Trapped under pressure: Anticyclonic gloom

High pressure is in charge, but it's often been cloudy and grey. Shouldn't high pressure bring sunshine?

We often associate high pressure with fine and settled conditions, whilst low pressure cloudy and wet. But that isn't always the case through the winter months.

Sometimes, cloud and moisture can get trapped under an area of high pressure, as we've seen in recent days. This leads to the aptly named term "anticyclonic gloom", or a "dirty high".

Low clouds or fog can become trapped as a cool area of high pressure moves over moist land. The moisture then evaporates, resulting in a deck of low clouds. As we've had a lot of rain recently, the land is rather saturated.

A temperature inversion, as explained in the video below, can also trap moisture, which can cause low level clouds or fog to persist. On Thursday December 1st for example, fog and low cloud remained stubbornly overhead for the entirety of the day.

The actual pressure value can have an impact too. A deep, dry high pressure system is far less likely to have any clouds at its core.

While it's frustrating to not see any sunshine, it also creates a surprisingly tough job for Meteorologists. This is because computer weather models struggle to calculate what will happen to the cloud cover, which can cause amplified errors at a larger scale.