Storm Floris brought with it extreme winds, but were they recorded breaking?
When we experience extreme weather, such as Storm Floris recently, it's understandable that interest in weather records increases. However, in order to fully understand how records are recorded or broken, we first need to look at how measurements are taken and why, in certain scenarios, some extremes aren't considered.
How wind is measured
Wind is measured using instruments called anemometers, which record wind speed, direction, and gust strength. The standard unit of wind speed is the knot (nautical mile per hour), equivalent to 0.51 metres per second or 1.15mph. Wind direction is measured relative to true north and is reported from the direction the wind is coming from. For example, an easterly wind blows from the east (90 degrees), while a westerly wind comes from the west (270 degrees).
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Wind speed typically increases with height above the Earth's surface. However, it is also influenced by local factors such as terrain roughness, buildings, trees, and other obstacles. For accurate and consistent readings, the ideal site for wind measurement is over level ground with uniform roughness and no large obstructions within 300 metres of the measuring tower. While it can be hard for each station to meet these criteria perfectly, most locations in the observing network are sufficiently representative of open terrain.
Why measurements are taken below 500 metres
While higher elevations, such as mountainous regions, often experience stronger gusts due to their altitude, these readings are not used for setting wind records or issuing weather warnings. The reason is that meteorologists focus on locations where strong gusts are less common but more impactful.
Wind measurements taken below 500 metres are more representative of conditions experienced by the general population and infrastructure. These sites are typically where people live and work, and where transportation networks and buildings are located. Strong winds in these areas are more likely to cause disruption, damage, and pose risks to safety. It's wind speeds in these areas that help influence the issuing of weather warnings, not those at the summits of mountains.
Wind direction is measured using a wind vane. This consists of a horizontal arm with a vertical flat plate at one end and a balance weight at the other. The arm is mounted on a spindle that allows it to rotate freely in response to wind direction. Both the anemometer and wind vane are mounted on a horizontal arm at the top of a 10-metre mast.
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By reporting data from lower-altitude sites, the Met Office ensures that wind data is relevant for operational forecasting and public warnings. This approach helps meteorologists identify unusual wind events that may have significant consequences, rather than recording high gusts that are typical of elevated terrain and less likely to affect daily life.
Types of anemometers used
The most common instrument used to measure wind speed is the cup anemometer. This device consists of three or four cups mounted symmetrically around a vertical spindle. As wind blows into the cups, the spindle rotates. The design ensures that the rotation rate is proportional to wind speed. To maintain accuracy, these instruments are calibrated in wind tunnels and any deviations from the manufacturer’s specifications are corrected to ensure reliable data.
In extreme environments, such as mountaintops, heated sonic anemometers are used. These instruments have no moving parts and measure wind speed and direction using acoustic signals transmitted between transducers. While the supporting structure can distort airflow, corrections based on wind tunnel calibrations help minimize this effect.
Capturing gusts and wind intensity
Wind is highly variable over short time periods, so it is sampled frequently, every 0.25 seconds, to capture gusts. Gusts are short-lived peaks in wind speed that can cause significant damage during storms. The gust speed and direction are defined by the maximum three-second average wind speed recorded during any period.
To assess overall wind intensity, the average speed and direction over the ten-minute period leading up to the reporting time are used. Other averaging periods may also be applied depending on the context. A gale is defined as a surface wind with a mean speed of 34-40 knots (39-46mph) over ten minutes. Stronger winds are classified using terms such as ‘severe gale’ or ‘storm’, which refer to speeds of 41 knots (47mph) or greater.
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