Measuring Wind Direction and Speed

Weather Vane

Weather vanes have been used since antiquity. Contemporary reports from the 1st century BC describe one honouring the Greek god Triton. The word “vane” comes from the Anglo-Saxon word “fane”, meaning flag. Originally, fabric pennants would show archers the direction of the wind in order to assist their accuracy. Compass points were not added until several centuries later.

The design of a weather vane is more complicated than at first might be thought. In order to catch the wind properly, a vane’s back portion must have a larger surface than the front portion but in order to swing freely, the smaller front portion must weigh about the same as the larger back.

The weather vane could be argued as one of the most useful instruments for a weather station. It is one instrument that could be made by a competent DIY person with simple materials and a compass for positioning. As discussed in detail within chapter 2, it is important to know from which direction the wind is coming (the direction to which the vain is pointing) and thus an indication of the approaching weather. A pointer made from plastic or thin metal, with a modest surface area, using the proportions and weighting as set out above, can give adequate results. This must be balanced on a vertical rod providing minimal friction to assist free movement.

Manufactured weather vanes come in a variety of shapes, sizes and patterns that just need to be positioned accurately using a compass.

Beaufort Scale

If the cost of purchasing an anemometer is not possible when first setting up the station, a record could be maintained using the Beaufort Scale. Rear Admiral Sir Francis Beaufort devised the scale in 1805 for maritime use, since adapted several times. The scale runs from 0 to 12 representing wind speeds of 0 mph to about 74mph. There are associated descriptions with each wind force. An example is Force 2 being a ‘light breeze’ and manifests itself on land with ‘leaves rustle and wind felt on exposed skin’ whereas on water it has the effect of ’small wavelets that do not break’.

Anemometer

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There is a wide range of individual anemometers that are portable and hand-held. These instruments produce figures of instantaneous wind speeds at a point just above ground level. But for a meteorologist, this data is not as useful or relevant as an anemometer placed in a raised position, preferably above roof level, and away from objects that would reduce its speed. There are two basic hand-held units. The first is a small replica of the professional anemometers that are set high above ground level. These consist of three identical cups that rotate and are calibrated to provide results on a scale set out below the cups and indicated via a moving marker. An alternative instrument is often rectangular in shape and within the top section is a circular hole that contains a small circular bladed unit that is rotated by the wind with the results displayed electronically on a panel below the propeller.

Weather Vane and Anemometer Combined

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There are a variety of manufactured units that combine both instruments. These instruments are designed for placement in a raised position for best results and thus the live data is transmitted to a monitor either by cable or wirelessly. Such equipment can provide a wealth of information in addition to wind direction. The most advanced units provide instantaneous readings of wind speed and maximum gusts in a set period. Alternative types of combined units will provide information on, for example, average wind speed over a specified time, usually ten-minute periods, that give a useful figure to indicate if the maximum gusts are a one-off incident or if the wind is consistently strong. Additional data is often provided of average speed over a 24-hour period and perhaps over an extended period such as a month.

There are occasions, fortunately not frequent, that natural occurrences affect an instrument. During an occasional summer’s evening when it is warm and the wind drops out entirely, I have found that many strands of web woven overnight by a spider have secured my anemometer, some four metres atop a mast above the ridge. These tiny strands have been very difficult to remove to release the anemometer and take a considerable gust of wind to allow it to break free.

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