In 1875 the Royal Meteorological Society organised a network of Climatological Weather Stations, regularly inspected and using verified thermometers. The instruments were exposed in the Standard Stevenson Screen, a procedure also encouraged by the Scottish Meteorological Society and the British Meteorological Office. Summaries of observations from these standard sites were subsequently published in the Meteorological Record of the Royal Meteorological Society, the Journal of the Scottish Meteorological Society, and the Monthly Weather Report of the Meteorological Office.

Since 1875 the highest temperature of the year in Britain has been registered as early as 14th May 1965 and as late as 27th September 1895 (for full details see Journal of Meteorology, 9(87), 69-72); a table of Britain's highest recorded temperatures for each day of the year was originally published in Journal of Meteorology, 9(90), 169-176; a revised list and associated article was published in Weather, 55, pp 298-315.

The highest mid-winter temperatures are the result of a 'Fohn', where very mild air advected on a south or south-westerly airstream is warmed (by adiabatic compression) on descent from mountain ranges. Outstanding examples include the temperatures of 18.3°C recorded in North Wales in January 1958 and January 1971. East Devon, the Welsh borders, North-west and North-east Scotland have also experienced temperatures above 16°C in December or January in similar circumstances.

During early spring the occurrence of very high temperatures still depends on the continuous advection of air from a warm source. This is now most effective across Eastern parts of England when a warm southerly airstream arrives across a relatively narrow stretch of sea. The latter is also the most favourable situation for high autumn temperatures when sea temperatures are considerably higher but the fading power of the sun ensures that temperatures decline steadily once a supply of warm air has been cut off. North-facing resorts along the East coast of England like Cromer and Whitby (both backed by hills to the South) feature in several of the highest recorded spring and autumn temperatures (e.g. 23.3°C at Cromer on 9 March 1948). This demonstrates how cooling (or, in Spring, chilling!) sea breezes are inhibited by a steady offshore prevailing wind.

Between April and September the direct advection of a very warm air mass is not the only source of high temperatures; progressively rising temperatures occur during any spells of calm and sunny anticyclonic weather when the highest values are often recorded in Central England, e.g. in the Thames, Severn-Avon and Trent Valleys. The highest values of all occur when a very hot air mass stagnates over the country. Both the two generally hottest days in the central England record (29th July 1948 and 3rd August 1990) and the day of the absolute highest temperature readings (10th August 2003) occurred with very light, variable winds, following the previous advection of very hot, tropical continental air. The record high temperatures in 1911, 1976 and 1990 were all preceded by a long period of dry, warm weather. The consequent lack of ground moisture resulted in a high proportion of the sun's energy being utilised to heat the air, rather than being used for evaporation.

As during Spring and Autumn very high summer temperatures can be recorded even along the coast when an appreciable offshore gradient wind hinders the development of a sea breeze. The development of these refreshing coastal breezes is also suppressed by the atmosphere stability associated with anticyclones which inhibits the ascent (convection) of hot air inland. In more favourable unstable conditions a `sea breeze' blows inland to replace these ascending currents of warm air. Temperatures above 32°C were recorded at numerous coastal locations during the predominantly anticyclonic heat-waves of June-July 1976 and August 1990.

The occurrence of numerous extreme Spring, Summer and Autumn maxima in the London area may reflect the "urban effect" but also the fact that the capital is favourably sheltered to the south by the North Downs. The highest July temperature, 36.0°C in 1911, was also recorded in the lee of the Downs at Epsom with a south-south-westerly breeze; on the same day 34.9°C was recorded at Eastbourne, one of the few south coast resorts with shelter to the south-west.

Absolute `highs' for each British County were published in Weather, 48, 282-291. A remarkable British `heat-wave' occurred on 6th August 1910 when 28°C was registered in the Shetland Isles, while nowhere elsewhere in Britain exceeded 20°C. The extraordinary weather situation responsible is described in Journal of Meteorology, 9(91), 211-213.

TORRO contact member: Jonathan D.C. Webb.