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There are several kinds of anemometer for measuring the surface wind. Figure 14.3 shows a simple device used by dinghy sailors, and at some weather stations. The cup anemometer has been commonly used since its invention in 1846; cups on each of three radial arms from a vertical axis are driven by the wind, and their rotations are counted. The number of rotations multiplied by the distance around the cups' circle is proportional to the wind run, the distance that a parcel of air would travel. International agreement in 1956 fixed the knot as the unit of wind speed, though subsequent metrication of units leads to the use of kilometres per hour or, preferably, metres per second (Note 1.J).

Figure 14.2 The effect of various degrees of surface roughness on the wind-speed profile.

Alternatively, one can use a landlubber's version of the Beaufort Scale, devised in 1896 by a British admiral to determine wind speed from the appearance of waves at sea (Table 14.1). (It is hard to measure the wind speed from a small boat at sea, because of its rocking and the eddies caused by the superstructure.) Beaufort scale 4 (a moderate breeze) is enough to make a dinghy 'plane', while 8 on the Beaufort scale is a gale, which is too strong for dinghy sailing, so a red triangular pennant is often flown as a warning. Two such pennants mean a strong gale.

A new method of determining the surface winds anywhere at sea involves an orbiting satellite which sends a radar beam onto the waves and then measures the back-scatter. The character of this depends on the waves' size, being different when waves are large, i.e. winds are strong. In particular, the back-scatter of radar at wavelengths of 2 cm responds rapidly to changes in wind speed and direction.

Nowadays, there are also 'Doppler radar' and 'acoustic profile' equipments, which detect wind by its effect on the echoing of pulses of radiation or sound sent from the instruments. For instance, air approaching the equipment returns an echo at a slightly higher frequency, just as the sounds of a train when it is coming towards us are at a higher pitch than when it is departing. Other methods of measuring winds aloft were mentioned in Section 12.3, including observing the sideways deflection of rising balloons. This is still common, partly to check more modern methods.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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