The annual mean temperature at a given location is the average of all temperature measurements taken at regular intervals during all days of the year. In this case, a distinction is no longer made between the highest daytime temperatures and the lowest nighttime temperatures, which are sometimes summarized separately.
The mean daily temperature as the first step
At the meteorological stations of the German Weather Service, which are all technically structured in the same way according to a defined standard, the temperature profiles, among other things, are continuously recorded. In order to generate a relatively accurate average value of the daily temperature, it is already sufficient to note the temperature reading every half hour and add it up:
Tdaymean = Mean daily temperature = Sum(Ti)/48
The Ti values are the temperature readings that are taken every 30 minutes. The average daily temperature determined in this way comes very close to the scientifically exact value that results from the time integral of the continuous temperature curve in relation to the daily interval.
Extension of the temperature observation to include the whole year
The mean annual temperature at a specific location can be calculated in a very similar way. For this purpose, all mean daily temperatures (Tdaymean-i) at this measurement position from January 1st to December 31st of the same year are added up and divided by 365 days:
Tyearmean = Mean annual temperature = Sum(Tdaymean-i) /365
This result can vary significantly from year to year. If you look at these values over a very long period of time, for example 100 years, you can see trends or wave-like fluctuations in the figure. When discussing climate change, for example, such results are used to support arguments. One thing becomes clear when looking at such long-term curves: there has indeed been a clear trend towards an increase in mean annual temperatures, especially in recent years.
How the mean annual temperature can still be determined
The annual temperature wave penetrates the ground, but not very deeply. When laying water pipes, for example, you can limit yourself to soil depths of around 60 to 80 centimeters, because in our latitudes the frost never penetrates to this depth. Nevertheless, with a certain phase shift, it is somewhat cooler there in winter than in summer. If you penetrate further into the ground, and this also applies to aquifers, you come to a depth of a little more than two meters in zones where the ground temperature in the subsoil no longer changes at all with the seasons. It will not come as a surprise to the reader that the local mean annual temperature “accidentally” settles in there quite exactly.
Regional annual mean temperatures
This brings us into a rather complex field. Anyone interested in the mean annual temperature of a small country like Luxembourg may find what they are looking for in the available meteorological data. But what about the average annual temperature in Germany or the USA? The question alone is more than questionable. Everyone knows that the climate in Schleswig-Holstein is very different from that in the Allgäu, for example. That's why you always have to narrow down the question of the mean annual temperature very locally, for example to a specific city or district.
Isotherms depict interesting structures
Imagine the Federal Republic of Germany in Divide into squares of 25 square kilometers each and assign an average annual temperature to each of these squares. We connect the same numerical values with lines of the same temperature, the isotherms. This does not result in a knotted tangle of lines, but in an isoline plan with clear structures. An important line, for example, is the 10-degree isotherm, which runs roughly through Frankfurt am Main, extends into south-eastern Bavaria and goes north-west through roughly Düsseldorf. Its course is more broadly a result of the Gulf Stream and has been slowly moving northeast for years, also due to climate change.