thermometer

A meteorological instrument that measures air temperature. There are different types, all of them based on the variation of some physical property experienced by certain substances in thermal equilibrium with the air, as the thermodynamic state of the air changes. Traditional thermometers contain a liquid (mercury or alcohol) inside a glass capillary tube connected to a small reservoir or bulb. Variations in air temperature cause the liquid to expand or contract, with the resulting rise or fall of the liquid in the tube. Thanks to the graduated scale it incorporates, a direct reading of the degrees reached by the air temperature surrounding the instrument can be taken. In weather observatories there is a great variety of thermometers, some of which - such as maximum and minimum thermometers, or those making up the psychrometer (the dry-bulb and wet-bulb thermometers) - are used for specific purposes. The use of the mercury thermometer is restricted to regions where cold is not extreme, since that liquid begins to freeze below –38.8°C and there are places on Earth where the air is at a lower temperature relatively frequently. To solve that problem, alcohol thermometers are used in weather observatories in the coldest regions. They usually contain ethanol (ethyl alcohol), whose freezing point is –114°C. Apart from mercury and alcohol thermometers, others such as the bimetallic strip thermometer, the resistance thermometer, or digital ones are also used, which incorporate sensors based on the same principles as some of those mentioned and which, thanks to an electronic circuit, convert temperature variations into a signal that can be displayed as digits on a liquid-crystal screen.