Low pressure area
A low pressure area (short: the low , also the disturbance ) is a part of the earth's atmosphere with lower air pressure compared to its large-scale environment. Conceptually and spatially, it faces the high pressure area . One speaks of a low pressure area only if a center can be identified from which the pressure increases horizontally in every direction. In other cases it is a trough .
A distinction is made between thermal low pressure areas and dynamic low pressure areas ( cyclones ).
species
Thermal low pressure area: soil low, altitude low
A thermal low pressure area is formed by differences in air density that are caused by warming ( solar radiation ) or by cooling .
The air pressure at any point in the earth's atmosphere is the hydrostatic pressure of the air that prevails at that location. This pressure is created (illustrated) by the weight of the air column that stands on the earth's surface or on a body . Under a column of air with a lower density, the hydrostatic pressure is also lower and therefore less air pressure is measured with a barometer , which is referred to as low pressure. In contrast, in the high pressure position, the air column is heavier; relatively higher pressure - high pressure - is measured.
Depending on the affected air layer, a distinction is made between ground level and altitude level.
Ground deep
A depth in the ground occurs when the density of the air near the ground decreases due to warming (solar radiation). Surrounding and overlying colder and therefore denser air creates a lift of the heated, lighter air due to the hydrostatic pressure . The colder air flowing in is continuously warmed up and swelled by the sunlit ground, so that a continued upward flow of warm air is established (thermal).
If heated air masses rise from the ground, they cool down by approx. 1 ° C per 100 m. One speaks of the " dry adiabatic temperature gradient ". Initially, the amount of water vapor contained in the air package remains unchanged. At a certain altitude, the temperature of the air mass reaches the dew point . This is where condensation sets in and a cumulus cloud forms. The cloud base is exactly at this height.
The condensation heat released during condensation also leads to a reduction in density (see moisture adiabatic temperature gradient ), so that further buoyancy arises from the hydrostatic pressure of the colder ambient air. In this area, a column of less dense air with a correspondingly lower weight force gradually arises, a low pressure area with a large-scale influx of foreign air (winds, storms).
High altitude
An altitude low (also altitude low core , HTK or Upper Level Low , ULL) describes the situation over a (ground) high pressure area. It is created by cold air falling from great heights, reducing the air pressure in the higher layers of the air and increasing it on the ground. Altitude lows are several kilometers high and are characterized by - compared to the surroundings - low temperatures. From this, conclusions can be drawn about the age of the low. Often there is no equivalent near the ground to an altitude low, but they are sometimes very weather-effective. High altitude lows determine the weather activity in the Mediterranean area, especially in autumn in the form of cut-off lows or cold air drops from high troughs, because the warm water of the Mediterranean Sea warms the lowest air masses and enriches them with moisture. There, altitude lows are indicators of abnormal rainfall. On the altitude weather map (e.g. 500 hPa topography) the altitude low is indicated by a few closed isobars .
Dynamic low (cyclones)
A dynamic low pressure area (also called a cyclone ) occurs when the air flow converges ( converges ) in the deeper layers of the atmosphere and diverges again at high altitudes, thereby losing speed. In a low pressure area, the air flows inwards due to the pressure gradient. In the northern hemisphere, this flow is deflected to the right by the Coriolis force, resulting in a counter-clockwise rotation.
Often the names for a low pressure area (the cyclones , plural the cyclones) and a tropical cyclone of the Indian Ocean (the cyclone , plural the cyclones) are confused.
Wind currents
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Instead of flowing radially towards the low pressure area from all sides , the inflowing air rotates around a vertical axis into the low pressure area. This is due to the earth's rotation ( Coriolis force ) and the earth's spherical shape. In the northern hemisphere of the earth, a moving air mass is deflected to the right in the direction of movement by the Coriolis effect, which adds up to the deflection in the direction of the greatest pressure gradient ( Baric wind law ) and in the southern hemisphere it is the other way round.
Winds flowing into a low pressure area are diverted to the right. They rotate (viewed from above) against the clockwise - so mathematically positive direction of rotation. Low pressure areas are therefore also called cyclones .
Winds flowing out of a high pressure area are also diverted to the right. They rotate (viewed from above) in a clockwise direction - ie in the mathematically negative rotation. High pressure areas are therefore also called anticyclones .
The Coriolis force disappears at the equator . Towards the Poles it increases.
On a global scale, winds at altitude run in the opposite direction to the winds on the ground due to the Coriolis force, because warm air flows from the tropics towards the poles within the framework of the planetary circulation . Due to the Coriolis force, it is deflected in an easterly direction so that strong westerly winds ( jet stream ) prevail at the altitude . The polar cold air flowing back on the ground is deflected in a westerly direction by the Coriolis force ( polar east wind ).
On a regional scale, thermal low pressure areas also form over warm water surfaces, over which moist warm air rises and cools down in the process. Circulations such as land wind and sea wind arise locally . The same thing happens on mountain slopes with mountain wind and valley wind (which allow air to be exchanged with fresh air). On a larger scale, the tropical cyclones that go hand in hand with heavy rainfall occur , whereby one speaks of a tropical low in the case of a lower level. At high wind speeds (from wind force 12) the storms can wreak havoc. Depending on the continent, one speaks of hurricanes or typhoons .
Dynamic low pressure areas are responsible for the polar easterly winds ( polar vortex ) and equatorial trade winds ( intertropical convergence zone ).
