How To: Pressure (synoptic) weather charts explored

Following on from the discussion on hectopascals and atmospheric pressure:

Let’s use the same chart again.

This time you know what the numbers mean, various high and low pressure areas.

But there are different kinds of lines to be seen also. In this chart we are concerned with five types

  • light black line with numbers
  • short curved bold black line 
  • line with hemispheres
  • lines with triangles
  • lines with paired hemispheres and triangles 

Some of the lines have triangles for a short length, then hemispheres for another length. These can be treated as different lines and are different from the lines where the hemispheres and triangles are beside each other in pairs.

  • In the previous related post, the plain lines (above) with inset numbers running in curves all over the chart were explained as isobars, the different pressure contours.
  • Next is the plain bold black line, which is a Trough, a longer area of lower pressure, associated with increased risk of rain and cloud
  • Next are the lines with a series of hemispheres. This a Warm Front, usually at the front end of an advancing warm air mass. It can often bring cloud and increased humidity. In coloured charts it easier to remember as these are red as seen in the example above.
  • Next is an advancing Cold Front, the line with triangles, which in coloured charts is blue.
  • Next is the Occluded front. This means the front of a cold mass catches up with a warm front and pushes it upwards. Usually brings rain.
These aren’t all the lines and markings used on pressure charts, but are the ones most relevant for the understanding and forecasting we require.
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