[chris (OP)]

Weather forecasters talk about areas of high and low pressure moving around and causing weather to happen.

But what causes these pathes of high and low pressure?

[Colin2B]

Sun shining on the earth heats air which expands, lighter, lower pressure, warm air rises. Air cools at altitude, becomes denser, air falls, higher pressure.
That's the simple bit. Add In Coriolis + global circulation patterns and those areas of higher/lower pressure begin to rotate, fronts form etc, weather happens.

[alancalverd]

It's more a matter of temperature differentials than absolute temperature. Whilst UK winter highs (1030 mb today!) are generally higher than summer ones, a summer Arctic high of 1000 mb will give you clear, hot , dry weather, whilst a winter Atlantic low of 1000 mb can be grey, cloudy and drizzly.

[chris (OP)]

Thank you both. I'm glad, Alan, that you have highlighted the key point which is that it's pressure differences that drive flow.

It's fascinating how these air masses move about...