What is a sting jet and could Storm Eunice be deadly?

Storm Eunice is set to be the worst in over 30 years, meteorologists have warned, as scientists said a rare “sting jet” similar to the one seen during the Great Storm of 1987 might hit the UK.

Coastal communities face a severe flood risk and 90mph coastal winds are expected. There are warnings of a threat to life across several parts of the UK.

Dr Ambrogio Volonté, a storm scientist from the University of Reading, said the “bomb” cyclone, so-called because of its rapid development over the Atlantic Ocean, was similar in structure and shape to the notorious storm of 1987, which killed 18 people - though wind speeds were unlikely to reach the 120mph gusts seen in that event.

“It was the sting jet that caused the catastrophic damage associated with the ‘87 storm',” said Dr Volonté.

“Eunice, in particular, started its growth in a particularly favourable region for storm development, on the right side of the entrance of the strongest part of this jet stream – an area meteorologists call the ‘jet streak’.”

But what exactly is a sting jet? We asked the Met Office.

What is a sting jet?

A sting jet is a small area of very intense winds (often 100mph or more) that can sometimes form in powerful weather systems crossing the UK. Relative to the size of the storm, the sting jet is narrow, often 30 miles across, and only lasts three to four hours.

Regardless, with gusts of wind exceeding 100mph the Met Office says there is clearly still the risk of damage.

How does a sting jet form?

Warm air rises and cold air sinks; that’s what creates cloud and rain. It also helps to form areas of low pressure, sometimes turning them into storms. Areas of low pressure almost always have weather fronts as part of their structure, unless they’re a tropical storm. These fronts separate areas of warm and cold air and it’s their interaction that creates and develops wet and windy weather.

Close to the fronts there tends to be more focused streams of warm and cold air that run parallel to them. They are known as conveyor belts; the warm conveyor rises and the cold conveyor falls. These wrap around the area of low pressure and help develop it by feeding warm air and moisture into the system.

The cold conveyor brings its cold air from higher in the atmosphere and from being in a cold air mass. Sometimes it has help from rain and snow as they fall into it and evaporate. This change from liquid to gas requires heat, which is removed from the conveyor, cooling it further. Now we have even colder air falling along the conveyor, speeding up as it does so, like a rollercoaster taking the first drop.

As this wind reaches the surface it can often produce much stronger gusts would otherwise be made by the storm. However, the cold conveyor catches up with itself after a few hours and consumes the sting jet, keeping the length of time and area of potential damage quite small.

How can the Met Office forecast a sting jet?

A spokesman said sting jets were difficult to forecast because of their relatively small size, and the way each individual low-pressure system develops. However, there are tell-tale signs in weather models that are now able to spot cores of very strong winds.

It is also possible to spot the sting jet developing on satellite images, as the end of the cold conveyor is marked by a hook-shaped cloud with a point at the end. This often looks like the sting in a scorpion’s tail, hence the name sting jet.

There are also types of satellite images that show the amount of water vapour, made up of light and dark shades. The darker the shade the less water vapour there is.

Clouds are made when water vapour condenses into water droplets, but around the sting jet there are often dark fingers of air stretching out, telling us that the rollercoaster of wind is also very dry. A small core of fast-moving, cold, dense, dry air.

Four quick facts about sting jets

• The 'sting in the tail' is produced by a distinct jet of air - the sting jet

• It starts out three or four kilometres above the ground and descends over three or four hours

• Snow and rain falling into it evaporates and cools it as it descends, helping to accelerate it to high speeds

• It can accelerate to more than 100 mph