Wind is the movement of air across the Earth’s surface. This air flow is rarely a steady, smooth current; instead, it is often erratic and turbulent. The sudden, momentary increases in speed that cause objects to sway and people to stumble are known as wind gusts.
The Difference Between Sustained Wind and Gusts
Meteorologists distinguish between two primary measurements of wind speed: sustained wind and wind gusts. Sustained wind represents the average speed of the air flow over a specific duration. In the United States, this measurement is typically calculated by averaging the wind speed observed over a two-minute period. Internationally, weather agencies often use a ten-minute average to define the sustained wind speed.
A wind gust, by contrast, is a rapid and brief spike in the wind speed, characterized by a sudden onset followed by a quick decrease. These bursts of wind generally last for less than twenty seconds. The official speed of a gust is determined by recording the maximum wind speed averaged over a very short interval, usually three seconds.
This distinction is important because sustained wind indicates the overall strength of the air mass, while the gust represents the highest momentary stress placed upon structures and objects. Gusts are formally reported when the peak speed reaches at least 18 miles per hour, and the variation between the peak speed and the lowest speed (the lull) is 10 miles per hour or greater.
What Creates Atmospheric Gusts
Gusty conditions arise from various atmospheric processes that create turbulence, preventing the air flow from being perfectly smooth. A primary cause is the friction between the moving air and the Earth’s surface, which creates mechanical turbulence within the planetary boundary layer. Obstacles such as trees, buildings, and uneven terrain disrupt the steady flow, generating swirling pockets of air called eddies or vortices.
These eddies constantly mix the air near the ground, causing localized, chaotic changes in speed and direction. When a rotating eddy brings a pulse of faster-moving air down toward the surface, it is felt as a gust. Conversely, a lull occurs when a slower-moving part of the turbulent air hits the surface.
Thermal differences also contribute significantly to gust formation, especially on sunny days. Solar heating warms the ground, which in turn heats the air directly above it, causing the warm, less dense air to rise in columns known as thermals. To compensate for this rising air, cooler, faster-moving air from higher altitudes sinks rapidly toward the ground. This vertical transfer of momentum brings the higher wind speeds found aloft down to the surface, resulting in a pronounced gust.
How Gusts Are Measured and Reported
The accurate measurement of both sustained winds and gusts relies on instruments called anemometers. These devices, often featuring rotating cups or utilizing ultrasonic technology, are designed to record the speed of the air flow. Because a gust is a short-duration maximum, the measurement system requires a high rate of data sampling to capture the brief spike in speed.
Meteorologists communicate the presence of gusty conditions by reporting both the sustained wind speed and the maximum expected gust speed. A typical forecast will state something like “sustained winds of 15 miles per hour with gusts up to 30 miles per hour.” This format provides the public with a complete picture of the expected wind environment.
The reported gust value is crucial safety information for the public, particularly for activities like aviation, boating, and construction projects. The momentary, high-speed impact of a gust often causes damage, such as knocking down power lines or making driving difficult. Warnings, such as a High Wind Warning, are issued when sustained winds or stronger gusts are expected to reach hazardous thresholds.