Temperature inversions are common
The subtleties of temperature forecasting
In yesterday's topic of the day (02/14/2019), the current high pressure period itself and its effects on the daily course of temperature, on developments in nature and on pollen pollution were discussed. Since the DORIT high pressure area continues to shape our weather, today's topic of the day will pick out one of these sub-aspects and illuminate them in more detail: the forecast of the daily high and low temperatures in winter high pressure situations.
Extensive high pressure areas with a focus on Central Europe usually lead to less spectacular weather processes in Germany. In summer, for example, these prevent showers and thunderstorms from forming and ensure plenty of sunshine. In winter, on the other hand, when the air pressure is high, fog or high fog can lead to cloudy weather conditions in some areas. In such weather conditions, the warning activity is therefore often limited to the output of nocturnal fog or frost warnings. However, both the forecast of the minimum values and the maximum daily temperatures, especially in such calm winter high pressure situations, are subject to certain regional characteristics.
During longer periods of high pressure, so-called "temperature inversions" often occur in the winter half-year. The temperature does not decrease with altitude as usual, but increases. Such inversions result from two important processes: On the one hand, the cold and therefore heavier air collects in the valleys and depressions during the night and can only be warmed up there slowly during the day by the sun, which is low in winter. On the other hand, in a high-pressure area, the air sinks from greater heights and is heated at around 1 degree per 100 m (dry adiabatic heating). Often, however, this warmed air cannot penetrate into the valleys or the plains and is therefore only effective in the middle locations. In contrast to the previously mentioned "soil inversion", this effect is called "free inversion" or "sinking inversion". However, the result of both processes, which often occur in combination, is identical: in the mountains, especially at night, it is significantly warmer than in the valleys or in the plains. Last night, for example, a low of -6 degrees was measured in Arnstein-Müdesheim on the lower Main, while on the Wasserkuppe in the Rhön it was +3 degrees. There is a similar example in NRW: -5 degrees in the valleys of the Sauerland and +5 degrees on the Kahler Asten.
Such an inverse temperature situation does not only have an impact on the lowest values, but also on the highest values to be expected on the following day. Especially in midwinter, when the position of the sun is low, there is too little energy available to significantly warm the layers of air close to the ground. This also prevents thermal mixing of the lowest layers of the atmosphere. The result is cold air lakes in the valleys and depressions.
The formation of fog fields is closely related to these cold air lakes. If the air close to the ground is very humid and the nighttime cooling is noticeable, fog or high fog fields can form during the night. These now prevent solar radiation from penetrating to the ground during the day. This increases the formation of a cold air lake through fog. However, this problem will be in the background for the next few days, as the probability of fog is extremely low due to the very dry air mass.
Another peculiarity must be taken into account with the lows. The long-wave radiation of the ground at night is directly dependent on its emissivity. However, this differs significantly, for example, between normal ground or a built-up area and a snow-covered surface. Snow has significantly better radiation properties and thus leads to significantly lower night temperatures. However, not all snow is created equal. While freshly fallen snow is the best way to promote the cooling at night, this effect is reduced with aging snow. Therefore, the lows over snow will be higher day by day during this high pressure period.
At this point, only a section of the effects to be considered in the temperature forecast can be described. However, this clearly shows that you can invest a few thoughts in the regional and local temperature forecast even in calm weather conditions. However, we are now supported by very good statistical forecast models.
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