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imageIt's hot – but some people aren't too bothered.Andy Rain / EPA

A heatwave has hit the UK, largely due to a flow of very warm air from southern Europe, and in particular the Iberian peninsula. Temperatures are expected to rise into the mid-30s Celsius. The media has picked up on the term “Spanish plume” to describe the current weather setup but it’s a phrase often misused. So what exactly is a “Spanish plume” and are we actually experiencing one?

The origins of the term aren’t completely clear, although a 1968 paper on severe convective storms can probably claim the first usage. Regardless, by the 1980s the phrase “Spanish plume” was certainly in widespread use among UK weather forecasters, so the headlines over the past few days aren’t referring to anything new.

As the Met Office pointed out last summer, a Spanish plume is a catchy name for a rather complex set of circumstances. It does indeed involve a flow of hot dry air from the Iberian peninsula across western France and into the UK. But this in itself isn’t enough to constitute a Spanish plume in the full meteorological sense. To make a true Spanish plume, it’s crucial what happens to this hot dry air as it tracks northwards and how it interacts with air flowing from other directions.

During the summer months, the air over Spain can become very hot if it is able to sit over the elevated plateau in the centre of the Iberian peninsula for several days. It will also be very dry as there isn’t a source of moisture near the surface. As an example, the air near Madrid at midnight on June 30 (Sunday night) had a relative humidity of only 24% (which is very dry) and a temperature of 38°C. Also important to note is that Madrid sits at about 600 metres above sea level.

As this very hot dry air moves north on southerly winds it passes above the Bay of Biscay and western France where the air near the surface is still very warm but much moister; at midnight on June 30, it was 23°C in Bordeaux but relative humidity was 61%.

The combination of these two air masses on top of each other, with warm and moist air near the surface and hot and dry air above, can be very unstable. In order to release this instability, the whole column of air, with the warm moist air near the surface and the hot dry air aloft, to be lifted in the vertical. Once this occurs then clouds, rain and potentially thunderstorms can form very rapidly.

This can indeed happen as the air flows northwards, with the air being lifted not directly vertically but on a rather gentle upwards slope as it moves north and runs into the colder, denser air of northern France and the UK. This combination of circumstances can occur when there is a slow moving depression situated over the Atlantic to the west of the UK, with high pressure over central Europe.

On some occasions this lifting can lead to the rapid development of intense thunderstorms over northwest Europe or the UK. A true Spanish plume occurs when all these circumstances conspire together to lead to intense storms. Such storms are being forecast – so it looks like the UK is indeed currently experiencing a true Spanish plume.

These events are typically well-forecast several days ahead, as they evolve rather slowly, although it is usually more difficult to pinpoint exactly where any thunderstorms may actually occur. A Spanish plume will usually occur at least once a year over the summer period and sometimes much more frequently.

Some of the heaviest rain that has fallen in the UK, such as the 279mm that fell on Martinstown, Devon in July 1955 and for many years held the UK record for the most rain in one day, was caused by the Spanish plume scenario. However it’s important to realise that not every UK heatwave or summer thunderstorm is down to a Spanish plume.

Peter Inness does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

Authors: The Conversation

Read more http://theconversation.com/explainer-how-spanish-plume-set-off-a-heatwave-in-the-uk-44109

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