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Fig. 4.2 Plan of a meteorological station for the northern hemisphere (dimensions in metres). (Reproduced with permission from WMO 2008. Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8, 7th edn. World Meteorological Organisation, Geneva, 681.)

read at 0900 h GMT each day in the UK and, at this time, the maximum and minimum thermometers are reset. At some meteorological stations, the dry and wet bulb thermometers are read again at 1500 h, but where more detailed observations are required, manual measurements can be supplemented with an automatic weather station (Section 4.4.3).

screen

The enclosure also includes a sunshine recorder and anemometer. A standard Campbell-Stokes sunshine recorder has a glass sphere that focuses the Sun's rays on to a specially treated calibrated card where they burn a trace. The accumulated lengths of burnt trace give a measure of the total length of bright sunshine in hours. Three sizes of cards are used with the recorder according to the season, i.e. over the winter or summer solstice or the equinoxes. A single card records a day's sunshine and is therefore changed each day at the normal observational time 0900 h GMT with the sunshine before and after 0900 h being credited to the correct days. Within the UK, sunshine duration sensors (e.g. Kipp and Zonen CSD-3, which uses three photodiodes) are now used in preference to the Campbell-Stokes (Met Office, 2006).

The direction and speed of the wind are some of most important features of the weather. Although the hydrologist concerned with evaporation may not be unduly worried by wind direction, other duties with regard to real-time hydrological events should encourage the installation of an instrument to measure both characteristics. For measurements in the meteorological station enclosure, a cup anemometer is recommended and are often cup generator anemometers incorporating a remote-indicating wind vane. The instrument is fixed on a pole 2 m from the ground and attached to a simple counter or a data logger. The cup anemometer can give instantaneous readings of wind velocity (knots or ms-1) or provide a run-of-the-wind, a collective distance in kilometres when the counter is read each day.

4.4.3 Automatic weather stations

The reliability of automatic weather station (AWS) has improved over the last two decades, so that measurements formally made manually at reference meteorological stations can be obtained at a higher frequency with data logged sensors. An annotated photograph of an AWS is given in Fig. 4.3. The instruments, excluding the rain gauge, are mounted on an aluminium mast with two cross arms.

A net radiometer or net pyrradiometer (Fig. 4.4) is installed at the end of one of the arms to measure net radiation. Specifically, they measure the short- and long-wave radiation from around 0.3 to 70 /m via the heating of a black surface. One black surface faces down horizontally, another faces upward; a thermopile measures the difference in their temperatures and thus the energy exchange (Strangeways, 2003). Net radiation values required within evaporation calculations of the energy budget and combination methods (Chapter 10) may be derived from measurements of shortwave radiation alone, so sometimes the cheaper pyranometer (also called a solarimeter) replaces a net radiometer on the AWS. There are two types of pyranometer used for measuring short-wave radiation: the thermal solarimeter and the photodiodes sensor. The thermal solarimeter, like the net radiometer, senses radiation by measuring the heating effect of a black surface. The cheaper photodiode sensors use a light-sensitive diode, photodiode or photovoltaic cell to measure solar radiation, where the response is to individual incoming photons directly, rather than a heating effect of the thermal sensors (Strangeways, 2003).

Automatic weather stations typically use cup anemometers (Fig. 4.4) to measure wind velocity required by the mass transfer and combination methods of calculating evapotranspiration (Chapter 10).

Fig. 4.4 A net radiometer or net pyrradiometer. (Reproduced by permission of Kipp and Zonen BV.)
Fig. 4.5 A combined air temperature and relative humidity probe. (Reproduced by permission of Vaisala Ltd.)

Air temperature can be measured electronically using resistance thermometers or thermocouples. Resistance thermometers (either platinum resistance thermometers or thermistors) change their resistance as temperature changes. In contrast, thermocouples measure the potential difference resulting from the temperature difference between two different metals. Consequently, the latter device is less suited to use on an AWS.

Several methods can be used to measure the relative humidity of the air, though most AWS systems use thin-film capacitive sensors. As the name suggests, these devices measure the change in capacitance of a cell comprising an organic polymer layer sandwiched between a metal electrode and an upper metal film (Strangeways, 2003). The air temperature and relative humidity sensors need to be housed in a small specially designed thermal radiation screen (Fig. 4.5) instead of a conventional-type Stevenson screen.

All the instruments are wired to a data logger. The Met Office maintains a network of over 150 such automatic weather stations across the UK, with an average density of more than 1 every 1500 km2.

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