A stationary area of high pressure has meant that we’ve had dry and stable weather during the past week, with temperatures up to 27 °C on Wednesday, but a persistent layer of cloud has spoilt what would otherwise have been wall-to-wall sunshine for many. Aren’t high pressure systems meant to mean cloudless skies, so what caused this frequent cloud? Ironically, the answer is the strength of the high pressure itself and how it traps moisture underneath it!
In a standard atmosphere, temperature normally falls with height, at a rate of around 6.5 °C/km. Air warmed at the surface rises and cools, forming clouds. It will continue to rise as long as it’s warmer (less dense) than its suroundings.