Tuesday 14 July 2020
Description of instruments

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.

kep3 Precipitation measurements

Precipitation gauge is a collector vessel with an exactly defined collecting surface. The collected precipitation is poured in a much smaller glass cylinder with scale, so the quantity of precipitation can be defined with a tenth mm accuracy. The precipitation amount in millimetres means on a territory of 1 m2 1 litre water/millimetre. In case of solid precipitation (snow, ice, etc.) the quantity of the melted precipitation is measured.

From the beginning of the 1900s, during 2-3 decades, we changed gradually from the precipitation gauge with a collecting surface of 1/10 m2 to the use of 1/20 m2 and then of 1/50 m2 precipitation gauges.

In the beginning of the 1920s, in the course of the reorganization of the network we changed to the 1/50 m2 Hellmann-type precipitation gauge with bronze ring. Other European countries began to use precipitation gauges with the same collecting surface. We were forced to use the 1/50 m2 Hellmann-type precipitation gauge with aluminium ring as copper was a scarce article during the Second World War.

Later Mihály Csomor and his colleague elaborated the 1/50 m2 Hellmann-type double with double wall aluminium precipitation gauge and the stations were installed with these instruments from the autumn 1962.

By 1966 the full exchange of instruments was finished. In our days we are still using this type in traditional precipitation measurements.

kep4 Measurement of sunshine duration

In our observation network we have been using the Campbell-Stokes sunshine recorder since the beginnings which uses the heating effect of solar radiation. The instrument consists essentially of a glass sphere mounted concentrically in a section of a spherical bowl with a diameter of 96 mm. The Sun’s rays are focused sharply on a card held in grooves in the bowl. Where the collected Sun’s rays fall the cards are burnt strongly or less strongly depending on the strength of the radiation according to the advancement of the Sun on the Sun’s path.

Three types of record cards are used according to the different heights of the orbital curve in the different seasons: summer, winter and spring-autumn. The burn cards have to be exchanged every evening.

The sunshine duration calculated based on summarizing the burns on the burn card with an accuracy of tenth hours.

Measurement of global radiation

The most commonly measured solar radiation parameter at the meteorological stations is global radiation. It is the most important component of radiation balance, also a basic climate parameter. It is the sum of direct radiation received from the Sun and diffuse radiation incoming from the whole sky.

Global radiation is measured by pyranometer. This instrument normally uses thermoelectric sensor. The radiation causes temperature difference between the instrument body, the sensor and the glass dome. An electric current is initiated as a result and its intensity is closely related to the intensity of the incident radiation.

The radiant energy of the Sun is measured in W/m2, daily sum is given in J/cm2.