Tornado and funnel cloud formation. - May 14, 2014
After the recent funnel cloud sightings from around the country, we have decided to put together a little post explaining the fundamentals as to why we get funnel clouds and tornadoes.
Before we go into the details, we have some pictures of the recent sightings for you to view.
The first image of a funnel cloud was taken by Martin McKenna aka (nightskyhunter) near Maghera in NI on May 13th 2014 at 12.55. Martin is a professional weather photographer and has witnessed numerous funnel clouds and tornadoes over recent years.
This impressive funnel cloud was spotted on the 12th May 2014 near Alston, Cumbria. Credit to Debbie Elliot.
Finally a photograph of a funnel cloud on the 12th May 2014 in the town of March, Cambridgeshire.
How are funnels and tornadoes formed?
Tornadoes often develop from huge thunderstorms known as supercells. Within these supercells we have mesocylones, these are rotating updrafts within the storms that produce the tornadic activity.
From the diagram below you can see that wind shear is needed to start the air spinning, then the intense updrafts start to lift the column of air into a vertical position and then rapid rotation starts to develop and a mescocylone is born.
As the mesocyclone lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm. This convergence of warm air in the updraft and cooler air from the downdraft causes a rotating wall cloud to form. As the updraft intensifies, it creates an area of low pressure at the surface, this then pulls the mesocyclone down, in the form of a visible condensation funnel and eventually a tornado.
Not all funnel clouds and tornadoes are formed from a mesocyclone. Some can form purely from single cell thunderstorms that form along a convergance zone. A convergance zone is an area of 2 opposite winds that run alongside each other for example, a northerly and southerly would be ↑ ↓ and in between the 2 arrows, a cell could build and the convergance winds can rotate the updraft just like in a supercell but on a much lesser intensity. Usually only funnel clouds are formed but not exclusively. This kind of set up is very common in the UK and usually small upper troughs are the cursor. We do see a lot of water spouts as well and basically it’s the same scenario only this time off shore breezes producing the convergence.
Typically the UK sees up to 35 tornadoes a year, which by land mass is one of the highest tornado rates in the world. Another point to make is that tornadoes are more common in autumn than in spring/summer. This is due to very strong cold fronts creating squall lines which can produce tornadoes. On the 23rd of November 1981, 105 tornadoes were reported within a 6 hour radius as a strong cold front passed across the country. This still remains the highest European outbreaks to date, although most of these tornadoes were weak and very short lived.
Why doesn’t the UK get tornadoes like the USA.
Generally this is due to the fact that we don’t have rich warm moisture from the gulf of Mexico and we don’t have the Rockies producing the dry colder air, both of which are needed to produce the big supercells that develop these tornadoes. We do however when the conditions are right, produce smaller supercells capable of producing tornadoes. We don’t need to look back to far for examples, the image below is a tornado from a supercell storm in Lincolnshire in 2012.
We can’t talk about UK tornadoes without mentioning the devastating storm of 2005 Birmingham. This image says it all, a wedge tornado rated F2 on the fujita scale.
These storms need very strong instability to form, usually formed in an uncapped atmosphere with very warm moist conditions. We usually get these conditions from a ‘Spanish plume’ event. The image below shows how a low pressure located to the south west can start to drag very warm air up from Spain, mix this with the uncapped atmosphere over the UK and thunderstorms will fire. Then if wind shear is a factor the huge updrafts within these storms may start to rotate and produce tornadoes.