Monitoring the weather

Temperature (continued)

Measuring temperature
Air temperatures are usually measured with a mercury-in-glass thermometer. When the bulb is heated, mercury is forced further up the fine bore of the tube, and when the bulb is cooled the mercury retreats. There are now sophisticated electrical instruments for measuring temperature, but liquid-in-glass thermometers are still used for most meteorological observations.

If a thermometer is exposed to the sun's rays, however, it will absorb radiation and give a higher reading than it would if the sun were obscured by clouds. Also, if the thermometer is near the ground, it will often read higher than if it were further away. Air temperatures are therefore measured in the shade under strict standard height conditions (1.5 m).
Together with other measuring instruments, thermometers are placed in a special meteorological or Stevenson screen. Its double-angled white-painted louvres block direct sunlight from the interior, but allow air to circulate freely.

Recording maximum and minimum temperatures (of considerable public interest, especially during heat waves or during frosts) requires special thermometers. The maximum thermometer has a constriction in the bore just above the bulb. When the temperature rises, the mercury is forced past the constriction, but when the temperature begins to fall the mercury is prevented from returning due to the constriction. Thus the thermometer will show the highest temperature reached. The clinical thermometer used to register your body temperature is also a maximum thermometer.

Alcohol is used in minimum thermometers because it has a lower freezing point (- 130°C) than mercury (- 39°C). These thermometers contain a small dumbbell shaped metal index. As the temperature falls the index is drawn along with the alcohol as it retreats. After the lowest temperature is reached and the temperature starts to rise again, the alcohol rises in the tube but flows past the index, leaving its position fixed. The minimum temperature is read where the upper end of the index joins the alcohol meniscus.