Cut-off process

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The cut-off process , German Abschnürprozess is in meteorology a phenomenon that affects the airflow and is responsible between the 35th and 65th latitude for the development of certain weather conditions. The cut-off processes lead to an independent low altitude or cold air droplets, they are linked to large-scale advances of cold polar air to the south or warm tropical air to the north. And stable high and low pressure areas , such as the Azores high and Iceland low , affecting the West wind zones , which pull in big waves around the earth.

Causes of the wave formation

The radiation from the sun heats air masses at the equator, at the poles the radiation of heat into space predominates and the atmosphere cools down. The temperature difference between north and south leads to an energy flow from the tropics to the poles. Air masses move, following the gradient force , from high altitude above the equator to low altitude above the poles. You cross the west wind zone.

The westerly winds meander in the frontal zone between the 35th and 65th parallel in powerful, planetary waves, also known as Rossby waves . The cause is first of all the isotherms as lines with the same temperature, which - shaped by land and sea - do not run parallel to the circles of latitude and influence the winds directly because the isobars with the same air pressure follow the isotherms.

Second, mountain ranges compress the flowing air parcels and increase their speed because of the detours. This also increases the deflecting Coriolis force , and behind the mountain range the air parcels rotate towards the equator . The resulting low pressure trough transports colder air towards the equator. Conversely, there are high pressure ridges in which warmer air is transported towards the poles.

circulation

If the isobars are very close together, there is a zonal circulation , also high-index circulation . Due to the processes outlined above, larger waves begin to form. If the amplitude increases, one speaks of low-index circulation or wave circulation . In the first case, relatively little energy is exchanged meridional (north-south). In the second case, this exchange is much larger.

With very strong wave movements, stable cyclones or anticyclones can become detached. It's a cut-off . As a result, the west wind drift is interrupted, called atmospheric blocking ( blocking action ). This means that there is no longer any energy exchange between the pole and the equator. The opposites build up, a new frontal zone develops and the cycle starts all over again.

literature

  • W. Lauer, J. Bendix: Climatology. 2nd, revised and corrected edition. Westermann, Braunschweig 2006, ISBN 3-14-160284-0
  • W. Weischet: Introduction to general climatology: physical and meteorological basics. 6., revised. Edition. Borntraeger, Berlin 2002, ISBN 3-443-07123-6