The recent spell of stormy weather that brought flooding rains to northern and central parts of Italy is now easing away thanks to high pressure spreading in from the north. However, as we look at the weather across Poland, Germany and the northwest of Europe, it may be dry and the winds light, but the skies are grey and it’s cold and frosty.
Dozens of flights were cancelled at London’s Heathrow Airport on Wednesday and Thursday due to a blanket of fog over southern England. That same area of fog, some of it freezing, stretched across the Low Countries into Germany and Poland on Thursday leading to numerous car accidents. This was all due to the same large area of high pressure.
High pressure acts like a “lid” on the atmosphere so it will tend to suppress the cloud which prevents them from growing higher. In the summer, there is usually sufficient turbulence in the atmosphere to break the cloud up altogether giving blue skies and plenty of warm sunshine.
However, in the autumn and winter, the sun is too weak and the nights too long to allow that process to happen. At the same time, the clearer skies allow what heat there is to radiate away during these lengthening overnight periods, and things will cool off further.
Eventually, the air near the ground becomes colder than the air above it so any moisture and pollution in the atmosphere will become trapped beneath that “lid” leading to grey and murky skies, and when it touches the surface, we have fog.
This sheet of dull, gloomy and misty air can sit in place for several days until the high pressure system eventually weakens or a weather front break through, introducing cleaner air in the process. This is starting to happen now as milder unstable air begins to nudge its way in from the Atlantic.