The shape of the blade cross-section, which for most modern horizontal axis wind turbines is designed to enhance the lift and improve turbine performance. (US Dept. of Energy)


Unit for the quantity of electricity obtained by integrating current flow in amperes over the time in hours for its flow; used as a measure of battery capacity. (US Dept. of Energy)


A device to measure the wind speed. (US Dept. of Energy)

Average wind speed

The mean wind speed over a specified period of time. (US Dept. of Energy)


The aerodynamic surface that catches the wind. (US Dept. of Energy)

Boundary layer

That part of the atmosphere that is adjacent to the Earth’s surface and which is affected by the properties of that surface. (CarbonTrust)


Various systems used to stop the rotor from turning. (US Dept. of Energy)

Canopy layer or sublayer

The part of the atmospheric boundary layer occupied by the roughness elements (buildings in the urban case). (CarbonTrust)


See Inverter. (US Dept. of Energy)

Cut-in wind speed

The wind speed at which a wind turbine begins to generate electricity. (US Dept. of Energy)

Cut-out wind speed

The wind speed at which a wind turbine ceases to generate electricity. (US Dept. of Energy)


Mass per unit of volume. (US Dept. of Energy)


On the opposite side from the direction from which the wind blows. (US Dept. of Energy)


The area upwind of a site, over which the air has travelled. (CarbonTrust)

Flow separation

The process by which an eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides. (US Dept. of Energy)


Rate of transport. (CarbonTrust)


A passive protection for the turbine in which the rotor folds either up or around the tail vane. (US Dept. of Energy)

Guarantee of Origin

An electronic certificate representing 1 MWh of electricity production used for the purpose of proving to final customers the share or quantity of renewable energy that was supplied to them. (Directive 2009/28/ec)


Horizontal axis wind turbine. (US Dept. of Energy)


The central part of the wind turbine, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle. (EERC)

Hydrostatic equilibrium

The state of balance between the force of gravity and the vertical component of the pressure gradient force. It is a state of the atmosphere in which there is no vertical acceleration of the air. (CarbonTrust)

Inertial sublayer

The part of the atmospheric boundary layer that is much lower than the boundary layer depth but much higher than the surface roughness elements. (CarbonTrust)


A device that converts direct current (DC) to alternating current (AC). (US Dept. of Energy)


Based on the form of the surface i.e. based on the dimensions and distribution of roughness elements. (CarbonTrust)


The body of a propeller-type wind turbine, containing the gearbox, generator, blade hub, and other parts. (US Dept. of Energy)

O&M costs

Operation and maintenance costs. (US Dept. of Energy)

Obukhov length

A quantity that characterizes the relative importance of mechanically and thermally produced turbulence. (CarbonTrust)

Pitch control

Pitch control allows the wings of a wind turbine to be adjusted to suit the prevailing conditions.

Power coefficient

The ratio of the power extracted by a wind turbine to the power available in the wind stream. (US Dept. of Energy)

Power curve

A chart showing a wind turbine’s power output across a range of wind speeds. (US Dept. of Energy)

Rated output capacity

The output power of a wind machine operating at the rated wind speed. (US Dept. of Energy)

Rated wind speed

The lowest wind speed at which the rated output power of a wind turbine is produced. (US Dept. of Energy)


The rotating part of a wind turbine, including either the blades and blade assembly or the rotating portion of a generator. (US Dept. of Energy)

Rotor diameter

The diameter of the circle swept by the rotor. (US Dept. of Energy)

Rotor speed

The revolutions per minute of the wind turbine rotor. (US Dept. of Energy)

Roughness layer or sublayer

The part of the atmospheric boundary layer that is not much higher than the surface roughness elements. (CarbonTrust)


Reduction of wind speed through obstacles (trees, houses, other wind turbines etc). This creates turbulence, which means that systems cannot deliver full power. Shadowing is taken into account when preparing earnings forecasts. (EcoVane)

Stall control

In contrast to pitch control, stall control is mainly used on smaller wind turbines. Here, the blades are locked in place and do not adjust during operation - instead, they are designed and shaped to increasingly ‘stall’ the blade’s angle both to maximize power output and to protect the turbine from excessive wind speeds. (EcoVane)

Start-up wind speed

The wind speed at which a wind turbine rotor will begin to spin. See also Cut-in wind speed. (US Dept. of Energy)

Surface layer

For the large scale meteorological community, this is synonymous with inertial sublayer. However the urban meteorological community often uses the term to mean the inertial sublayer and roughness sublayer combined. (CarbonTrust)

Tip speed ratio

The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity. This is typically a design requirement for the turbine. (US Dept. of Energy)


The base structure that supports and elevates a wind turbine rotor and nacelle. Modern turbines are typically constructed using a tubular steel tower. Older wind turbines and windmills used the latticetype tower, which consists of a crisscrossed network of steel or wood members. (EERC)


The changes in wind speed and direction, frequently caused by obstacles. (US Dept. of Energy)


On the same side as the direction from which the wind is blowing—windward. (US Dept. of Energy)


Vertical axis wind turbine. (US Dept. of Energy)

Weibull distribution

In order to calculate the mean power delivered by a wind turbine from its power curve, it is necessary to know the probability density distribution of the wind speed. The Weibull distribution method is used for this purpose, simply showing the distribution of the proportion of time spent by the wind within narrow bands of wind speed. (EcoVane)

Wind Assessments

The wind speed at a site is calculated by experts and compiled in a report, which takes data from site and local weather stations. The assessment of a specific site surrounding obstacles, hills, mountains, trees are also considered. At the end of an opinion is the average wind speed of a location in meters per second at hub height, and the Weibull distribution. (EcoVane)

Wind farm

A group of wind turbines often owned and maintained by one company. Also known as a wind power plant. (US Dept. of Energy)

Wind power class

A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7, with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources. (EERC)

Wind power density

A way to define the amount of wind power contained in a given area for use by a wind turbine, measured in watts per square meter. (EERC)

Wind vane

A device used to measure wind direction. (EERC)


The movement of the tower top turbine that allows the turbine to stay into the wind (US Dept. of Energy)