Having looked at depressions on a stylised synoptic chart, it’s time we looked at a real one.
Synoptic weather charts show the pressure at the earth’s surface. They are rather like contour maps, but instead of contours of constant height they have isobars showing lines of consistent pressure. Areas of low and high pressure appear as concentric rings. The chart also shows fronts - boundaries between warm and cold air masses.
This chart shows an especially nasty area of low pressure (marked L 944) centred north of Scotland. We can see a cold front (black line with triangles) marking the boundary between warm air in front and cold air behind, as well as a warm front (black line with semicircles) which has warm air behind and cold air ahead. The area between the two is called the warm sector. It is the wedge of warm air that has pushed into the cold air to the north. The line with both triangles and semicircles is called an occluded front. It shows where the warm air has been lifted completely away from the ground.
Where the isobars are close together, we expect strong winds. Here, storm force winds were blowing over much of the UK, whilst Italy would have experienced light winds. The wind will blow anticlockwise around low pressure areas, clockwise around high pressure. The wind direction will be angled 15-30˚ inward towards low pressure - so we would expect wind to be blowing from WSW over Wales given the chart above.
In the area marked ‘H 1029’, some perturbations are appearing in the cold front. Perturbtions like this can develop into new depressions, in the same way that we described a depression developing along the polar front.