How does a ‘Spanish plume’ enhance our thunderstorms? Firstly lets look at what a ‘Spanish plume’ is and how it gets all the way to the UK
A ‘Spanish plume’ is a weather pattern in which a plume of warm air moves from the Iberian plateau or the Sahara, then advects northwards over France and into the United Kingdom. This layer of air in the atmosphere is rather warm but dry, so once over our shores the temperatures will rise as this air acts as a lid keeping the heat stored, we call this a cap. Below is a pressure pattern that typically spawns a ‘plume’ event. High pressure to the East of the UK, with an area of low pressure to the West, this scenario will start to draw very warm air up from Spain, thus raising the temperatures as it does so.
So how does this ‘plume‘ enhance our thunderstorms? The first enhancement will be the temperatures, these will rise significantly during a plume as warm air above will obviously lead to very warm temperatures below, however, due to this heat above acting as a lid or cap, the heat builds and builds creating a vast amount of energy. In normal thundery situations this energy is gradually released as the cap is usually very weak, so the build up of energy is reduced.
So now we know that although temperatures will rise with a ‘Spanish plume’, thunderstorms are not always guaranteed. There is one final element needed for the storms to build, a trigger. This will usually come in the form of an upper trough or cold front. Remember the low pressure to the west of the UK we mentioned earlier, this low pressure system will have associated frontal systems with cooler air in behind and if one of these interacts with the warm air over the UK, then the cap will be eroded and all the stored energy will be released.
The best way to explain this would be to imagine you have a pan of boiling water with a lid on, now remove the lid and all the steam and heat rises very quickly. This is what enhances our thunderstorms during a ‘plume’. Once the cap has eroded all the energy and heat rushes into to the atmosphere to create huge thunderstorm clouds (cumulonimbus), this is called explosive development.
Explosive development like this will cause frequent lightning as well as large hail and in some cases, severe convective wind gusts and even tornadoes if the conditions are right, as they were in 2005 (Birmingham Tornado).
We will of course be monitoring how this weekend’s Spanish plume will be shaping up and with activity expected during the night for some areas, we are expanding our working hours to cover this 24 hours a day, with regular updates on our Facebook page.