Pressure charts

A guide to pressure charts

They give us an overall view of what's happening with the weather, are the starting point for working out winds and waves.

Differences in pressure
Pressure gradients generate winds, which in turn generate waves.

Isobars are lines of equal pressure
The closer the isobars are to each other, the stronger the winds and the bigger the waves being generated.

Pressure charts plot the air pressure at a given altitude. For surfers & windsurfers, the ones of most interest are the surface pressure charts, which are for sea-level.

Pressure charts show the strength of high and low pressure areas (often marked with a H or L) and plot the lines of equal pressure.

It's simplest to think of a pressure chart as similar to a geographical map, with highs like mountains and lows like valleys.

The lines of equal pressure (isobars) on a pressure map are equivalent to the lines of equal altitude (height contours) on a geographical map.

The two important relationships between isobars and winds are:

• The wind blows almost parallel to the isobars.
• The closer the isobars, the stronger the wind and the bigger the waves being generated.

What are all those triangles and circles?

The (red) semicircles indicate a warm front, which is the leading edge of a low pressure (warm air is advancing to replace cold air).

The (blue) triangles indicate a cold front, which is the trailing edge (cold air is advancing to replace warm air)..

You'll also see lines that have both triangles and semicircles. These are "occluded" fronts, where the cold front has combined with the warm front.

North Atlantic pressure charts for today and tomorrow
From the UK Met Office

These charts also show isobars (lines of equal pressure) and weather fronts for today and tomorrow.
UK Met Office Atlantic storm forecasts are widely regarded as the most accurate available.

North East Atlantic pressure charts for today and tomorrow
From the Irish Meteorological Service

Advanced pressure chart tip

The pressure at the centre of a storm is not enough to tell us how strong the winds are around that storm.

This is because wind speed is determined by:

• The pressure gradient - how closely spaced the isobars are.
• The latitude of the storm.

Which means that:

• You can get strong winds and big waves from a weak low if it is next to a powerful high.
• A 980mB storm off the coast of Portugal will produce bigger winds & waves than a 980mB storm off the coast of Scotland.

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