Isobars, wind, and the front symbols on a synoptic chart

Overview

Synoptic chart and letters L and H.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a synoptic chart?

   Model answer

   A synoptic chart, also called a surface pressure chart, is a map showing the state of the atmosphere over a wide area at one particular moment. It plots atmospheric pressure using isobars, marks the positions of centres of high and low pressure and of fronts, and often includes station circles giving observed wind, temperature, cloud cover and weather at individual places. Forecasters use it to identify weather systems and to work out how they will move.

2. 2

   What is an isobar, and at what interval are isobars usually drawn?

   Model answer

   An isobar is a line joining all places that have the same atmospheric pressure, corrected to sea level so that altitude does not distort the pattern. Values are given in millibars, also written as hectopascals, and on British charts isobars are normally drawn at intervals of four millibars, for example 992, 996, 1000, 1004. Every point on a single isobar has the same pressure.

3. 3

   What does it mean when the isobars on a chart are packed closely together?

   Model answer

   Closely spaced isobars show that pressure is changing rapidly over a short horizontal distance, which is a steep pressure gradient. A steep pressure gradient means a large pressure gradient force, so the wind is strong, and on the tightest gradients around a deep depression it will reach gale or storm force. Closely spaced isobars are therefore the first thing to look for when judging wind strength.

4. 4

   What does it mean when the isobars on a chart are widely spaced?

   Model answer

   Widely spaced isobars show that pressure is changing only slowly over distance, which is a gentle pressure gradient. The pressure gradient force is weak, so winds are light or even calm. Wide spacing is characteristic of anticyclones and of the centres of ridges, and it usually goes with settled conditions.

5. 5

   How do you work out the wind direction around a low pressure centre on a chart of the British Isles?

   Model answer

   Find the centre marked L, then remember that in the Northern Hemisphere air spirals anticlockwise and slightly inwards around a low. Place yourself at the point of interest and read the direction the air is flowing along the isobars in an anticlockwise sense about the centre, then turn it a little towards the low. The wind is named for the direction it comes from, so air arriving at a station from the south-west is a south-westerly. A useful check is Buys Ballot's law: standing with your back to the wind in the Northern Hemisphere, low pressure lies to your left.

6. 6

   How do you work out the wind direction around a high pressure centre on a chart of the British Isles?

   Model answer

   Find the centre marked H, then remember that in the Northern Hemisphere air spirals clockwise and slightly outwards around a high. Read the direction of flow along the isobars in a clockwise sense about the centre and turn it slightly away from the high, then name the wind for the direction it is coming from. Because the isobars around a high are widely spaced, whatever direction you find will be a light wind.

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