Low pressure vortex
A low pressure vortex is caused by the fact that the opposing winds in the middle latitudes do not slide against each other in a laminar (vortex-free flowing) flow, but form eddies in a turbulent flow (see also Rossby wave ). By their nature, these eddies have a more vertical axis of the eddy so that the winds on the ground and in the height blow in almost the same direction. A low pressure vortex typical of Europe is the Icelandic Depression .
Construction of a classic extra-tropical low pressure zone (ideal cyclones)
When cold and warm air from each high pressure area flow together into a low pressure area (usually a low pressure vortex), fronts are formed . In addition to these fronts, there are special names for different areas of a low pressure area.
Front and back side
Due to the prevailing westerly winds in the mid-latitudes, low pressure areas move from west to east. Therefore, the east side of the depression, which usually arrives earlier at a certain location, is also referred to as the front, the west side as the back.
Warm front
At the front of the depression, warm - and therefore relatively light - air slides onto the colder, heavier air in front of it and is lifted in the process. A warm front is created . The sliding air cools adiabatically , there is condensation and first ice crystal clouds ( cirrus ) and then layer clouds ( stratus , nimbostratus ) form, from which it can rain continuously. Because the cooled air joins the cold air at the front of the lows, the warm front (in the frame of reference rotating with the lows) increasingly migrates towards the back of the lows. The overall counterclockwise rotation (in the northern hemisphere) of the low pressure area creates the impression that the warm front is slowly rolling in at the front of the low pressure area.
Warm sector
The warm sector is located between the cold and warm front . Often clouds dissolve in this area and there is no precipitation . However, the air is also often moist, warm and unstable stratified, so that showers and thunderstorms can form. Since the cold front usually advances faster than the warm front, the warm sector gradually becomes narrower.
Cold front
With a cold front , cold air is pushed under the warm air, which cools down adiabatically . It is raining. Because here the cooled air strikes the cold air on the back of the deep, the cold front (in the reference system rotating with the deep) increasingly migrates towards the front of the deep. The overall counter-clockwise rotation of the low pressure area creates the impression that the cold front is rapidly rolling in at the front of the low pressure area.
The cold front is characterized by convective cloud, also known as cumulus clouds ( Cumulus , Cumulonimbus ). There will be showers, and thunderstorms can follow.
occlusion
When occlusion is defined as the union of a cold and warm front. Warm air is lifted from the floor and turbulence is created. The formation of an occlusion usually occurs because the cold front catches up with the warm front after a few days as a result of the different rotational movements in a low.
The occlusion is particularly rainy because the effects of the warm and cold front cumulate. Since it is comparatively close to the core of the low pressure, the heaviest local precipitation often occurs in the older lows relatively close to their trajectory (track).
Cold sector
The cold sector follows behind the cold front . This is where the air masses flowing southwards (or northwards in the southern hemisphere) on the back of the deep begin to circle around the deep core. In the cold sector the sky clears up again, so it becomes bright after the cold front has passed through, but with a drop in temperature. Cloud areas also arise here, which are heavily called Enhanced Cumulus (EC, extended cumulus cloud formation). They typically bring snow to Europe at the autumnal Atlantic lows.
When the cold sector has penetrated into the core of the deep behind the occlusion, the deep disintegrates.
Weather pattern when an extra-tropical cyclone passes through
With their instability, the cyclones largely influence the weather in Central Europe. The approaching cyclones can be seen long before the warm front arrives. Advection clouds and falling air pressure are associated with the hesitant gliding of the lighter warm air over a heavier preceding cold air. The speed of the air pressure drop when the warm front approaches is also an indicator of the expected intensity of the wind. If the air pressure falls by more than 2 hPa per hour, a storm is announced, according to a rule of thumb. Hook-shaped bent-up spring clouds ( Cirrus uncinus ) are the precursors of an approaching warm front. They seal themselves first to form high, later powerful and deeper layer clouds ( stratus ), from which increasingly rich and prolonged drizzle , so-called land rain , falls if the cold air remains stationary. The cessation of the sliding movement after the warm front has passed through leads to the cessation of condensation and the cloud cover may dissolve. Warm air from the south flows into the warm sector and rises to great heights, where cloud formation and possibly local shower-like precipitation can occur again. The warm sector leads to the highest temperatures in the cyclone passage.
The following cold front can be clearly felt by a temperature drop of a few degrees. The warm air displaced upwards by the cold air cools down quickly. High convection clouds ( cumulus ) are created. Heavy showers with large drops and sometimes hail formation are the result. There are also several thunderstorms. After the cold front has passed through, the clear air gives you excellent visibility when the air pressure rises again. The backside weather after a cyclone passage depends on whether other cyclones follow. The precipitation often stops again after the cold front has passed through.
Naming for Europe
The names used in Germany and some neighboring countries for low and high pressure areas that influence the weather in Europe are given by the Meteorological Institute of the Free University of Berlin .
See also
literature
- Seydlitz: Geographie 11. ISBN 3-507-52325-6 .
Web links
Individual evidence
- ↑ Legend for the weather map , ZAMG
- ↑ low altitude. In: AgrarMeteorologie, Wetterlexikon , Rural Areas Service Centers, Ministry of Economics, Transport, Agriculture and Viticulture, Rhineland-Palatinate (ed.).
- ↑ Compare e.g. B. Tief Quinton , June 2009: Forecast 20090628 , DWD, met.fu-berlin.de - heavy rainfall on this day in the Eastern Alps-Carpathian region.
- ↑ Enhanced Cumulus , KNMI, ZAMG, DHMZ: Manual of Synoptic Satellite Meteorology , Version 6.8, on zamg.ac.at, accessed November 16, 2014.
