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Lows versus highs

We talked earlier about high and low pressure, but why do low pressure systems develop? Well let’s go back to looking at the big picture.

In the six graphics below, remember that we’re looking down at the atmosphere from above (plan view).

Here we have the cold, polar air meeting the warm tropical air, with a boundary between the two. In the beginning: A diagram showing cold air to the north, warm air to the south and a slightly wavy boundary between the two

If we have a small perturbation or disturbance somewhere along the boundary between the cold and warm air, such as an island in the otherwise flat ocean, or a mountain on a continent … Something alters the balance:  If a kink develops along the boundary between air masses, pressure will fall resulting in a warm and cold front

… this disturbance begins to develop. Things start to develop: Where the pressure falls, a circulation starts to develop

Eventually a low is formed. A low pressure area: The circulation becomes a low pressure system

The cold front travels more quickly than the warm front, so catches up with it, and an occlusion is formed. The beginning of the end: An occlusion develops near the low centre

Eventually the system matures, and the low starts to decay. Decaying depression: The occlusion become longer as the low starts to fill

High pressure and blocking patterns

Generally lows are compact and often fast moving. High pressure systems on the other hand, tend to be large and slow moving, and can lead to a block in the weather pattern for days and even weeks on end.

If you would like more information on this watch the Met Office video on blocking weather patterns.

This image shows a situation when high pressure was dominating the weather across the UK. Some fronts were trying to push in from the northwest, but the high acted to deflect these fronts away to the north, keeping the weather settled across the bulk of the UK.

Weather graphics showing a map of the North Atlantic and Northwest Europe. A large area of high pressure covers much of the UK, with a low centred over Iceland and cloud and rain to the west of the UK

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