What is temperature?

Temperature is a physical quantity that describes heat and cold and is proportional to the average kinetic energy of a mass.
Thermometers are used to measure the temperature. These can be standardised to one of three scales:

  • the Fahrenheit scale (° F), which is especially popular in the US
  • the Celsius scale (° C), which is the most commonly used
  • the Kelvin scale (K), which represents SI unit for temperature.

The Kelvin and the Celsius scale have exactly the same intervals for a temperature difference of 1 °, they differ only by their zero point.

In meteorology, temperature is differentiated into air temperature, surface temperature, soil temperature, water temperature and temperature in altitude. In meteorology, as a rule, the air temperature is considered, so all other details refer to this ambient temperature, but are referred to in the following only briefly with temperature. Often day or night temperatures are given to describe the weather of a day.

In addition, it should be noted that temperature is highly dependent on altitude. This means, that the air cools by about 1°C per 100m height increase. Therefore, it is important to know on which altitude the measuring station is and in which height the temperature sensor is fixed, so that the height and thus also the temperature difference to other sensors or stations can be considered correctly.

How is temperature measured?

All thermometers take advantage of various physical properties of fabrics that change with changes in temperature. This can be, for example, the increase in volume of a liquid, the electrical resistance of a metal or the change in the volume of a metal.

According to WMO (World Meteorological Organization), weather stations for measuring air temperature are the most commonly used devices with electrical resistance. The temperature is measured in an interval of a few seconds, from which then a 5-minute average is formed. At the full hour, these 5-minute funds are then aggregated to an hourly average. For non-automated - mostly private - stations, the measurement interval can also be one hour or even one day.

How should a thermometer of a weather station be set up?

Figure 1: Weather station in Cham, Switzerland

A thermometer for measuring the air temperature has to be placed on a level surface (ideally short cut grass) and two meters above the ground according to WMO standards. The measuring environment has to be able to be illuminated directly by the sun, and no major obstacles (trees, buildings) affecting the circulation of air must be nearby. For the measurement to actually measure the temperature of the air, the thermometer has to be in a well-ventilated, white protective box. This protection ensures that no direct radiation can hit the device, which heats it more than the ambient air. The ventilation can be provided either naturally with the help of slats or artificially with the help of fans. To measure the temperature at different heights, measuring masts are used up to about 300m high. For measurements above these 300m, weather balloons are used.

Which measuring instruments are used to measure the air temperature?

National meteorological services (such as MeteoSwiss) mostly use one of the two following devices to measure temperature:

Figure 2: Protective white box of a temperature sensor

  • An instrument with a copper constantan thermocouple
  • An instrument with a platinum resistance thermometer (PT100)

The device with the PT100 sensor is the newer of the two used and slowly displaces the copper constantan thermocouple.

The PT100 sensor responds to the change in conductivity of the metal platinum. The copper-constantan element, on the other hand, generates a very small voltage proportional to the temperature of the element's solder joint due to the Seebeck effect.

Which other measuring methods are available?

Figure 3: High precision mercury thermometer

The sensors found in any digital home thermometer are based on the same technology as the professional equipment, but with much cheaper materials. It is a conductive material used, the resistance of which is temperature dependent.

The classic old-fashioned thermometers use mercury, which - thanks to its low heat capacity - can increase or decrease its volume even at low temperature changes. However, these thermometers can not be automated and the accuracy is highly dependent on the observer.

Another outdated method of temperature determination is the bimetallic method. Two metal plates with different coefficients of thermal expansion are stapled together. When heated, the plates deform differently and you can calculate the difference in temperature by the difference of the plates.

Difficulties of temperature measurement

The biggest measurement errors are based on the wrong installation of the instruments. If the measuring instruments are fastened too close to the ground, they not only measure the air temperature, but also the heat which is emitted by the earth. When a device is irradiated directly from the sun, the measurements of this unit are far too high. Guaranteed ventilation must not be neglected, because standing air heats up a lot faster than moving air. Thus, if there is complete calm in the box of the measuring instrument, the measured values are always too high in the case of solar irradiation.

If the temperature sensor is installed over a tar or asphalt surface, the measurement results will also be falsified as these surfaces (unlike grass) will heat up extremely as a result of solar radiation. For uniform and thus comparable measurement results, it is therefore essential to follow the rules of the WMO.

How many measuring stations are there?

In Switzerland, there are 154 official measuring stations from MeteoSwiss which measure the air temperature. However, this is not enough for a nationwide measuring network, which could represent Switzerland, since there are large differences in altitude in Switzerland and the temperature changes significantly with altitude.

Therefore, meteoblue also uses numerous private measuring stations, which are evaluated and checked for completeness of the data in order to offer services such as Nowcast or the weather maps in more detail. However, these metering stations may fail as they do not necessarily conform to WMO standards, so all measurements undergo automated quality control before being used for forecasting or other purposes.

Figure 4: Worldwide distribution of WMO-Weather Stations

According to the National Oceanic and Atmospheric Administration (NOAA), there are currently over 14,000 active weather stations operating daily updated and meet all WMO standards. Most of them are in the US and Europe. meteoblue uses about 70'000 weather stations worldwide, where in addition to the WMO stations, other public and private networks are used. There are well over 100,000 private weather stations that measure and record weather data. Looking at all temperature sensors installed in cars, smartphones and other electronic devices, several million temperature sensors are in circulation, with a strong upward trend.