A 'Spanish plume' is a weather setup which brings an increased risk of thunderstorms within an air mass which travels north from Iberia.
The storms can form over the UK, or can move towards it having developed over Spain, western France or Biscay.
There are three main ingredients which are typically involved in a Spanish plume event:
The very warm air moving northwards from Spain towards the UK will rise as hot air is less dense than cold air. As this unstable air rises it cools leading to the formation of clouds, in this case cumulonimbus or thunder clouds.
As cooler air spreads from the west instability increases and the very warm air rises through the cooler air, enhancing the formation of thunderstorms.
The very warm air from Spain forms a layer at height and acts as a lid, initially preventing warm air rising from the surface. However, as the air at the surface increases in temperature eventually it warms enough to break through this barrier and air can then rise from the surface up to the higher parts of the troposphere.
This air is very unstable and the dramatic release of energy that occurs results in the formation of cumulonimbus clouds, lightning, thunder, hail, downpours of rain and sometimes gusty winds.
These storms can last for several hours if there is enough energy and moisture to sustain the process and can form a large complex of thunderstorms known as a Mesoscale Convective System (MCS).
Spanish plumes most often affect southern areas of the UK, with southeastern and southern England most at risk. These areas are closest to the source of the warm plume of air and so it is here that the contrast between warm and cool air masses is greatest.
info From the met office