We are increasingly hearing the term 'Spanish plume' used by the popular media now. This feature is not that uncommon in some warm summers, whilst in some others they are almost completely absent. Basically they occur when a plume of very warm air is advected north from the Iberian area reaching the UK on a southerly airflow but this is usually replaced fairly quickly by cooler weather moving east from the Atlantic with a cold front seperating the two air masses. When the two air masses meet, the very warm 'plume' air is forced to rise vigorously over the cooler Atlantic air and as a result produces thunderstorms. Storms may well coalesce and can give widespread, heavy rainfall, with hail.

A very good example of a classic spanish plume occurred on the 18th-19th June 2005. A very warm and moist air mass was advected north across the majority of the UK through the period. There was unabated sunshine in the clear skies initially across much of the UK with a very warm upper plume of air located across much of England and Wales throughout the 18th. A weak surface cold front began to approach from the west, though later in the day introducing a much fresher air mass. Temperatures though soared across much of the south east with many London stations including Heathrow Airport and Northolt already reporting temperatures of 27°C by 09Z (10BST). By mid-afternoon a pleasant sea-breeze had initiated across coastal parts of East Anglia and the South coast – however much of the rest of England, Wales, East Scotland and more especially central London were baking, temperatures were locally as high as 33.7C at Wyton, Beds, 33.1C at the old London Weather Centre in High Holborn at 15Z and between 28C and 32C across much of England and Wales. 33.1C was the highest June maxima in the capital since 1976.

By early afternoon a convergence zone across the north of England set up (usually where two sea breezes meet giving a boost to the upward convection) and a line of deep convection formed ahead of a weak cold front in a line extending north east from Northeast Wales through Cheshire, Greater Manchester, Lancashire, and across much of Yorkshire. The thunderstorms generated brought a sudden drop in temperature of up to 10C (reported in places in a matter of minutes) accompanied by gusty winds, large hail (above 1cm in diameter) and almost continuous thunder and lightning. Numerous reports were received suggesting hail of up to 2cm in diameter falling locally, As the afternoon wore on the storms continued to progress slowly NE and intensified further – over North Yorkshire by around 16Z some particularly violent storms brought torrential falls of rain. This caused some considerable disruption to the area. The worst affected area being located to the south of the North York Moors national park with villages such as Sutton-under-Whitestonecliff, Thirlby and the town of Helmsley particularly badly affected by floodwaters. Local totals in the area included at Hawnby 59.8mm in 1 hr and a 5 minute total of 12.0mm, at Church Houses (10 miles NNE of Helmsley in Nth York Moors) 44.1mm; at Westerdale (15 miles NNE of Helmsley on the North York Moors) 36.0mm and at Brompton, Low Moor 33.2mm.

In the south of England the skies remained relatively calm and clear and the night that followed was again warm and oppressive for the southeast. Temperatures barely fell below 20°C by dawn and not even as low as that in the central urban area of London itself where London Weather Centre only fell to 22.1C. Temperatures remain high the following day in the SE (up to 28C) but somewhat cooler, fresher air gradually moved east across the whole country during the afternoon of the 20th though without storms being initiated here. This can often be the case with plume events -some areas see heavy rain and thunderstorms whilst other areas miss the action completely. In this instance though in a few weeks there was another plume event on the 4th July 2005 when the south did see the rain and thunderstorms.