Book - School of Science and Technology

12 FundamentalsTable 1.7 Wind-chill equivalent temperatures ( C)*Air temperature ( C)Wind speed (m/s) 10.0 5.0 0 2.0 4.0 6.0 8.0 10.02 11.0 6.0 1.0 1.0 3.0 5.0 7.5 9.54 14.5 9.0 3.5 1.5 1.0 3.0 5.0 7.56 17.5 11.5 6.0 3.5 1.0 1.0 3.5 5.58 20.0 14.0 8.0 5.5 3.0 0.5 2.0 3.010 22.0 16.0 9.5 7.0 5.0 2.0 0.5 3.012 24.0 17.5 11.0 8.5 6.0 3.5 1.0 2.015 26.5 19.5 13.0 10.0 7.5 5.0 2.0 0.520 29.5 22.5 15.5 12.5 10.0 7.0 4.0 1.5* Dixon, J. C. and Prior, M. J., `Wind-chill indices ± a review'. The Meteorological Magazine, 1987, 116, 1.wheret res ˆ dry resultant temperature ( C)t ri ˆ mean radiant temperature ( C)t ai ˆ room air temperature ( C)v ˆ air velocity (m/s)It will be noted that, when the air velocity is 0.1 m/s, this expression may be simplifiedgreatly for use in the general run of calculations as t res ˆ (0:5 t ri ‡ 0:5 t ai ).In addition to the items noted, wet bulb globe temperature, wet resultant temperature andthe equitorial comfort index could be added. These, however, have narrow applications tospecific circumstances and are not in general use. Also excluded from the list are two scaleswhich are not comfort indices but which relate to the calculation of heat losses and gains.The more important of these is environmental temperature, a concept shown by theBuilding Research Establishment to provide a valuable simplification of the relationshipbetween air and mean radiant temperatures within a space. The second is sol±air temperatures,a scale which increments outside air temperature to take account of solar radiation.Wind-chill indicesAlthough the expression wind-chill will be familiar to most readers, as a result of occasionaluse by TV weather forecasters, it is not generally appreciated that the concept ofwind-enhanced cooling pre-dates the First World War. A paper by Dixon and Priorprovides a full history, including a digest of both the empirical and theoretical evaluationsand suggests that analyses of wind-chill indices by wind direction may be useful indeciding upon orientation and layout of new buildings, shelter belts, etc. Wind-chillequivalent temperatures for wind speeds of 2±20 m/s (5±45 mph) and air temperaturesbetween 10 C and ‡10 C are listed in Table 1.7.Methods of measurementThe descriptions which follow relate to very basic instruments which in many respectshave been superseded by electronic or other devices. They do, nevertheless, serve toillustrate requirements and to warn of pitfalls.