The annual mean temperature at a specific location is the average value from all temperature measurements that are recorded at regular intervals during all days of a year. In this case, a distinction is no longer made between the maximum daytime temperatures and the lowest nighttime temperatures, which are sometimes summarized separately.

## The mean daytime temperature as the first step

At the meteorological stations of the German Weather Service, all of which are technically set up in the same way according to a specified standard, the temperature profiles are continuously recorded, among other things. In order to generate a relatively accurate average value of the daytime temperature, it is 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 mean 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 the whole year

The mean annual temperature at a specific location can be calculated in a very similar way. For this purpose, all mean daytime temperatures (Tdaymean-i) at this measurement position from January 1st to December 31st of the same year are added 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, for example, 100 years, trends or wave-like fluctuations can be seen 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 increasing 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 deep. When laying water pipes, for example, you can limit yourself to a depth of 60 to 80 centimeters, because in our latitudes the frost never penetrates this depth. Nevertheless, with a certain phase shift, it is a little 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 temperature in the subsurface does not change at all with the seasons. The reader will not be surprised that the local mean annual temperature “coincidentally” regulates itself quite exactly there.

## Regional annual average temperatures

We are thus entering a very 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 mean 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, for example, in the Allgäu. Therefore, the question of the mean annual temperature always has to be limited locally, for example to a specific city or district.

## Isotherms depict interesting structures

Imagine dividing the Federal Republic of Germany into squares of 25 square kilometers each and assigning an average annual temperature to each of these squares. We connect the same numerical values with lines of the same temperature, the isotherms. The result is not a knotted tangle of lines, but an isoline plan with clear structures. An important line is, for example, the 10-degree isotherm, which runs roughly through Frankfurt am Main, extends into southeastern Bavaria and goes to the northwest roughly through Düsseldorf. In a broader sense, its course is a result of the Gulf Stream and it has been migrating slowly north-eastwards for years, which is also due to climate change.