What is the benefit of the potential temperature

The potential temperature and the foehn observation

"Across Germany, the maximum values ​​today are between 17 and 23 degrees". These or similar statements can be found in the daily weather reports. No wonder, because there is great public interest in the development of temperature. Do you have to reckon with a long-lasting heat wave, does Father Frost cause the freezing cold or does a mild late summer evening invite you to grill in the fresh air?

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In meteorology, the air temperature is of particular interest. This is measured 2 m above the ground, protected in a white weather hut, away from external influences such as the ground heat flow or solar radiation. If one follows the temperature over several days, one obtains the corresponding daily curve of the temperature between day and night. If one extends the series of observations and plots the monthly mean temperatures, then one can determine the annual course of the temperature.

In meteorology there are also many other temperatures, such as the perceived temperature or the wet temperature, although the so-called "potential temperature" will now be discussed in more detail below. This is a fictitious, i.e. calculated value. The aim of this calculation is to be able to compare temperature measurements from different altitude ranges with one another or, to put it another way: the potential temperature indicates what temperature an air parcel assumes when it is brought to sea level.

Not infrequently, when preparing the weather report, one asks the question of which altitude level an air mass originates from. An important property of the potential temperature is its independence from rising or falling processes. How can you imagine that? If you look at a particle of air and follow how it flows over a mountain, for example, the following can be observed: As soon as the particle begins to rise, it cools down and when it sinks back down on the other side of the mountain, it warms up again. In contrast, however, the potential temperature of this particle does not change. So it can be said that the air, apart from a few exceptions, which are not discussed in detail here, flows along lines of equal potential temperature (the so-called "isentropes"). It all sounds very complex, but this fact can be illustrated well using an example from reality.

The picture shows the effects of a striking foehn situation, which affected the eastern part of Lake Constance at the beginning of March 2017. The left picture shows the measured air temperature 2 meters above the ground. Where the hair dryer broke through, the temperature climbed to 14 degrees or more. The foehn breakthrough can also be seen in the picture on the right, where the wind broke through with gale force, sometimes with hurricane-like gusts down to the lower elevations. Now one can wonder at what altitude the blow-dryer air comes from. This question can be answered at least partially with the help of the potential temperature (see middle picture). You can see that where the hair dryer broke through, values ​​of the potential temperature of 18 to 21 degrees were calculated, while away from the hair dryer they were around 10 Kelvin lower. The hair dryer definition of the researcher H. v. Ficker from 1920 says that "... the potential temperature in a valley location is the same as that of any higher, foehn-coated locations [...]". Since the air flows along isentropes, one now has to find similar or identical values ​​in different altitude ranges. In our example, the potential temperature on the 2500 m high Säntis (air temperature at -3 degrees) is similar to the potential temperature on the southeastern Lake Constance (Altenrhein at 400 m with an air temperature of +16 degrees). A mass of air that sinks after flowing over a mountain range dries and warms up dry adiabatically (see: http://bit.ly/2r4ZNS9), i.e. by 1 degree per 100 m. This would be a temperature difference with an altitude difference of 2100 m of 21 Kelvin. Altenrhein would have to report +18 degrees. But since the potential temperature in Altenrhein is still slightly lower than that of the Säntis, the air mass did not drop from 2500 m for the time being and could not warm up as much. Just 1 hour later, however, the 18 degrees were reached in many places in this region and you could say that the foehn air had broken through from the height of the Säntis to the ground.

This example of the foehn observation with the help of the potential temperature shows only one possible use out of many and therefore it is not surprising that the potential temperature is an important and frequently used parameter in the weather forecast.