Wind speed is a dynamic and highly localized measurement, making the height at which it is taken a crucial variable in any weather report. The wind speed in a forecast is not the same as the wind felt on the ground, because velocity changes significantly with altitude. To ensure wind data is useful and comparable across different locations, meteorologists rely on standardized measurement heights. This standardization allows for consistency in fields like weather forecasting, aviation, and engineering.
Standardized Wind Measurement
The universally accepted standard for measuring surface wind speed is 10 meters (approximately 33 feet) above the ground in an open area. Organizations like the World Meteorological Organization (WMO) and the National Weather Service (NWS) uphold this convention to ensure data can be reliably compared worldwide. This specific height is a practical compromise chosen to minimize the effects of surface friction and turbulence. Measuring wind at 10 meters ensures the readings are representative of general air movement, rather than being skewed by small, localized obstacles. A site is considered “open” if the distance between the wind sensor and any obstruction is at least ten times the height of that obstruction.
The Specific Context of 20-Foot Winds
While 33 feet (10 meters) is the global standard, the measurement of “20-foot winds” is a specific reference height used primarily in wildland fire weather forecasting. This 20-foot (approximately 6-meter) measurement is frequently cited in fire behavior models because it provides a relevant wind estimate slightly above the vegetative canopy. The 20-foot wind speed is the primary input for calculating the midflame wind speed, which is the velocity that actually drives a surface fire. This calculation involves multiplying the 20-foot speed by an adjustment factor to account for the drag and sheltering effects of the terrain and vegetation. In the context of fire danger indices, the 20-foot wind is the specific data point that directly feeds into fire behavior predictions.
The Physics of Wind Speed and Height
The importance of measurement height is due to the physical phenomenon known as the atmospheric boundary layer. This is the lowest layer of the atmosphere, extending up to a kilometer or more, where the air is directly influenced by the Earth’s surface. Within this layer, friction from the ground, trees, and buildings significantly slows the wind speed. This reduction creates a vertical gradient called wind shear, which is the change in wind speed or direction over a short vertical distance. As altitude increases, frictional drag decreases rapidly, causing the wind speed to increase until it reaches the less obstructed flow aloft. The 10-meter and 20-foot measurements are distinct points on this vertical wind speed profile, and the difference between them is a direct result of surface friction.