However polar lows moving south can produce large amounts in the north, and winter depressions running east through the English Channel give copious snow over southern England and Wales. They are always enhanced if the isobars are cyclonically curved and also when pressure is falling. Showers are inhibited when isobars are anticyclonically curved and also when the pressure is rising. Often high surface temperatures are required to trigger their parent clouds. This means that inland they are much more frequent from late morning through the afternoon, and are generally less common in winter.
Hilly areas trigger more showers than flat areas.
Thunderstorms - These occur when shower clouds are particularly deep and may be regarded simply as a type of heavy shower. The most widespread and most violent thunderstorms occur in summer when hot and humid low-level air is overridden by cooler air aloft ahead of fronts, usually with falling pressure. Fog - Fog forms on radiation nights when skies clear and winds are light, which happens most often in anticyclones or ridges of high pressure. Long winter nights can produce freezing fog which the weak sun may not manage to clear during the day. Summer fog on the other hand is short-lived and often the precursor of a hot day.