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A “cool change”, late-September 2003

It snowed last Tuesday over the Scottish highlands, settling on slopes above the 300 metre level – the first significant snowfall of the season. Snow in September is not rare over the mountain tops, but it has become increasingly infrequent on slopes between 300 and 600 metres above sea-level during the last 15 years or so.

This wintry snap arrived in the wake of a vigorous cold front which swept south-eastwards across the whole country during Monday, and this same front was responsible for the dramatic arrival of autumn in southern and midland counties of England with a sudden drop in temperature, fierce squalls, the first widespread rain since the end of July, and there was even a minor tornado in Gloucestershire.

The temperature had continued abnormally high for the season until last Sunday when 28.1ºC was recorded at Gravesend, and even on Monday a reading of 23.3ºC was obtained at Norwich in spite of increasing cloud and a freshening wind. When the cold front passed the temperature plummeted to 11ºC in a matter of minutes, and the wind veered from southwesterly to northwesterly. The sudden temperature drop was observed nationwide, although generally of a slightly lesser magnitude than that seen at Norwich.

The accompanying downpour offloaded 10mm of rain or more across much of the UK, and at RAF Marham, near King’s Lynn, the day’s fall of 22mm contrasted with a total of 15.5mm during the period August 1 to September 21 inclusive. At one site near Letchworth in Hertfordshire more rain was collected in 15 minutes at the peak of the rainstorm than had fallen in the previous 51 days.

The northwesterly airflow in the rear of the cold front had originated inside the Arctic Circle, somewhere to the east of Greenland, so the cold snap was not unexpected and forecasters warned of widespread frost on subsequent nights. The lowest temperature to hand, –3.5ºC), was logged at Kindrogan, Perthshire, on Wednesday morning; this is the lowest September reading at any low-level (below 300 metres) site in the UK for ten years, though well short of the record for the month which stands at –6.7ºC at Dalwhinnie, Inverness-shire, on September 26, 1942.

In southern England several records of 40-50 years standing were broken, and in Hampshire, Berkshire and Wiltshire readings around –2ºC were certainly the lowest in September since 1919.

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