Apparent wind
Encyclopedia
Apparent wind is the wind experienced by a moving object.

Definition of apparent wind

The Apparent wind is the wind experienced by an observer in motion and is the relative velocity
Relative velocity
In non-relativistic kinematics, relative velocity is the vector difference between the velocities of two objects, as evaluated in terms of a single coordinate system....

of the wind in relation to the observer.

Apparent wind velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

is the vector sum of the true wind and the headwind an object would experience in still air. The headwind velocity in still air is inverse
In mathematics, the additive inverse, or opposite, of a number a is the number that, when added to a, yields zero.The additive inverse of a is denoted −a....

of the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the Velocity of the wind minus the Velocity of the object.

A simple example

Suppose you are riding a bicycle on a day when there is no wind. Although the wind speed is zero (people sitting still feel no breeze), you will feel a breeze on the bicycle due to the fact that you are moving through the air. This is the apparent wind. On the windless day, the apparent wind will always be directly in front and equal in speed to the speed of the bicycle.

Now suppose there is a 5 mph wind coming directly from the north. If you pedal at 10 mph due north, you will feel an apparent wind of 15 mph from the north. But if you pedal 10 mph due south, you will feel an apparent wind of 5 mph from the south. The apparent wind is not only different in speed than the true wind, but also different in direction.

In these simple examples, the motion is parallel to the true wind which makes it easy to calculate of apparent wind. When the motion is not parallel to the wind, one must use vectors and trigonometry to calculate apparent wind.

Apparent wind in sailing

In sailing
Sailing
Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...

, the apparent wind is the actual flow of air acting upon a sail
Sail
A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...

. It is the wind as it appears to the sailor on a moving vessel.
It differs in speed and direction from the true wind that is experienced by a stationary observer.
In nautical terminology, these properties of the apparent wind are normally expressed in knots and degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s.

Calculating velocity and angle

Where:
• = velocity (boat speed over ground, always > 0)
• = true wind velocity (always > 0)
• = true pointing angle in degrees (0 = upwind, 180 = downwind)
• = apparent wind velocity (always > 0)

The above formula is derived from the Law of cosines
Law of cosines
In trigonometry, the law of cosines relates the lengths of the sides of a plane triangle to the cosine of one of its angles. Using notation as in Fig...

and using .

The angle of apparent wind () can be deduced from the measured velocity of the boat and wind using the inverse cosine in degrees ()

If the velocity of the boat and the velocity and the angle of the apparent wind are known, for instance from a measurement, the true wind velocity and direction can be calculated with:

and

Instruments

The apparent wind on-board is often quoted as a speed measured by a masthead
-Media:* the masthead , a list, usually found on the editorial page of a newspaper or other periodical, listing the publisher, editorial board, advertising rates, etc....

transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

containing an anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...

and wind vane
Weather vane
A weather vane is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building....

that measures wind speed in knots and wind direction in degrees relative to the heading
In navigation, a vehicle's course is the angle that the intended path of the vehicle makes with a fixed reference object . Typically course is measured in degrees from 0° clockwise to 360° in compass convention . Course is customarily expressed in three digits, using preliminary zeros if needed,...

of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat velocity are input.

Implications on sailing speeds

In sailboat racing, and especially in speed sailing
Speed sailing
Speed sailing is the art of sailing a craft as fast as possible over a predetermined route, and having its overall or peak speed recorded and accredited by a regulatory body. The term usually refers to sailing on water, even though sailing on land and ice is progressively faster because of the...

, apparent wind is a vital factor, when determining the points of sail
Points of sail
Points of sail describes a sailing boat's course in relation to the wind direction.There is a distinction between the port tack and the starboard tack. If the wind is coming from anywhere on the port side, the boat is on port tack. Likewise if the wind is coming from the starboard side, the boat...

a sail-boat can effectively travel in.
A vessel traveling at increasing speed relative to the prevailing wind will encounter the wind driving the sail at a decreasing angle and increasing velocity. Eventually, the increased drag and diminished degree of efficiency of a sail at extremely low angles
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

will cause a loss of accelerating force. This constitutes the main limitation to the speed of wind-driven vessels and vehicles.

Windsurfers
Windsurfing
Windsurfing or sailboarding is a surface water sport that combines elements of surfing and sailing. It consists of a board usually two to four metres long, powered by the orthogonal effect of the wind on a sail. The rig is connected to the board by a free-rotating universal joint and comprises a...

and certain types of boats are able to sail faster than the true wind. These include fast multihull
Multihull
A multihull is a ship, vessel, craft or boat with more than one hull.-Description:Multihulls include: Proas, which have two differently shaped or sized hulls with lateral symmetry; catamarans, which have two hulls with longitudinal symmetry; and trimarans, which have a main hull in the center and...

s and some planing
Planing (sailing)
Planing is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift .-History:...

monohulls. Ice-sailors
Ice yachting
Ice yachting is the sport of sailing and racing iceboats, also called ice yachts. It is practiced in Austria, Germany, Great Britain, Poland, Norway and Sweden, to some extent, and is very popular in the Netherlands and on the Gulf of Finland, but its highest development is in the United States and...

and land-sailors
Land sailing
Land sailing, also known as sand yachting or land yachting, is the act of moving across land in a wheeled vehicle powered by wind through the use of a sail. The term comes from analogy with sailing. Historically, land sailing was used as a mode of transportation or recreation...

also usually fall into this category, because of their relatively low amount of drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

or friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

.

Other areas of relevance

In fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

, apparent wind is what is experienced on-board and it determines the necessary speeds for take-off and landing. Aircraft carrier
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

s generally steam directly upwind at maximum speed, in order to increase apparent wind and reduce the necessary take-off velocity. Land-based airport
Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...

traffic, as well as most mid-sized and large birds generally take off and land facing upwind for the same reason.

Online calculation and graphical impact presentation

Short introduction related to Kitebuggy, Kite-MTB, Kitesurf

Apparent Wind - Online Tool - Calculation of Speed, Angle and SW- windwindows

Uses Wind speed, Course in degrees and Course speed as an input to calculate apparent wind speed and apparent wind angle