Thursday 27th November
Usually the maximum temperature is during daylight hours and the minimum in the early hours of the following morning. However, yesterday did not fit that pattern. At 08.00, when my instruments were read, the thermometer registered -4.6 C, after a hard frost. The temperature very slowly rose, under the influence of weak sunshine, but struggled to get above freezing until 09.52. Thereafter, under the influence of the warmer southwesterly airstream and the sun, the temperature slowly rose to 6.6C at 13.24 and hovered around that figure during daylight hours. There was a slow rise after 17.00 to reach the maximum of 10.6C over the twenty-four period at 01.54 early Thursday being just 0.3C above average.
There were a couple of light showers during the evening that amounted to 3.2mm. The monthly rainfall total is now 125.0mm, being well above my 42-year record for November of 91.8mm.
Thursday revealed overcast conditions to start the new day thanks to weather fronts approaching from the west. Mist and light rain had by 08.00 reached Cornwall. The cloud will thicken all day with possible sporadic outbreaks of light rain this afternoon.
The deep depression in the Atlantic is relocating between Scotland and Iceland, which will continue the run of warm, most air under unsettled conditions.
Yesterday I attached the first part of an explanation of why snow is difficult to forecast for the UK.
The most common wind direction for the UK is southwesterly, bringing moist, mild air from the Atlantic. This typically results in rain rather than snow. For snow to occur, we need a combination of cold air and moisture, two ingredients that don’t always align in our climate.If we have a flow of colder from Continental Europe this will increase the likelihood of snow, especially when combined with high-pressure, which allows temperatures to fall steadily over several days.
Moisture is another key factor. For snow to form over the UK, cold air must either meet a rain-bearing weather front or pick up moisture during its short journey across the North Sea. This can lead to showers that fall as snow if temperatures are low enough.
Topography also plays a role. When air rises over hills and mountains, it cools and condenses, forming cloud and precipitation. Whether this falls as rain or snow depends on the freezing level, the altitude at which temperatures drop below zero.