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Recent storms locally in Reigate, around Surrey and across the UK have brought relatively brief periods of squally weather which have got people asking questions about what they have experienced.  In particular, people are wondering about the January 25 squall line and asking… “wow, was that a tornado?!”

In Reigate we recorded our maximum wind gust for at least 4 years of 52mph during the passage of the active “squall line” cold front at 5pm on Saturday 25 Jan. Biggin Hill recorded 68mph and Kenley 60mph peak gusts.  For Reigate 52mph exceeded StJude 2013 and the storms in December, however the duration of the January 25 gusty front was barely 10 minutes.  The weather phenomenon that produced this extreme short-lived gustiness is called a squall line.  Below is the 500hPa chart for temperatures at 5000m and the 850hPa chart (1500m) as the storm passed over Reigate and SE England.  Note the dramatic change in air mass temperature (colour) and pressure at height either side of the front, especially in the SE.

A squall line is a very active cold front.  This one migrated steadily from the NW to the SE during the course of the day (see radar pics above) causing electric storms across the Midlands and some unconfirmed tornadic activity reported there too.  The squall line appeared to break up a little on its passage down towards the SE but still caused significant gusts measured between 50-60mph.

The experience of fierce winds “coming out of nowhere” combined with lashing rain or hail impeding movement, causing tree branches to sway or snap alarmingly and even whole trees or garden walls and sheds to blow over in an instant, followed by power cuts in some areas locally, have all understandably prompted the search for explanations.  “Mini-tornadoes” have been mentioned in the media as a possible cause for this weather phenomenon.  So, was Surrey struck by a “mini-tornado” on Saturday January 25? This post looks at some local evidence and puts forward a round-up of what is “required” before weather phenomenon can usually be labelled a tornado.

It is worth mentioning that there is not really any meteorological phenomenon formally known as a “mini-tornado”.  Whilst spinning vortices of air can vary greatly in size and shape (e.g. vortices of rubbish on a street corner, dust devils, land /water spouts and gustnadoes) and can yield a great range of wind speeds, a “proper” tornado is only formally defined as a violently rotating column of air, in contact with the ground, usually underneath a cumulonimbus cloud and usually (but not always) visible as a funnel cloud that is intense enough to do damage.  So there is no such thing as a “mini-tornado”.  Wind speeds in tornadoes are unknown.  This is because traditional anemometers are destroyed even in weaker rotations.  It is suspected that wind speeds can exceed 200mph and even 300mph in larger Mid West tornadoes.

From evidence seen so far, few local eye-witness accounts from 25 January have revealed any reliable sightings of funnels or convincing sustained surface wind rotation and there are no photos of funnels or vortices, which isn’t surprising given the lashing rain and bad weather and gathering gloom at 5pm!  Any formal tornado identification must therefore rely on gathering evidence of damage and analysis of the prevailing conditions.  To get nearer an answer we must see how close recent events came to satisfying tornado producing conditions and whether any damage evidence points to tornadic activity in and around Reigate / Surrey area.  Tornado formation is a big topic so these are necessarily abbreviated explanations but they are a starting point from which we might be ready to observe and record tornadic activity even better the next time!

1.Were conditions conducive to tornado formation?

Yes, partly! Tornadoes are associated with severe thunderstorms. The biggest tornadoes are produced by storms called super-cells.  These are rare in the UK but relatively common in the Mid-West of the USA where about 1000 tornadoes occur every year usually in spring/early summer and autumn.  Thunderstorms are produced when moist humid air is rapidly lifted from the surface and huge amounts of water vapour condenses to form towering clouds called cumulonimbus. Latent heat is released which drives further lift and draws in further warm moist air in a feedback loop which builds cloud height.  Cb clouds form characteristic shapes like the ones below spotted in Reigate on 28 Jan 2013.  The UK experiences around 30-40 tornadoes per year, so thunderstorms and squall lines like the Jan 25 event can certainly produce tornadoes, albeit weaker than the US Mid-West variety.

