Wind is a fundamental element of weather that impacts daily life, safety, and commerce. Measuring wind requires standardization for accurate forecasting because wind speed is not constant throughout the lower atmosphere. The height of measurement is an important detail for meteorologists and various industries. Understanding how wind is measured near the surface is necessary for interpreting weather reports and assessing environmental conditions.
What Does “20 Foot Wind” Actually Mean?
The term “20 foot wind” refers to a specific measurement of wind speed taken at an elevation of 20 feet (approximately 6.1 meters) above the ground surface. This measurement is not the global standard for general weather reporting, but it remains an important reference height in specialized fields. In the United States, the 20-foot height is particularly relevant in wildland fire management and forestry applications.
The international standard for surface wind measurement used by most meteorological agencies is 10 meters (roughly 33 feet) above the ground. This standardized height ensures that wind speed data collected worldwide can be directly compared for global weather modeling and forecasting. The 20-foot measurement is often used by systems like the National Fire Danger Rating System (NFDRS), which specifies an anemometer height of 6 meters (20 feet) above vegetation.
Wind measurements are typically averaged over a specific duration to account for natural variations like gusts and lulls. The 20-foot wind speed is used as a baseline to calculate the wind speed at other application-specific heights, such as the mid-flame height for fire behavior modeling. Because of the height difference, a 20-foot wind measurement will naturally be lower than a simultaneous measurement taken at the 10-meter standard height.
Understanding the Wind Gradient
The reason wind speed changes between the ground and the 20-foot or 10-meter level is explained by the concept of the wind gradient. This gradient describes the increase in wind speed with increasing height above the surface. The lowest part of the atmosphere, where this effect is most pronounced, is known as the Atmospheric Boundary Layer (ABL).
The fundamental cause of the wind gradient is friction created by the Earth’s surface. Obstacles like trees, buildings, and uneven terrain create aerodynamic drag, which slows the airflow nearest to the ground. This friction generates turbulent eddies and swirls, decreasing the overall speed of the wind close to the surface.
As the height increases, the effect of surface friction diminishes significantly. Consequently, the air at 20 feet moves faster than the air closer to the ground, and the air at 33 feet (10 meters) moves faster still. This vertical change in wind speed is a defining characteristic of the ABL.
Specialized Uses of Lower Atmosphere Wind Data
Measuring wind at standardized lower levels, like 20 feet and 10 meters, provides necessary data for fields where surface conditions are paramount. In wildland fire forecasting, the 20-foot wind speed is a foundational input for predicting fire behavior. Fire management teams use this reading to calculate the mid-flame wind speed, which dictates the rate and direction of fire spread.
This specific data point is also important in agriculture, particularly for applications like aerial spraying and crop dusting. Knowing the wind speed and direction at a low elevation helps farmers and pilots manage pesticide or fertilizer drift. This ensures the chemicals land on the intended target and do not contaminate surrounding areas.
Aviation also relies heavily on accurate low-level wind data, especially at airports for takeoffs and landings. Surface wind reports, often obtained from Automated Surface Observing Systems (ASOS), provide pilots with information on wind direction and speed near the runway surface. This data helps pilots compensate for crosswinds and turbulence upon final approach and initial climb-out.