The wind blowing into the cups causes the spindle to rotate. Wind speed is normally measured by a cup anemometer consisting of three or four cups, conical or hemispherical in shape, mounted symmetrically about a vertical spindle. In practice few sites in the observing network meet this requirement exactly for all incident wind directions, but most are reasonably representative of an open site. The optimal exposure for the measurement of wind is over level ground of uniform roughness with no large obstacles within 300 m of the tower. Wind speed normally increases with height above the earth's surface and is much affected by such factors as the roughness of the ground and the presence of buildings, trees and other obstacles in the vicinity. An easterly wind blows from the east or 90 degrees, a southerly from the south or 180 degrees and a westerly from the west or 270 degrees. Wind direction is measured relative to true north ( n o t magnetic north) and is reported from where the wind is blowing. 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.The normal unit of wind speed is the knot ( nautica l mile per hour = 0.51 m sec-1 = 1.15 mph). 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. There are occasions, fortunately not frequent, that natural occurrences affect an instrument. Additional data is often provided of average speed over a 24-hour period and perhaps over an extended period such as a month. 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. The most advanced units provide instantaneous readings of wind speed and maximum gusts in a set period. Such equipment can provide a wealth of information in addition to wind direction. 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. ![]() There are a variety of manufactured units that combine both instruments. 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’. ![]() There are associated descriptions with each wind force. The scale runs from 0 to 12 representing wind speeds of 0 mph to about 74mph. Rear Admiral Sir Francis Beaufort devised the scale in 1805 for maritime use, since adapted several times. 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. Manufactured weather vanes come in a variety of shapes, sizes and patterns that just need to be positioned accurately using a compass. This must be balanced on a vertical rod providing minimal friction to assist free movement. 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. 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. It is one instrument that could be made by a competent DIY person with simple materials and a compass for positioning. ![]() The weather vane could be argued as one of the most useful instruments for a weather station. 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 design of a weather vane is more complicated than at first might be thought. Compass points were not added until several centuries later. Originally, fabric pennants would show archers the direction of the wind in order to assist their accuracy. The word “vane” comes from the Anglo-Saxon word “fane”, meaning flag. Contemporary reports from the 1st century BC describe one honouring the Greek god Triton. Weather vanes have been used since antiquity.
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