Isohypse (meteorology)

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An isohypse ( Iso , from Greek : “ isos ”; to Germanequal ” and hypsos , from ancient Greek ὕψος: height , hill ), also contour line , is in meteorology “a line, the points of the same geopotential height on one A certain area, usually an isobar area. ”Like the isobar, it is an isoline , but in simplified terms, in contrast to the isobar (line with the same air pressure in relation to sea level in weather maps ), it shows the points projected onto a map or a geographic scheme at which a certain reduced air pressure is reached at the same altitude.

The best known are probably the isohypses maps for an air pressure of 500  hPa , which are used to represent the weather at high altitude - conclusions can be drawn about the three-dimensional development of the weather on the ground from this. They are also used in marine weather maps, e.g. B. for offshore sailing regattas , and thus outside of specialist circles.

General

With isohypses, the geopotential of the air masses is shown over a section of the earth's surface, i.e. their distribution in height. An energy potential is expressed in relation to the force of gravity . Since the mass of the air column u. a. Depending on its temperature (warm air expands and is lighter than cold) and moisture (moist air is lighter than dry), conclusions can be drawn about these two values.

Isohypses are better suited than isobars to represent the weather at high altitude: While the uniform reference of the weather on the ground to a contour line (usually sea level) can be easily established, the weather at high altitude takes place at different seasons and at different latitudes at significantly different altitudes; the representation of an isobar map for, for example, 10,000 m above sea level would result in large gaps in the informative value. Isohypses can better represent the air pressure conditions at altitude, since a point is reached above every point on the earth's surface where the air pressure has reduced to a certain value - depending on e.g. B. thermodynamic processes, however, at different levels.

If for the 500 hPa isohypses z. For example, if an average altitude of a little over 5500 m in the standard atmosphere is to be assumed, the values actually move between approx. 5000 and 6000 m due to the weather . Isohypses maps therefore represent a three-dimensional structure in a reduced, flat area.

The values ​​shown in isohypses are based on a relatively small number of actual measurement data using radiosondes . The measured values ​​temperature, air pressure and humidity are linked to their altitude and position by GPS sensors on the probe or from the earth via radar measurements .

Although the recorded data for the desired representations must be strongly selected, transformed and massively extrapolated in relation to standardized pressure levels , this form of representation for the altitude-air pressure is so successful that not only analysis maps , but also prognosis maps for over 14 days in advance are publicly available (see weather services ).

Typical isohypse maps and their uses

In synoptic meteorology , the various isohypses maps with their standard pressure areas between 925 and 200 hPa each serve to predict certain temperature, precipitation and wind developments in different "floors" of the weather genesis. The combination of the different surfaces creates a relative topography of the air layers .

In seafaring (see sea ​​weather ), the use of the 500 hPa isohypses map in conjunction with an isobar map of the soil conditions for predicting large-scale weather, v. a. established in wind development.

In air traffic (see aviation weather ), such maps are also used to correct the altimeter (for cross-country flights at low altitude) and often directly show the relative local deviation from a standard altitude (e.g. 5000 m for 500 hPa isohypses).

units

The measured values ​​in isohypse maps are still given differently.

As modern units come into question

  • J / kg or m² / s², which include the acceleration due to gravity of 9.81 m / s²
  • the geopotential unit derived from this (1 gpu = 10 J / kg).

The older units used are:

  • geopotential meters (gpm)
  • geopotential decameter (1 gpdam = 10 gpm).

When using the newer units, the standard isohypse of the 500 hPa pressure area (often drawn a little bold) has the designation 5520 J / kg or 552 gpu, when using the older units 5640 gpm or 564 gpdam (both times corresponding to the 1013 hPa -Normal isobars of the ground air pressure).

From this line, the structures of the troughs and ridges (roughly high and low ) could be seen, which are inversely related to floor level and floor level .

Representation variants

With the isohypses shown in black, the map shows the lines of the same height for an air pressure of 500 hPa in relation to sea level, plus the wind conditions with blue arrows and the
vorticity as colored areas

Since the air pressure in the lower layers of the troposphere decreases by an average of 1 hPa per 8 meters, sea weather maps are usually drawn with isohypse lines at a distance of 40 J / kg = 4 gpu, so that their distance corresponds to the distance between the ground isobars, which usually drawn at a distance of 5 hPa. As with the isobars, lines approaching each other mean higher dynamics (more wind).

Isohypses are used in weather maps for practical use i. d. Usually shown combined with other values ​​in order to achieve greater informative value:

  • In the 500 hPa isohypse maps for sea weather, the ground isobars and the temperatures at the 500 hPa surface are often also entered.
  • In order to be able to estimate the course of the jet stream, the 500 hPa isohypses map is also offered with streamlines and wind speeds .
  • If the isohypses are shown together with the vorticity (vorticity, automatic angular momentum of rising and falling air masses), the thunderstorm tendency, etc., can also be read off well.

Individual evidence

  1. Glossary at Eumetcal.org ( Memento of the original from March 3, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.eumetcal.org
  2. See the overview on Seewetter-Kiel.de

See also