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.

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 …

… this disturbance begins to develop.

Eventually a low is formed.

The cold front travels more quickly than the warm front, so catches up with it, and an occlusion is formed.

Eventually the system matures, and the low starts to decay.

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.