Low pressure systems - depressions

You need to know:

- what a depression is
- how it forms
- what it looks like on a satellite image and a synoptic chart
- the type of weather it brings

REMEMBER: a depression is a low-pressure system (if you are feeling depressed, you are sometimes said to be feeling 'low').

A depression forms when cold polar air meets warm tropical air at a front. The less dense warm air starts to rise over the colder air, and this starts to lower the air pressure. Depressions are common over the UK as we are on the boundary between the warm and cold air masses. They move from west to east across the UK, bringing rainy weather with them. The air in a depression blows anti-clockwise and upwards.

The boundary between the two masses is called a front. There are 2 fronts in a typical depression:

1. the warm front - this passes first, and has the warm air behind it (i.e. it is the front of the warm air). It is where the advancing warm air is forced to rise over the cold air

2. the cold front - this follows, and has the cold air behind it (i.e. it is the front of the cold air). This is where the advancing cold air undercuts the warm air in front of it.

On a weather map, the warm front is shown as red semi-circles and the cold front as blue triangles. To remember this, think about the colour on bathtaps (cold=blue, red=hot) and the shape of a lady's chest in hot/cold weather! The isobars are closely spaced, indicating strong winds, and the numbers decrease to a low in the centre. An occluded front is shown by alternating blue triangles and red semicircles.

The sequence of a depression

We used a mnemonic to remember this:

Chesterton Army Went Fighting With Sticks, Coleridge Fled Cowardly Away

which stands for:

Cold Air, Warm Front, Warm Sector, Cold Front, Cold Air

This mnemonic can also be useful when remembering the pattern of weather that a depression brings. This is shown in the PowerPoint below.

View Upload your own

In class, we used Mr Cassidy's fantastic Postman Pat movie and PowerPoint to help us with the depressions topic. You can download the movie and presentation from here. It's a huge download (over 50mb) so may take a while - but it's definitely worth it!

We also used the interactive files from the NGFL in class. You can work through them here.

We didn't make out own movie about the passage of a depression, but if we had it would probably have turned out something like this one from Teale Green school.