An unstable atmosphere is a prerequisite for most thunderstorm formation.  Instability is a condition where warm air rises and keeps rising until it hits an inversion layer or the top of the atmosphere, (itself an inversion layer called the tropopause) whichever is the lower.  Inversions occur where temperatures increase with height, instead of the more usual decrease.  An inversion will prevent thermals rising and any increase to cloud height will cease.

In unstable conditions, warm bubbles of rising air (thermals) will expand with height, cool down and moisture will condense into water droplets creating the bubbly explosive cloud formations that can be seen growing at the top of young thunder clouds (“Turkey towers”).  Skew-T graphs show temperature change with height.  The graphs below show recently unstable conditions in Reigate during a period of thundery activity and the passage of the squall line in Nottingham on 25 Jan.  Note the relatively steep “lapse rates” shown where temperatures fall rapidly with height.  Steep lapse rates are a critical ingredient for thunderstorm development. Steep lapse rates allow convective thermals of rising air to lift rapidly.  US tornadic thunderstorms in the mid-west often have an additional atmospheric feature called a “cap” that “turbo-charges” thunderstorm development.  A “cap” is an extreme temperature inversion created by a warm dry wind aloft that prevents further uplift of thermals until energy at the surface has built sufficiently for thermals to “bust through the cap” and rise unabated, building explosive thunderstorms extremely rapidly. There is no evidence of an inversion cap on the 25 Jan squall line.

The movie below shows how violent super-cells erupted after the cap has been broken last year over El Reno, OK, USA, producing one of the most violent tornadoes ever recorded.

Another essential process in the production of tornadoes is rotation.  Air can start to rotate in a thunderstorm due to changes in wind speed and/or direction with height. This is called wind shear and it is an essential ingredient in creating severe tornadoes.  A twisting motion in rising air masses can be the result of winds from different directions meeting and “rolling” the air into a horizontal sausage. Try rolling a pencil between your hands moving them in different directions… this is essentially what winds from different directions can do to the air.  Note the absence of directional wind shear in the Nottingham Skew-T chart during the passage of the squall line: all the winds at different heights were blowing in much the same direction. There appears to have been some vertical speed shear. Nevertheless, the absence of directional shear seems to reduce the chances that this storm was a tornado producer.

Clouds can show evidence of rotation in the atmosphere: shelf clouds (horizontal rotation, rarely produce tornadoes directly), wall clouds (vertical rotation: often an imminent precursor to a tornado) and funnel clouds (visible tornado condensation funnel in early stages of development that has not yet “touched down”) are all signs of very disturbed rotating air that is a precursor to tornado activity on the ground.  No evidence of distinctive wall clouds or shelf clouds or funnels has emerged from the Reigate storms 25 January.

Horizontally rolling air aloft can be twisted into a vertical spiraling vortex, a tornado, by strong warm updrafts of air in thunderstorms.

Warm updrafts (thermals) can be lifted for various reasons.  During the summer, heating of the surface can trigger thermals  to rise and, in unstable conditions (where rising thermals keep rising as it is warmer than surrounding air), this can trigger thunderstorms.  During the winter, with less sun available to heat the surface, warm air can still be forced off the surface by active fronts or extremely unstable and moist converging air masses. A coastline or range of hills might be sufficient to lift air to unstable levels and create thundery activity.


