Temperature

measurements are normally made at 09.00 Greenwich Mean Time (GMT) each day. Most of the instruments are housed in a characteristic Stevenson's Screen although there are usually other instruments on the designated ground or mounted on poles nearby (see Figure 2.1).

Horizontal Min Max Thermometers
Figure 2.12 Stevenson's Screen showing Max-Min Thermometer (horizontal) and Wet and Dry Thermometer (vertical).

The normal method of measuring the air temperature uses a vertically mounted mercury-in-glass thermometer that is able to read to the nearest 0.1°C. In order to obtain an accurate result, thermometers used must be protected from direct radiation i.e. the readings must be made 'in the shade'.

In meteorological stations, they are held in the Stevenson's Screen (see Figures 2.1 and 2.12), which is designed to ensure that accurate results are obtained at a standard distance from the ground. The screen's most obvious feature is the slatted sides, which ensure that the sun does not shine directly on to the instruments (see radiation p26) whilst allowing the free flow of air around the instruments. The whole screen is painted white to reflect radiation that, along with its insulated top and base, keeps the conditions inside similar to that of the surrounding air. In controlled environments such as glasshouses the environment is monitored by instruments held in an aspirated screen, which draws air across the instruments to give a more accurate indication of the surrounding conditions (see p116).

The dry bulb thermometer is paired with a wet bulb thermometer that has, around its bulb, a muslin bag kept wet with distilled water. In combination, they are used to determine the humidity (see below). Robust mercury-in-glass thermometers set in sleeves are also used to determine soil temperatures; temperatures both at the soil surface and at 300 mm depth are usually recorded in agro-meteorological stations.

The highest and lowest temperatures over the day (and night) are recorded on the Max-Min (maximum and minimum) thermometers (see Figure 2.12) mounted horizontally on the floor of the screen. The maximum thermometer is a mercury-in-glass design, but with a constriction in the narrow tube near the bulb that contains the mercury. This allows the mercury to expand as it warms up, but when temperatures fall the mercury cannot pass back into the bulb and so the highest temperature achieved can be read off ('today's high'). Shaking the contents back into the bulb resets it. The minimum thermometer contains alcohol. This expands as it warms but as it contracts to the lowest temperature ('tonight's low') a thin marker is pulled down by the retreating liquid. Because it is lightly sprung, the marker is left behind whenever the temperature rises. Using a magnet, or tilting, to bring the marker back to the surface of the liquid in the tube, can reset the thermometer. In addition to the screen reading, there are other lowest-temperature thermometers placed at ground level giving 'over bare soil' and 'grass' temperatures (see Figure 2.1).

Precipitation

The term precipitation covers all the ways in which water reaches the ground as rain, snow and hail. It is usually measured with a rain gauge (see Figure 2.13).

Improved Dipstick

Figure 2.13 Rain gauges. A simple rain gauge consists of a straight-sided can in which the depth of water accumulated each day can be measured with a dipstick. An improved design incorporates a funnel, to reduce evaporation, and a calibrated collection bottle. A rain gauge should be set firmly in the soil away from overhanging trees etc. and the rim should be 300 mm above ground to prevent water flowing or bouncing in from surrounding ground.

Figure 2.13 Rain gauges. A simple rain gauge consists of a straight-sided can in which the depth of water accumulated each day can be measured with a dipstick. An improved design incorporates a funnel, to reduce evaporation, and a calibrated collection bottle. A rain gauge should be set firmly in the soil away from overhanging trees etc. and the rim should be 300 mm above ground to prevent water flowing or bouncing in from surrounding ground.

Simple rain gauges are based on straight-sided cans set in the ground with a dipstick used to determine the depth of water collected. Accurate readings to provide daily totals are achieved with a design that maximizes collection, but minimizes evaporation losses by intercepting the precipitation water in a funnel. This leads to a tapered measuring glass calibrated to 0.1 mm. These gauges are positioned away from anything that affects the local airflow e.g. buildings, trees and shrubs. They are set in the ground but with the rim above it to prevent water running in from the surroundings.

Recording rain gauges are available which also give more details of the pattern of rainfall within twenty-four hour periods. The 'tipping bucket' type has two open containers on a see-saw mechanism so arranged that as one bucket is filled, it tips and this is recorded on a continuous chart; meanwhile the other bucket is moved into position to continue collection.

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