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An isohypse (contour)..
  • A
    indicates the true altitude of a pressure level.
  • B
    is the limit between two air masses of different temperature.
  • C
    is the longest slope line of a frontal surface.
  • D
    indicates the altitude of the zero degree isotherm.

Refer to figure.

A constant pressure or contour chart is a chart where the pressure is constant everywhere.

Take for example a 900 hPa pressure level, this pressure level normally varies with height over different locations.
These heights are plotted as contour lines (also known as isohypses) with the reference being MSL.
The heights give us an indication of the distance that a pressure level is from MSL (so the true altitude of a pressure level).

Isohypses are plotted on upper air charts such as the 850, 700, 500 and 300 mb analysis and forecast charts.

If the contours are high values (in comparison to other values on the chart) then we can assume a high pressure exists.

The other way if the contours are lower values then we can assume a low pressure.
If the contours are closely spaced, we can assume a high-pressure gradient exists.
This wind speed is proportional to the distance between the contour lines.
Closely spaced is high pressure gradient means higher winds.

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