Once rotation is moved vertically through the storm then a tornado is possible given the continued updraft of warm air.  Thunderstorms will decay if the cold downdraft of air created by heavy rain or hail interrupts or cuts off the rising warm inflow.  This decay occurs in single cell thunderstorms that sit still over the same rising thermal source.  To build a severe storm requires a strong wind shear and motion across the ground to separate warm inflow from cold downdrafts.  Once this separation occurs then the storm engine can keep running and produce long lasting severe weather with a greater chance of tornadic activity. Although the squall line on 25 Jan did not produce super-cells it nevertheless provided lengthy displays of lightning and thunder, especially across the Midlands.

life cycle of a tornado

  1. The early stages with a rotating wall cloud (counter-clockwise in NH)
  2. development of a funnel cloud,
  3. damage at the surface
  4. descent of the funnel cloud to the ground.
  5. width of the funnel increases to some maximum,
  6. then begins to shrink,
  7. finally reaching a rope-like dissipation.
  8. after several seconds or an hour

Mature super-cell with no problems separating inflow and outflow

2. What do tornadoes look like in the UK and were any spotted during the 25 Jan event?

Classic photos of US tornadoes do not help identification in the UK.  Tornadoes in the UK tend to be smaller and short-lived.  However, the appearance of a tornado is not a reliable indicator of its intensity. Some narrow rope tornadoes can contain extremely violent winds whilst wide established rotations might be weaker.  In the UK tornadoes often initially appear as small funnel clouds, sometimes hard to spot, emerging from a lowering in the base of a cumulonimbus cloud ( a wall cloud).  Funnels appear to drop from the cloud, though this is not really the case. The funnel is actually condensing water droplets progressively becoming visible at lower altitudes as pressure falls in rapidly rotating rising updrafts of air. The rotating air beneath a rotating thunderstorm remains invisible until pressure and humidity allows condensation to make it “appear”.  It is possible to have an “invisible” tornado, for a period at least.  The other reason funnels become increasingly visible is when they pick up dust and debris in their rotation. Funnel clouds are quite commonly observed in the UK but few “touch down”.  Here is a selection of UK funnels and tornadoes.  Unfortunately there were no similar sightings on 25 Jan so this reduces confidence in tornadic activity on this occasion.

3. Was any convincing tornado-like damage left behind?

Despite the impressive trees knocked down, the damage type did not suggest a tornado. The type of damage can be conclusive in identifying tornado activity.  The damage left behind by a tornado depends on the speed of vortex rotation, the duration and length of the path across the ground and the nature of the objects along the path. Severe storm meteorologists have a keen understanding of what kind of damage tornadoes do on the ground. The US Enhanced Fujita scale and the UK based TORRO both estimate wind speed from the damage inflicted by tornado events.  Meteorologists investigate possible tornado sites and assess the damage using these scales.

The damage left behind by a tornado usually includes a ‘surgically’ well-defined and narrow damage-path, along which significant damage is seen, but away from which there is little sign of damage.  A damage PATH of sorts is therefore an essential piece of evidence in pinning down tornadic activity.  In addition, it is common for projectiles to be thrown to the left of the direction in which the vortex advanced (i.e. looking down-track)

When visiting WIND DAMAGE a diverging pattern of damage with several throw directions, with no clear path, perhaps moving out from a single point in several directions or otherwise isolated damage limited to single trees or blown objects is a good indication of straight line winds or downbursts (severe, but non-tornadic).

To confirm a tornado the existence of a damaging circulation / evidence of rotation at the surface must be found before the event can legitimately be called a tornado. With tornado damage, destruction is generally along one direction or path, debris along the ground is twisted or has spiral characteristics, and often small arcs where the top-soil has been removed are visible.  If such a confirmation cannot be made, the event must be considered to be either straight line winds, or possible a funnel cloud which did not touch-down or simply a “possible” tornado with no sighting.

Damage in and around Reigate and Surrey from the 25 jan episode is not representative of tornado damage (see below).  It is more likely that the damage below was caused by straight line winds and down drafts during the passage of the cold front.

We have to agree with the Met Office conclusion from January 25th 2013 which was that “we have had a line of very intense thunderstorms with strong, squally winds today and it is possible that these conditions could have produced small tornadoes. We have had reports of small-scale events consistent with tornadoes but at this stage it’s mostly hearsay.”  RGSweather would finally add that it is unlikely any damage around Surrey was caused by a tornado on this occasion. 

