High Pressure: How high is high?

Our meteorologist Chris Page explains more about pressure patterns

We always talk about areas of high and low pressure in weather forecasts but do you actually know what it is?

Like any fluid, our atmosphere exerts a pressure on anything immersed in it.

Putting it simply, low pressure is where air rises from the earth's surface, where as high pressure is where air sinks towards it.

We often associate bad weather (strong winds and heavy rainfall) with areas of low pressure.

Whereas, high pressure usually brings us dry and settled weather.

Often in summer, this is what provides us with those clear blue sky days with light winds.

Pressure is measured in hectopascals (hPa) also known as millibars (mb).

The average pressure value at sea level is 1013 hPa (mb) and it's generally higher during the summer months than the winter months.

However, it may seem a bit of a paradox that the extremely high values of pressure occur during the depths of winter.

Why?

The reason is cold air is denser, and therefore heavier than warm air, so changes in temperature at various levels in the atmosphere result in changes in the pressure near the ground.

The highest pressure ever recorded in the UK dates back to 1902, when 1053.6 hPa was recorded in Aberdeen, Scotland.

To put that into a bit of context. The highest pressure ever reached on Earth is 1083.8 hPa and not surprisingly this was over central Siberia during the winter back in 1968.