For most of the United States, a 90 mph 3-second gust wind speed is equivalent to a 73.8 mph fastest-mile wind speed. This conversion is based on a factor of 0.82, meaning the fastest-mile wind speed is approximately 82% of the 3-second gust wind speed (90 mph * 0.82 = 73.8 mph).
Understanding Wind Speed Measurements
Wind speed can be measured and reported in various ways, often leading to different numerical values for the same wind event. The key difference lies in the averaging period over which the wind speed is recorded.
- 3-Second Gust Wind Speed: This measurement represents the peak instantaneous wind speed recorded over a very short duration, typically 3 seconds. It captures the sudden, most intense bursts of wind, which are critical for assessing short-term loads on structures.
- Fastest-Mile Wind Speed: This measurement refers to the speed of a "mile of wind" passing a point. It's an average speed over a longer period, specifically the time it takes for one mile of air to pass the anemometer. This averaging period typically ranges from 30 seconds to several minutes, depending on the actual wind speed. For instance, a 60 mph fastest-mile wind means it took exactly one minute for a mile of wind to pass.
The difference in these averaging periods means that a 3-second gust will almost always be higher than a fastest-mile wind speed for the same storm event, as it captures a momentary peak rather than an average over a longer duration.
Conversion for Design and Safety
The conversion between 3-second gust and fastest-mile wind speeds is crucial for engineering design, especially for structures like buildings, bridges, and signs. Building codes and standards often specify design wind speeds based on one of these metrics.
Below is an example of the specific conversion mentioned:
| Measurement Type | Wind Speed (mph) | Notes |
|---|---|---|
| 3-Second Gust | 90 | Represents the peak instantaneous wind burst. |
| Fastest-Mile Equivalent | 73.8 | Calculated as 90 mph * 0.82, applicable to most of the US. |
This relationship helps engineers and designers translate between different wind speed criteria to ensure structures can withstand local wind conditions effectively, accounting for both sustained winds and sudden, powerful gusts.
