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Low / high pressure
Source: Physics, Uni - Munich
Low pressure areas
The surface of the earth warms up due to the sun's rays and acts like a heater on the surrounding air. The air expands when it is heated and, due to the expansion, has a lower density than the cold air. As a result, the warm air rises and also warms the layers of air above. At the same time, it moves away from "the heater" - the floor - and thus cools down.
If this process takes place in a larger area, a large package of warmer, expanded air is created, which is now higher stratified (piled up) than the surrounding air. When this difference in height increases, the warmer air begins to flow away laterally via the adjacent cooler air parcels. As a result, it cools down further, becomes heavier and eventually sinks back to the ground when it hits less cool layers.
This process creates a cycle: the air rises under high pressure, spreads sideways, and flows off to the areas of lower pressure. The remaining air can cool down under certain circumstances and thereby collapse. Due to the drain, it has lost mass, so that its pressure drops.
As a result of the warming, the air can also absorb more water. These are transported upwards through the ascent. When the air cools down, the moisture escapes again: clouds form at high altitudes. A first sign of the lateral outflow of warmer air masses is the formation of cirrus clouds (veil clouds), which are increasingly condensing. Cirrostratus clouds are formed as a result of the continued flow of more warm and more humid air. If warm air is continued to follow (advection), Altostratus form, followed by Nimbostratus, which are usually associated with prolonged drizzle, the so-called "land rain". As the stratus clouds rain down, the cloud cover continues to break up until the sun finally reaches the ground again and the air begins to warm up again.
Thermal and dynamic low
A depth of ground occurs when the density of the air near the ground drops by heating the air near the ground. The warm air dissolves from the ground and rises (thermals), which leads to a pressure drop at ground level (in higher layers, however, the air pressure rises slightly due to the warm air flowing in from the ground). The pressure drop near the ground leads to a large-scale influx of foreign air (winds).
An altitude low occurs when cold air sinks from a great height, which lowers the air pressure in the higher layers of the air. On the other hand, the air pressure increases slightly on the ground. The altitude low is at a level of 5 km, and is characterized by low temperatures compared to the surrounding area. These are mostly aged systems, in the form of constrictions (cut-offs) or drops of cold air from vertical troughs, with the formation of deep convection clouds. On the altitude weather map (e.g. 500 hPa topography) the altitude low is indicated by a few closed isohypses, it is hardly recognizable on the ground weather map. For Europe, the low altitude is particularly important for the Mediterranean region in autumn.
A thermal low pressure area is formed by differences in air density, which are caused by warming (solar radiation) or by cooling. Depending on the affected air layer, a distinction is made between ground level and altitude level.
Dynamic low (cyclones)
Dynamic low pressure areas exist when the air flow converges in deep layers of the atmosphere and diverges again in height, lifting the air in the center of the depth.
High pressure areas
Air masses drop sharply in a high pressure area. The air warms up so that no condensation and therefore no cloud formation can take place. Near the ground, the air flows from the high pressure area towards the low pressure area, it diverges. As a result, there is no formation of fronts in height. As the air masses sink, an inversion is formed. The clouds dissolve in the process.
A high pressure area builds up relatively slowly. The circulation forces prevailing in the subtropical areas of our earth ensure that the most stable high pressure layers can form there.
Because there are differences in origin and development, the high pressure areas are divided into three categories.
- A cold spike occurs when the air cools down, which happens in winter, for example, over cold land masses (e.g. in the heights of Central Asia). The air then has a higher density and exerts greater pressure on the ground. In the middle latitudes it can also arise in the form of flat wedges on the back of cyclones as an intermediate high.
- A dynamic high is generated by Rossby waves (polar front jet stream). The dynamic Azores high has a great influence on the weather in Central Europe.
- A high altitude is a high pressure area that occurs at high altitudes and is therefore shown in altitude weather maps. It is always associated with a depth in the ground, since the vertical pressure gradient is lowered when surfaces are heated and the relative reduction in air pressure on the ground with increasing altitude is reflected in a higher pressure relative to the horizontal environment. In the opposite case, you can therefore derive a height low from a floor level (also a thermal high).
If we select the places on a map that have the same air pressure in a certain area and connect them with a line, a circle is created. Such circles are called isobars.
This process is repeated again and again with a different air pressure without the circles touching or crossing each other. An international standard for the distance between the circles is on weather crats 5 hPa (hPa = the unit with which the air pressure is measured). The closer the lines of the circles are to each other, the higher the pressure gradient.
In the low pressure area, the pressure is lowest in the core, i.e. the smallest circle in a low pressure area. The further you get away from the core, the more the air pressure rises.
In a high pressure area this is exactly the other way around. Here the highest pressure is at the core of the high and the further you get away from the high, the more the pressure drops.
High pressure in the northern hemisphere
Low pressure in the northern hemisphere
World map for relative air pressure / topography
Since the air can absorb more moisture when it is heated and rises due to the lower density, this moisture is raised in higher layers. There the air cools down again and its water holding capacity is reduced.
When the air rises rapidly over a warm area and reaches great heights, it has absorbed more water than it can carry when cooled. It flows over a large area to the side and excretes moisture in the process: first cirrus clouds (veil clouds) form at great heights, which condense more and more - until finally cirrostratus clouds form. These clouds are pushed away to the side by air flowing in further. As the warm air continues to rise (advection), Altostratus are now formed, followed by Nimbostratus, which are mostly associated with rain. As the stratus clouds rain down, the cloud cover continues to break up until the sun finally reaches the ground again and warms the air.
Questions about deepening
In the following you can query, test - and above all apply your knowledge. If you have not yet understood something, then take another look at the texts and illustrations above.
Now it's getting serious;) We - the meteoblue team - wish you a lot of fun!
Look at the map on the right. It is a section of a pressure area at a height of 850 mbar (approx. 1.5 kilometers). Can you tell from the pressure gradient whether it is a low pressure area or a high pressure area?
Now follow the animated maps as the pressure area approaches Dublin. Also pay attention to the pressure map in the higher layers (height at 500 mb, approx. 5 km). Is the same distribution of the pressure area also found here?
We are on the maps in Europe - that is, in the northern hemisphere. Try to use the animated cloud and precipitation maps to see in which direction the wind is blowing in the pressure area (you can also tell whether this is a high or a low).
Caution: In the southern hemisphere, the winds in pressure areas blow the other way around than in the pressure areas in the northern hemisphere.
In the following you can see 4 different images. of which 2 are high pressure areas and 2 are low pressure areas. Make a note of which of these images are high and low pressure areas.
Tip: Pay attention to the pressure of the isobars.
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