High pressure back

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In meteorology , the high pressure ridge describes an elongated, spacious area with relatively high air pressure in the lower earth atmosphere , which starts from a high pressure area (high). As a high pressure bridge, the high pressure ridge connects between two highs, as a so-called high pressure wedge it is the end of a high.

Ground weather map of the Northern Hemisphere (July 10, 1899) - the isobars ( mbar ) show u. a. the high pressure wedge of a North Pacific high (left) and a high pressure bridge between a high Azores and a high whose core lies over northeastern Europe (right)

Close to the ground formed back are on the bottom weather maps on the course of the air pressure lines ( isobars ) to recognize that at a wedge show not more closed curves with a curved portion. In the upper troposphere bulging high pressure ridges are also called ridges and are represented in altitude weather maps by air pressure-related contour lines ( isohypses ).

Altitude weather map showing the averaged 500 hPa - isohypses ( gpm ) around the North Pole - the pattern of formed waves (wave number: 4) increases with the ridge next to the high trough above Labrador (October 9-21 , 2010; reanalysis )

The development of ridges is influenced by high-altitude winds that occur as jet streams with a wave-like course on the polar front . Between warm air poleward leading ridge thus allowing them to level troughs forming cold polar air. High pressure ridges influence the weather in the lower atmosphere by sinking warm air, which can break up existing clouds .

Scheme of a shift in the polar jet flow (pink band) -
(a) recognizable Rossby waves
(b) increased wave characteristics
(c) beginning cold air separation.
Cold air masses are shown in blue, warm air masses in orange, directions of the jet flow are shown by white triangles. A high trough ( low pressure trough ; blue in c) lies
between two ridges .

Ridge and high winds

A ridge usually arches over (sub) tropical warm air and is formed in layers of air over 4,000 m. In altitude weather maps, it is represented by contour lines which, as isohypses, indicate the geopotential altitude for an area of ​​the same air pressure. For this, the 500 hPa isobar surface is usually chosen, with which air layers between 5,000 m and 6,000 m can be described in terms of atmospheric pressure. Ridge ridges can therefore be understood as a curvature of this pressure surface. Their bulge over warm air from a mountain ridge, which is directed towards the pole, influences high-altitude winds and promotes an oscillating course of the polar front jet stream . An intensive energy transport takes place over the back from the higher to the lower latitudes .

In the upper troposphere of the earth's atmosphere , high-altitude winds occur with wind fields that spread over several degrees of latitude and develop long-lasting strong winds as jet streams . The so-called polar beam currents shift and run around the earth in this way. Often an oscillating, spacious, undulating course develops, which is referred to as the atmospheric Rossby wave . The warm air of a high pressure ridge lies in a wave crest bulging towards the pole. This ridge borders on polar cold air, but is not surrounded on all sides by low pressure. The neighboring low pressure areas contain cold air and are located as troughs in a wave trough.

Meteorological laws

  1. In contrast to the enclosed high pressure areas close to the ground, the high pressure ridge is not completely surrounded by low pressure areas.
  2. A high pressure wedge always causes a divergent air flow in its wave crest , because the jet stream acts in the same direction as the gradient force. The sinking air masses and the air movement flowing out on the ground ( anti-cyclonic conditions) are a consequence, whereby the convection and thus the formation of clouds and precipitation are suppressed.
  3. Unstable air masses often flow towards the back of a high pressure ridge , and after hours or days a low pressure area or a low pressure trough follows , which is an extensive area of ​​relatively low atmospheric pressure. This location is characterized by the convergence that leads to the horizontal mass influx of air particles into the low pressure area. The jet stream now acts against the gradient force, and a slightly upstream gradient wind flows .
  4. High pressure wedges and low pressure troughs develop through the interaction of flow divergences and convergences at wave peaks and valleys of the jet stream when well-developed waves of the polar jet stream are present.
  5. The long waves as well as the ridges and troughs can be clearly seen on the weather map by the course of the isobars .
  6. Backs and troughs are particularly intense in blocking to north-south-facing (meridional) weather conditions with a negative NAO index . This index for the North Atlantic Oscillation measures the fluctuation in the pressure ratio between the Iceland low in the north and the Azores high in the south. It correlates with the strength of the westerly winds over the North Atlantic, the storm intensity and the storm region over the North Atlantic, as well as with the precipitation in Eurasia.
  7. A high pressure ridge controls the displacement of a high near the ground. The axis inclination plays a major role: With a vertical axis, the height remains stationary. In the case of an axis inclined eastward with height, the high increases or it shifts eastward. If the axis is tilted westward, the high is weakened or shifted westward.
  8. As the frontal zone becomes increasingly unstable between the 35th and 65th degrees of latitude and with a gradual transition from meridional circulation ( north-south directed, negative NAO index ) to zonal circulation (positive NAO index) of the jet stream, the high pressure wedge advanced towards the poles from the source area of ​​the warm air cut off by the beam current ( cut-off effect ). This high pressure area, which is now completely surrounded by low pressure, can no longer be filled directly with tropical warm air, but as an intermediate high it can still bring friendly weather to the places over which it moves, wedged between two low pressure areas or low pressure troughs.

Individual evidence

  1. ↑ M. Bahadir, H. Parlar, M. Spiteller: Springer Umweltlexikon. Springer-Verlag, 2013, ISBN 978-3-642-56998-2 , pp. 565f.

literature

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