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Low Pressure Area
A low pressure area is an area where the air pressure is lower than in surrounding areas. One differentiates between thermic and dynamic low pressure areas. A thermic low pressure area occurs when the air density varies, caused for example by insolation or a cool down period. A surface low is situated close to the ground, a surface high is high. The differences between a high pressure area and a low pressure area are not only horizontal but also vertical. A dynamic low develops when there are air movements that converge in lower layers of the atmosphere and expand again when they are at higher elevations. As a result the pressure drops at higher altitudes and there is a kind of pull which sucks the air up from the ground and upwards, which in turn leads to the pressure falling on the ground. All in all these are very complex processes for which large amounts of data must be evaluated in order to make a precise prediction.
When the air rises, which is common in low pressure systems, due to the lower temperature it leads to the clouds no longer being able to hold the moisture that they store. The water condenses and forms clouds from which rain falls to the ground. That is why one always associates “bad” weather with low pressure areas. Clouds block the sunlight and thus lead to lower temperatures and rain requires an umbrella, if one does not want to get wet. But rain in agriculture is frequently very welcome and in some areas even existential.
Like temperature, air pressure tends to constantly compensate when there are differences. Air from a high pressure system or a normal pressure system flows back in the direction of the low pressure system which causes a pressure compensation after a certain period of time. Due to the earth’s rotating, this movement, which we experience as wind, is not direct but in turn rotate themselves, which explains the
typical “low pressure eddies” one sees on weather maps. The low pressure systems themselves in our regions also generally move from West to East. Iceland is one of the places where low pressure systems frequently develop and then move towards Central Europe, which is why these low pressure systems are referred to as Iceland lows.
Lows can be predicted easily with barometers. When the air pressure falls, a low pressure system can be expected to arrive soon. When the air pressure rises again, the low has arrived and a high system is announcing itself. One must not use only the absolute figures but must keep the time sequence in mind.