For the next few days, a drop of cold air will lie over the center and south of the country. What is meant by this and how much rain is to be expected?
Weather maps, such as those contained in newspaper weather reports, show areas of high and low air pressure in the lowest layer of the atmosphere, i.e. close to the ground. In addition to these “usual” high and low pressure areas, so-called highs and lows also occasionally occur. What they have in common is that they only appear on the weather maps as circular pressure structures, i.e. with closed lines of the same air pressure (isobars), from a height of a few kilometers. The ground weather maps only give an idea of their presence in the form of a wave in the isobars.
At a certain altitude, that is to say viewed horizontally, highs and lows are associated with a more or less circular area of relatively warm and cold air, respectively. This area is typically several hundred kilometers in diameter. Since they simultaneously extend over a few kilometers in height, you have to add the vertical and imagine the whole thing spatially. This results in approximately cylindrical structures that are filled with warm or cold air. In the following we want to limit ourselves to the cold representatives, i.e. the high altitude lows or cold air drops.
Why can cold air droplets only be recognized well or at all in the high-altitude weather maps? There is actually a very simple reason for this. As is well known, the air pressure decreases as you go up in the atmosphere. However, this decline is not the same everywhere, because it depends to a very large extent on the air temperature. For example, the air pressure in cold air decreases faster with altitude than in warm air.
Now let’s imagine again the cold cylinder of air surrounded by comparatively warm air. In addition, the air pressure on the ground, i.e. in its immediate horizontal surroundings, should not vary. So there shouldn’t be any highs or lows. If we climb up inside the cylinder, the air pressure – because of the cold air – will drop relatively sharply. Everywhere around – in the comparatively warm air – there is only a relatively small drop in air pressure during such an ascent. The air pressure difference between and below and above increases faster in the cold area than in its warm surroundings. In order to obtain the air pressure at a certain altitude, this air pressure difference must be subtracted from the air pressure near the ground. Because the surface air pressure is assumed to be the same, there is a low in the area of the cold air, i.e. our cold air droplet, which becomes more and more pronounced with altitude.
Although cold air droplets are a special form of low-pressure area, they are usually accompanied by clouds, rain or showers. The current drop of cold air is no exception. Our figure shows the precipitation as it can be expected up to and including Friday. While in the center between 5 and 20 liters of rain per square meter (l/sqm) can accumulate (in the Ore Mountains up to 40 l/sqm), you can only expect around 10 l/sqm in places towards the south-west.