Measurement of temperature
The temperature is measured in Celsius scale with an accuracy of decimal grades. In the traditional measurements we use three types of thermometers, the station, the maximum and the minimum thermometer which are placed in an English-type screen. The station thermometer is in vertical position, its mercury bulb is by 2 m over the surface. The minimum thermometer is placed over it in a vertical support and directly over it the maximum thermometer mounted from the horizontal on its right end.
Station thermometer
We use it to determine the momentary value of the air temperature. The station thermometer is a mercury-in-glass thermometer with an internal scale used in vertical position. Its structure is simple, a vessel of spherical form continued in a so-called capillary. The vessel is filled by mercury so that it reaches the capillary. The volume of mercury increases when temperature raises and decrease it when temperature falls, so the mercury column in the capillary raises or sinks. The surface of the capillary is thin behind it there is a white scale with markings of 0.2 grades and the scale is between -35ºC and +45ºC in Central Europe.
Maximum thermometer
We use it to determine the maximum air temperature between two observations. It is working the same way as the fever thermometer and is a mercury-in-glass thermometer. There is a constriction in the bore between the bulb and the beginning of the scale. This constriction prevents the mercury column from receding with falling temperature. Thus, the end of the mercury column show the maximum temperature. After use it can be reset by holding it firmly, bulb-end downwards and swinging it until the mercury column is reunited similarly to the fever thermometer which is also a maximum thermometer. The scale is marked of 0.5 grades.
Minimum thermometer
We use it to determine the minimum temperature between two observation. It is a spirit thermometer with internal scale. The liquid is colourless or painted spirit. There is a blue or red glass index in the capillary near the surface of the spirit. In case the temperature falls, the surface of the spirit draws the index, and when the temperature raises, the spirit expands and advances in the capillary, but leaves the index on the lowest place. The thermometer must be in vertical position so that the index should not remove due to its weight. The scale is marked of 0.5 grades.
Vaisala HMP 35D, HMP 45D type electrical thermometer and hygrometer
In case of automatic stations we use a combined instrument for measuring air temperature and humidity which is placed in an English-type screen or under a screen protecting against radiation. The temperature is measured continuously by a platinum resistance thermometer. The continuous sampling makes it possible to determine temperature frequently. The automatons have been set so that the measured data are available in every 10 minutes. We receive in every 10 minutes the 10-minute mean, 10-minute maximum and 10-minute minimum values. Beside this frequency of measurements there is no need for separate instrument for determining extreme temperatures.
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