Weather Maps

Pressure is shown by lines called isobars. Each isobar joins points that have the same atmospheric pressure, just as the contours on a navigational chart join points that are at the same height or depth.

Pressure is measured in millibars: normal atmospheric pressure is about 1000 mB ±30 mB, and averages about 1013 mB.

Temperature is not shown as such on the synoptic charts available to yachtsmen, but the boundaries between air masses at different temperatures are shown by lines called fronts.

Depressions (Lows)

The weather around us is dominated by areas of low pressure called depressions, and their associated fronts. They grow out of distortions in the polar front which separates the warm tropical air south of about 50°N from the cooler polar air further north.

Sometimes the southwesterly wind pushes a little pocket of warm air into the cooler air to the north

The warm air is less dense than the cool air around it, so it creates a local area of low pressure. The surrounding air tries to fill the low, but is immediately affected by the Coriolis effect.

This sets up a swirling motion that exaggerates the original pocket of warm air.

In the northern hemisphere, the swirl is always anticlockwise. Buys Ballot's law says: "if you stand with your back to the wind, the centre of the depression is to your left".

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