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Module 1 Contd...
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DATA AND MEASUREMENT OF ELEMENTS OF CLIMATE 1) Measurement of Temperature Many surface air temperature records extend back to the middle part of the last century. The measurement of the surface air temperature is essentially the same now as it was then, using a mercury-in-glass thermometer, which can be calibrated accurately and used down to - 39°C, the freezing point of mercury. For lower temperatures, mercury is usually substituted by alcohol. Maximum and minimum temperatures measured during specified time periods, usually 24 hours, provide useful information for the construction and analysis of temperature time series. Analysis involves the calculation of averages and variances of the data and the identification, using various statistical techniques, of periodic variations, persistence and trends in the time series. Temperature is a valuable climate element in climate observation because it directly provides a measure of the energy of the system under inspection. For example, a global average temperature reveals information about the energy content of the Earth-atmosphere system. A higher temperature would indicate a larger energy content. Variations in temperature are also subject to less variability than other elements such as rainfall and wind. Perhaps most importantly of all, our own perception of the state of the climate are intimately linked to temperature. 2) Measurement of Rainfall Rainfall is measured most simply by noting periodically how much has been collected in an exposed vessel since the time of the last observation. Care must be taken to avoid underestimating rainfall due to evaporation of the collected water and the effects of wind (Folland, 1988; Mueller & Kidder, 1972). Time series can be constructed and analysis performed in a similar manner to those of temperature. The measurement of global rainfall offers an indirect or qualitative assessment of the energy of the Earth-atmosphere system. Increased heat storage will increase the rate of evaporation from the oceans (due to higher surface temperatures). In turn, the enhanced levels of water vapour in the atmosphere will intensify global precipitation. Rainfall is, however, subject to significant temporal and spatial variability, and the occurrence of extremes, and consequently, analysis of time series is more complex 3) Measurement of Humidity
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Transcript of Module 1 Contd...
DATA AND MEASUREMENT OF ELEMENTS OF CLIMATE1) Measurement of TemperatureMany surface air temperature records extend back to the middle part of the last century. The measurement of the surface air temperature is essentially the same now as it was then, using a mercury-in-glass thermometer, which can be calibrated accurately and used down to -39C, the freezing point of mercury. For lower temperatures, mercury is usually substituted by alcohol. Maximum and minimum temperatures measured during specified time periods, usually 24 hours, provide useful information for the construction and analysis of temperature time series. Analysis involves the calculation of averages and variances of the data and the identification, using various statistical techniques, of periodic variations, persistence and trends in the time series.Temperature is a valuable climate element in climate observation because it directly provides a measure of the energy of the system under inspection. For example, a global average temperaturereveals information about the energy content of the Earth-atmosphere system. A higher temperature would indicate a larger energy content. Variations in temperature are also subject to less variability than other elements such as rainfall and wind. Perhaps most importantly of all, our own perception of the state of the climate are intimately linked to temperature.
2) Measurement of RainfallRainfall is measured most simply by noting periodically how much has been collected in an exposed vessel since the time of the last observation. Care must be taken to avoid underestimating rainfall due to evaporation of the collected water and the effects of wind (Folland, 1988; Mueller & Kidder, 1972). Time series can be constructed and analysis performed in a similar manner to those of temperature.The measurement of global rainfall offers an indirect or qualitative assessment of the energy of the Earth-atmosphere system. Increased heat storage will increase the rate of evaporation from the oceans (due to higher surface temperatures). In turn, the enhanced levels of water vapour in the atmosphere will intensify global precipitation. Rainfall is, however, subject to significant temporal and spatial variability, and the occurrence of extremes, and consequently, analysis of time series is more complex
3) Measurement of HumidityThe amount of water vapour in the air can be described in at least 5 ways, in terms of:1) the water-vapour pressure;2) the relative humidity;3) the absolute humidity4) the mixing ratio5) the dewpoint.A full account of these definitions may be found in Linacre (1992). The standard instrument for measuring humidity is a psychrometer. This is a pair of identical vertical thermometers, one of which has the bulb kept wet by means of a muslin moistened by a wick dipped in water. Evaporation from the wetted bulb lowers its temperature below the air temperature (measured by the dry bulb thermometer). The difference between the two measured values is used to calculate the air's water-vapour pressure, from which the other indices of humidity can be determined.
4) Measurement of WindWind is usually measured by a cup anemometer which rotates about a vertical axis perpendicular to the direction of the wind. The exposure of wind instruments is important (Johnson & Linacre, 1978); any obstruction close by will affect measurements. Wind direction is also measured by means of a vane, accurately balanced about a truly vertical axis, so that it does not settle in any particular direction during calm conditions.
