect on external factors, such as the movement of tree branches, chimney smoke and ocean waves. If, for example, the surface of the ocean is smooth and chimney smoke rises straight in the air, the wind speed is less than one knot and the wind is described as "calm." If, on the other hand, large tree branches are moving and waves are between 8 to 13 feet high, the wind is 22 to 27 knots and is described as a "strong breeze." The Beaufort scale has 12 gradations, ending when the wind blows at over 64 knots––an official hurricane. A complete version of the scale is available, free, from the National Oceanographic and Aeronautic Association.
Determine the angle at which the wind intersects with the direction of travel. Use a 360-degree circle, with North as degree 0. For example, if you are traveling towards a harbor located at 270 degrees, the harbor is due west, or 3/4 of the way around the circle. When the wind blows "out of 90 degrees," it is blowing directly "out of," or from, the east.
Calculate the angle at which the wind intersects with the direction of travel using your compass. Calculate the difference between the angle of the direction of your travel (say a runway located at 270 degrees, or due west) and the current source direction of the wind (say 315 degrees, or northwest). For example, a destination at 270 degrees with the wind out of 315 degrees means an angle of crosswind of 315 minus 270, or 45 degrees.
You can roughly determine this number without a compass by estimating the angle created when you point one arm in the direction of the wind and the other in the direction of travel. You'll know you are looking directly into the wind when you feel a breeze blowing into the cavities of both ears.
Multiply the actual wind speed by the sine of the angle at which the wind intersects with the line of travel. This number is the crosswind speed. The sine is the mathematical function of an angle. It is used to determine the distance of the Y axis, or "rise," of a line stretching out from a particular angle. Most calculators have a button for calculating sine.