Forecasting the weather

 

  
Contents | PreviousBack|NextNext
 

Monitoring the weather

Temperature (continued)

Thermometer screen

Thermometers are protected from direct sun. The screen is painted white to reflect more sun, and must be correctly located away from walls, trees and other objects. Louvres allow air to move gently past the thermometers.

Maximum and minimum thermometers

Maximum and minimum thermometers, stylised to show functions clearly. A is a maximum thermometer; B a minimum thermometer; C is a section of a maximum thermometer; D a section of a minimum thermometer showing both the current air temperature and the minimum temperature.

(A) Mercury thread breaks at the constriction as the temperature falls. The thermometer is reset by shaking the mercury back into the bulb.

(B) The index is drawn back to the minimum position as the temperature falls. The thermometer is reset by tilting it, bulb end upwards.

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 (1.2 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.

 

How to forecast the weather up arrowTop | Contents | PreviousBack|NextNext