Low-pressure area

A low pressure area (near the low, majority lows, the disorder) is an area of lower air pressure relative to the large-scale environment. He is conceptually compared to the high-pressure area. The typical pressure difference is about 10 hPa, but there are also far greater pressure differences. From a low pressure area is called only when a core can be identified, seen from which the pressure in any direction except the height increases. Otherwise, there is talk of a low- pressure or high-pressure trench wall. A distinction thermal low pressure areas and dynamic low-pressure areas ( cyclones ).

• 4.1 the front and back
• 4.2 Warm Front
• 4.3 warm sector
• 4.4 cold front
• 4.5 occlusion

Types of lows

Thermal Low: soil depth, height depth

A thermal low pressure area formed by differences in air density due to heating (solar radiation) or be caused by cooling. Depending on the affected air layer, a distinction between surface low and upper low.

A surface low is formed when the density of the air near the ground decreases by heating (solar radiation). The heated air dissolves the ground and rise to ( thermal), which leads to a pressure drop near the ground ( in higher layers of the air pressure increases by the inflowing warm air on the other hand ( high altitude ) ). The pressure drop near the ground results in a large-scale foreign inflow air ( wind ).

A low level describes the situation on a (floor ) area of ​​high pressure. Like this it results from cold air that falls from a great height, which reduces the air pressure in the higher layers of the air and increases the ground. Height lows lie several kilometers height and are characterized by - compared to the surroundings - low temperatures. From this, conclusions can be drawn regarding the age of the lows. Especially in the fall determine height lows in the form of cut-off lows or cold air drops from a height of troughs in part the weather activity in the Mediterranean, because there the very warm waters of the Mediterranean warms the lowest air masses and enriched with moisture. At the height of the weather map (eg 500 hPa topography), the upper low apparent through several closed isobars.

Dynamic depth ( cyclones )

A dynamic low-pressure area ( also cyclones ) occurs when the flow of air flows together in deep layers of the atmosphere ( converges ) and apart again flows in height ( diverges ) and thereby loses speed. The air in the center of a depression is lifted, whereby an air mass deficit at the bottom, so a drop in pressure or low pressure is created. In a low pressure area, the air flows due to the pressure gradient inside. This flow is diverted to the northern hemisphere by the Coriolis force to the right and it results in an anti -clockwise rotation.

In many cases, the names for a low pressure area (the cyclones, cyclones Pl ) and a tropical cyclone in the Indian Ocean confused ( the cyclone, cyclones Pl ).

Flows in low pressure areas

Instead zuzuströmen from all sides radially to the area of ​​low pressure, rotating the air flowing around a vertical axis into the low pressure area. This is due to the Earth's rotation ( the Coriolis force ), and the spherical shape of the earth. In the northern hemisphere of the earth a moving air mass is deflected by the Coriolis effect in the direction of movement to the right, which is added to the deflection in the direction of the largest pressure gradient.

Winds that flow towards in a Tiefdruckgebeit are deflected to the right. They rotate ( viewed from above ) counterclockwise (see picture) - so in the mathematically positive sense of rotation. Low pressure areas are also called cyclones.

Winds that flow out of a high-pressure area, are also deflected to the right. They rotate ( viewed from above ) clockwise ( see picture) - so in the mathematically negative sense of rotation. High pressure areas are also called anticyclones.

The Coriolis force at the equator is less pronounced and increases towards the poles greatly.

On a global scale run -level winds due to the Coriolis force approximately opposite to the surface winds, because in the context of planetary circulation flowing warm air from the tropics towards the poles. Due to the Coriolis force, it is distracted in an easterly direction, so that in the amount of strong westerly winds ( jet stream ) prevail. The flowing back at the bottom of cold polar air is deflected by the Coriolis force in a westerly direction ( polar easterly wind ).

On a regional scale, thermal low-pressure systems also form over warm water areas over which ascends moist warm air and cools it. There arise the problems associated with heavy rainfall tropical cyclones, which is also called a tropical depression at a low severity. At high wind speed ( wind force 12) storms can cause severe devastation. Depending on the continent then it is called hurricanes or typhoons.

