"Spanish Plume"

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  • "Spanish Plume"
    16.07.2007 08:18

     

    Most people will have heard of the phrase "three fine days then a thunderstorm", possibly coined by George II, to describe a typical period of summer weather.

    Sometimes a spell of hot weather can last a week or more of course, but the phrase does sum up the sort of conditions the UK can experience during the phenomenon known as the "Spanish Plume".

    This typically starts off when a ridge of high pressure extends over the UK from the Azores. This area of high pressure then moves eastwards slowly over the course of a few days, giving increasingly sunny, hot weather. At the same time, a low pressure system attempts to drive in from the west but is slowed or held up by the high pressure and as a result, warm southerly winds are pumped northwards from Spain, and even North Africa.

    The air is very warm in its lower layers, and because of this, it picks up huge amounts of moisture at the surface as it tracks over the Bay of Biscay. Importantly, the air above the surface layer is warm, but much drier, as it has originated from the Spanish interior. This warm air aloft creates a "cap" on the atmosphere, preventing deep convection as it moves north. Sometimes, severe storms break out over France first if the cap is eroded by high surface temperatures. As the air moves north over southern England, the low pressure system to the west and its associated cold front usually encroach eastward with an upper trough and associated high-altitude cold air.

    This results in the warm, dry air aloft cooling dramatically creating a sharp drop in temperature with height.  The humid air near the surface is forced up by the massive instability, thus generating thunderstorms, sometimes severe if the conditions are right (with torrential rain, squalls and large hail possible - even tornadoes).

    July 1 1968 was a classic example of a Spanish Plume, with very warm air in the lower atmosphere, and a surface low pressure system near southwest England. These two factors created a very volatile atmosphere. Severe thunderstorms broke out later in the day, particularly in northern and western parts, after highs of 34C (93F) in Hampshire and 32C (90F) widely across the southeast. Slapton, in Devon, reported massive hailstones of around 7cm in diameter, caused by repeated circulation in the powerful updraughts and downdraughts associated with severe thunderstorms.

    The weekend brought a Spanish Plume, with obsidian skies, sonorous thunder, and riotous electrical fulmination; in short, thunderstorms.