Thursday 14th May,
Wednesday was notable for the brief, but heavy showers that began around midday and occurred at intervals during the afternoon, which were accompanied by strong gusty winds that peaked at 32mph. The total precipitation was just 3.6mm. Due to the minimal sunshine and very strong wind from the cool northwest, the maximum of 14.5C was 2.8C below my long-term May average. The wind died down in the evening resulting in a minimum of 4.8C just after midnight at 00.22, being 2.2C below average in the pool of cool air.
There was a cloudy start to Thursday, that is likely to continue all day, with showers dotted around southern England, likely to build late afternoon. Further troughs of low pressure, circling within the large depression over Scandinavia, will bring us more disturbed periods during the day, on a northwesterly flow.
Regrettably, the low-pressure system over Scandinavia will continue to dominate our weather up the weekend, pushing air around its western flank, originating from near the Arctic, down across the UK on a cool northwesterly wind. Temperatures by day and night are not likely to return to average values until Sunday into Monday when the current low pressure to the east moves away and a deep depression arrives from the west, that will then take charge. This will return our weather to a more moist, but crucially milder air stream from the Atlantic driven on by an airstream from a westerly quadrant.
I attach the second half of an article from the Met Office on why air frosts occur in late Spring.
Clear skies and rapid cooling
One of the most important ingredients for frost is a clear night sky. After sunset, the ground loses heat as it radiates energy back into the atmosphere and out to space. Cloud acts like a blanket, trapping some of this heat and slowing the cooling process.
On clear nights, heat escapes more efficiently, allowing the ground and objects close to the surface to cool quickly. Even during late spring, when days are noticeably warmer, clear skies overnight can still allow temperatures at ground level to fall below freezing.
Wind plays a key role in determining how cold it becomes overnight. Light winds allow colder, denser air to settle near the ground, forming a shallow layer of cold air where frost can develop. Stronger winds mix the air, drawing in slightly milder air from above and reducing the chance of frost forming at the surface.
This is why frost risk is often highlighted on calm nights, particularly beneath areas of high pressure, which typically bring lighter winds and clearer skies.
Ground cooling and grass frost
In May, ground frost is more common than air frost. At this stage of the year, the ground has not yet accumulated as much stored warmth as it does later in summer. This means that similar air temperatures can lead to colder ground conditions than they would in early autumn.
Different surfaces also cool at different rates. Short grass, soil and vegetation lose heat more quickly than materials such as concrete or roads, which retain warmth for longer. As a result, grass frost can occur even when nearby paved surfaces remain frost-free.