Predictions have suggested the UK could be set for a mini heatwave in September thanks to an ‘Iberian Plume’ bringing temperatures of up to 26C.

Weather maps had suggested that the country could experience the Iberian Plume between September 2 and September 6, with temperatures in the south-east of England rivalling Barcelona.

The Met Office has explained what causes an Iberian or Spanish Plume.

They say the three main ingredients are:

  • Very warm air pushing north from the Spanish plateau on a southerly airflow. This can happen at almost any time of year but during the summer months the extra warmth and moisture lead to increased energy available for storm development
  • Cooler air at height advancing from the west associated with upper troughs or cold fronts
  • Strong summer sunshine heating air at and near the surface across France and the UK

Will the UK be hotter than Barcelona in September?

However, The Met Office is not expecting temperatures to reach Barcelona levels during the first week of September.

They forecast highs of 24C on September 4 in parts of the south-east of England.

A Met Office spokesman told The Metro: “There are signs of high pressure starting to build in from the west of the UK through next week, however there is still uncertainty in the forecast due to Tropical Cyclone activity in the Atlantic which can influence the weather we experience in the UK.

“The longer range forecast does currently show more settled conditions especially in the south and south east of the UK, though with the chance of humid and thundery conditions in the far south or south east at times. Any more prolonged unsettled conditions confined to the far north and north west.

“There is no sign of any spell that would meet heatwave criteria in our outlook, and while settled conditions could lead to higher temperatures during the day these will be offset by cooler overnight periods.”

What happens during a Spanish plume?

The Met Office explains: “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.”