Due to their complexity, wind patterns are impossible to describe with a single concept or numerical value. It is common knowledge that wind speed varies from site to site, and also that it changes over time for a given location. However, there is another variation that is much less evident: wind conditions also change with altitude. For instance, a measurement taken 20 metres above ground level will yield a different result than one taken 50 meters over the same spot. This wind variation with altitude is called windshear.
Unless the wind is being blocked by a tall obstacle, its speed increases with altitude. Wind patterns are unique for each site, but they are determined by common physical principles.
Weather scientists have studied windshear in depth, developing many mathematical models to describe it. Windshear is normally combined with turbulence, which can be described as a random three-dimensional movement of the wind. Turbulence is caused when the wind interacts with natural or man-made obstacles.
There are many applications where wind speed may be required for a range of values as opposed to a single altitude value. The following are some common examples:
In theory, you could account for windshear by measuring the wind at multiple heights. However, this approach has practical limitations:
When wind speed is measured, it is important to specify the height at which the anemometer is installed. When no height value is provided, it is normally assumed that the measurement was taken at 10 metres.
You may recall from high school mathematics that you can calculate the slope of a linear graph using two points along the line. A similar principle applies for windshear, but just keep in mind that the math is much more complex!
Although this approach does not provide the same accuracy as measuring wind speed and direction at the required height, the confidence is over 95% if the method is used correctly. Experience is also very important: you will want to work with a provider that has an established track record in weather monitoring.
Windshear does not behave the same for all locations. In general, wind speed increases faster in proportion to altitude when there are less obstacles and uneven terrain around.
Ground-level obstacles are described based on a metric called the roughness length, which is a broad approximation of the height at which average wind speed becomes zero. This value ranges from less than one millimeter in the open sea, to several meters in urban locations.
Marine wind turbines are such a promising concept because the impact of terrain is eliminated almost completely: with no obstacles around, favorable wind speeds can be found at lower altitudes. This enables the use of shorter and less expensive towers.