Following the heavy showers and thunderstorms that dominated last week, there was some welcome sunshine over the Bank Holiday weekend.
This reversal in fortunes was due to a switch from low to high pressure across south-east England. High pressure means dry, descending air and often clear conditions, particularly during spring and summer. These clear skies meant night times were chilly though, with temperatures falling to -2C in colder spots early on Saturday morning.
Combined with light winds, clear skies allow so-called ‘radiation’ conditions to develop, where the ground is particularly efficient at radiating heat to space, quickly cooling down as a result.
Heat exchange with the surface air means this rapidly cools too, becoming ‘cut-off’ from warmer air higher up. Temperatures can fall by several degrees per hour after sunset, turning a pleasantly warm early evening into a decidedly chilly one soon after dark. Particularly prone to this rapid cooling are valleys in hilly terrain – Chesham being especially well known. These so-called ‘frost hollows’ can witness minimum temperatures 10C lower than on nearby higher ground. Unless cloud develops or a breeze picks up, the surface air in these frost hollows remains much colder than the air above until the next morning when the sun starts heating the ground.
Temperatures rapidly climb and within a few hours can be higher than on surrounding hills. High pressure with light winds and clear skies can result in some impressive diurnal temperature ranges in these frost hollows, with the difference between the daytime maximum and night time minimum temperature occasionally exceeding 20C. The world record 24 hour temperature range is a massive 57C measured on January 15, 1972 in Loma, Montana, US, when temperatures rose from -48C to +9C.
Back home and there’ll be no such temperature swings over the coming days. Instead, low pressure is now dominating our weather again, meaning an unsettled end to the week with sunshine and showers. Windy with more persistent rain during Saturday and then a return to sunshine and heavy showers on Sunday.