Tornado & Storm Research Organisation (TORRO) - United Kingdom
The United Kingdom (U.K.) experiences 15 days, on average, each year when the synoptic and meteorological conditions produce tornadoes. A single tornado may form on some days but on other days the conditions may give rise to a large outbreak of tornadoes. For example, on 23 November 1981, an outbreak of 105 tornadoes occurred across England and Wales during the 6-h passage of a very active cold front attached to a very rapid deepening depression. On average the U.K. experiences 33 tornadoes each year but the annual total has varied from only 11 tornadoes in 1989 to 152 tornadoes in 1981.
The Tornado and Storm Research Organisation (TORRO) has documented around 2,000 tornadoes that have formed in the U.K.. Tornadoes can occur throughout the year but most tornadoes and tornado outbreaks occur during the autumn and winter, with around two-thirds of tornadoes occurring between September and January. If sufficient information is available, the intensity of each tornado is rated using the TORRO Tornado Intensity Scale (T-scale). This is an open-ended scale but, for convenience, is usually listed from T0 to T10. Most U.K. tornadoes, as in the rest of the world, are weak (T0-T3) with wind speeds up to 51 m.sec-1 (114 mph). Approximately 6% of U.K. tornadoes are characterised as strong tornadoes (T4-T7). It is rare for a U.K. tornado to be rated in the violent category (T8 or more), the return period of such events being about one event every 500 years. Path length, width and direction are also recorded for each event together with the synoptic and meteorological conditions. This paper examines the most common situations in which tornadoes form in the U.K..
The strong (ana) cold front associated with a rapidly developing intense depression has historically produced the largest number of tornadoes across the U.K.. A combination of two factors is involved. Firstly, there is ‘line convection’ where cold air moving from a general northwesterly direction undercuts a conveyor belt of warm air moving from a southwesterly direction. This initiates a narrow band of rapidly rising air that, in turn, produces a narrow, but intense band of heavy rain or hail. Secondly, there is often a very sharp wind veer across the front, sometimes by more than 90 degrees. This veer can induce a spinning motion along the line convection, especially when the wind is strong and consequently several, or evens tens, of short-lived tornadoes may form.
The slack summer low pressure is a relatively common situation in the late spring and summer when a slow moving area of low pressure close to or over the U.K. has almost filled, and has left a legacy of stagnant, but still unstable, moist polar air across the country. Local intense convection and convergence zones left by old troughs or from sea breezes can augment this convection, concentrating it and forming weak and short-lived tornadoes.
The ,zone of wind shear‘ combined with lee effects is another situation that produces tornadoes. It develops across the south of England in autumn and winter, and is probably one of the reasons why towns along English Channel experience the occasional strong tornado. This situation occurs where light northwesterly winds created by a land breeze coexist along side strong west to southwesterly winds, creating a line of shear, effectively a coastal front. Convergence, where these two wind streams meet, enhances the already intense convection giving a line of heavy showers or thunderstorms. The differences in the wind speed along this zone of shear give each shower cell a cyclonic motion that may lead to waterspouts or tornadoes. Additionally, if these storms travel across the Isle of Wight, friction causes the wind in the lowest part of the atmosphere to back and thus increase the potential for tornado formation to the lee of the island. Most tornadic activity appears to occur during the night because of the increased temperature difference between the sea and adjacent land.
Other tornado-producing situations explored in the paper include showery, southwesterly airstreams; the severe summer thunderstorm; and the triple point (i.e. point of occlusion) associated with an old tropical storm or hurricane that becomes entrained into a developing wave depression along the polar front.