Dynamic low-pressure areas are responsible among other things for the polar easterlies ( the polar vortex ) and equatorial trade winds ( Intertropical Convergence Zone ).

Low-pressure systems

A low pressure vortex is due to the fact that the opposite midlatitude winds not slide on one another in a laminar ( irrotational fluid ) flow, but in turbulent flow vortices (see also Rossby wave ). These vortices have, by nature, a more vertical vortex axis, so that the winds on the ground and in the amount blow the same direction in nearly. A typical low-pressure systems for Europe is the Iceland low.

Building a classical extratropical low pressure zone (ideal cyclones )

When cold and warm air of one high pressure area together in a low pressure area (usually a low- pressure systems), flow to form fronts. In addition to these fronts, there are special names for different areas of a low pressure system.

Front and back

Due to the midlatitude total of prevailing westerly winds low pressure areas move from west to east. Therefore, we designated the Thus, in a particular place as a rule earlier incident east side of the lows as well as the front side, the west side as the back.

Warm front

At the front of the low sliding warm and thus relatively little air to the cooler lying in front of it, to heavier air and is lifted thereby. The result is a warm front. The aufgleitende air cools adiabatically from, condensation occurs and make it yourself first ice crystal clouds ( cirrus ) and then layer clouds ( stratus, nimbostratus ), from which it can rain continued. Because the cooled air strikes to the cold air to the front of the lows, the warm front ( in the rotating frame of reference with the depth ) moves increasingly toward the back of the lows. Due to the generally counter-clockwise circular motion ( in the northern hemisphere ) of the low -pressure area gives the impression as if the warm front slowly curl at the front of the lows.

Warm sector

Is the warm sector between the cold and warm front. Often clouds dissolve in this area and it falls no precipitation. However, the air is often layered warm and humid and unstable, so that showers and thunderstorms to form. As the cold front usually pushes forward faster than the warm front, the warm sector is gradually narrower.

Cold front

With a cold front cold air pushes under the warm air that it cools adiabatically. It 's raining. Because here the cooled air strikes for cold air on the back of the lows, the cold front ( in the rotating frame of reference with the depth ) moves increasingly toward the front of the lows. Due to the generally counter-clockwise circular motion of the low pressure area of the impression as if the cold front quickly curls at the front of the lows produced. The cold front is characterized by convective clouds, also known as cumulus clouds (cumulus, cumulonimbus ). It comes to showers, thunderstorms can follow.

Occlusion

When occlusion is defined as the union of a cold and warm front. Warm air is lifted off the ground and there are turbulences. In order to form an occlusion occurs usually by the fact that catches due to the different rotational motions in a deep cold front after a few days, the warm front.

Weather flow during the passage of extratropical cyclones a

With its inconstancy the cyclones largely influence the weather process in Central Europe. Long before the arrival of the warm front, the approach of a cyclone can be seen. With the hesitant sliding of the lighter warm air over a heavier previous cold air Advektionsbewölkung is connected. Hook -shaped bent- clouds ( cirrus ), the precursor of an approaching warm front. They seal off initially too high, mighty and later deeper layer clouds ( stratus ), from which increasingly productive and prolonged drizzle, so-called steady rain falls, when the cold air remains stationary. The cessation of Aufgleitbewegung after the passage of the warm front only converted once for exposing the condensation, and it can come to resolve the clouds. Warm air from the south flows into the warm sector and rises to great heights, where it may lead to cloud formation and possibly to local showery precipitation again. The warm sector leads to the highest temperatures of the cyclone passage.

The following cold front can be clearly felt by a drop in temperature by a few degrees. The cold air displaced by the upward warm air cools quickly. It creates high -reaching convective clouds (cumulus ). Strong rain rain with big drops and some hail formation are the result. Several times it comes to thunderstorms. After the passage of the cold front has an excellent distance vision due to the clear air. The rear weather by a cyclone passage depends on whether follow more cyclones. Often the rainfall after the passage of the cold front to listen to again.

Naming Europe

The names used in Germany and some neighboring countries for low and high pressure areas that influence the weather in Europe are assigned by the Meteorological Institute of the Free University of Berlin.