For reference: how to stay safe in tornadic conditions: for next time!

  • Do NOT open windows to “equalize pressure”: useless and waste of time and you should stay away from windows (NOAA)
  • Do NOT stop under bridges: debris will be blasted under bridges and impaled into people hiding; chances of survival not good as people ripped out from under bridges and fly-overs; bridge may fail
  • Vehicles are extremely risky in tornadoes

Here is a selection of tornado clips showing the formation, typical examples and scale of damage and frightening experience that can occur in Mid-West tornadoes.

Devastating tornadoes hit Oklahoma suburbs schools and towns today.

News at

supercell thunderstorm

Info @RGsweather

What makes a thunderstorm?

The conditions for lightning occur when powerful updrafts in cumulonimbus clouds force water droplets and ice crystals to rub against one another, creating massive amounts of positive- and negative-charged particles. The updrafts cause these two types of charged particles to separate, with the top of the thundercloud usually becoming positively charged as the lower part becomes negatively charged.

Here are the ingredients which formed the heavy “April showers” and first thunderstorm (TS) of 2013 over Reigate on Friday afternoon, 12 April 2013.  Whilst only a minor storm, it possibly still contributed to a multi-vehicle non-fatal accident on the M25, closure of the motorway for several hours, poor visibility, local flooding and hail across the area. Convective isolated rainfall events like these are important but tricky to forecast accurately: predicting exactly how much rain will fall, what type and precisely where and when isolated showers and thunderstorms will take place has a lower success rate than other elements of forecasting, like temperature predictions, for example.  Here is a round-up of the key indicators that enabled @RGSweather to issue a forecast for possible thundery activity more than 4 days before and monitor it’s development thereafter and issue a local forecast warning of a thunderstorm risk with very marginal low level tornado risk on the morning of 12 April.

A single relatively minor thunderstorm developed mid-afternoon with hail and lightning crossing north of Reigate on April 12, following a line roughly along the M25 between Leatherhead and Reigate.  The morning saw scattered and heavy showers but little organised severe weather.  Skies darkened over Reigate by 3pm under thicker cumulonimbus cloud and this thunderstorm caused some hazardous driving conditions on the M25 and a non-fatal multi-vehicle accident coincided exactly with the time the storm passed over the M25 which was closed clockwise for several hours thereafter.

Synoptic Situation: An upper trough over the UK and surface low (pictured) across southern UK moved slowly east during Friday, with an unstable airmass building cumulonimbus and increasingly heavy showers during the day as April sunshine heated the surface and increased instability. Here are the figures for yesterday and brief analysis…

LAPSE RATES: +29°C: Cold temperatures at 500mb heights and warming at the surface in the April sunshine caused steep LAPSE RATES of 29ºC. Lapse rates are the drop in temperature with height usually measured between 850hPa (1500m) and 500hPa (5000m). Steeper lapse rates indicate an unstable airmass where parcels of warm air heated at the surface in spring sunshine will rise rapidly and remain warmer than the environmental air surrounding them. Such air parcels will condense, releasing latent heat, which causes further rapid uplift and potential for the formation of cumulonimbus clouds given the absence of any inhibiting factors, like a cap (inversion or isothermal layer… see link below).

CAPE and Lifted Index: 378j/kg; LI -1: Convective Available Potential Energy (CAPE) is a measure of the energy in the atmosphere for convection (j/kg).  Figures in MidWest USA approaching 6000 j/kg cause tornadoes. Here in the UK, CAPES above 300j/kg can cause thunderstorms.  Lifted index is the difference in temperature between the environmental air at 500hPa and a parcel of air lifted to that height: a negative LI indicates buoyancy in rising air parcels and instability and significant convection.

Vorticity at 700hPa: upper air velocity at mid levels means that air is rising. April 12 has UVV: upwardly mobile air at mid-levels.

PWAT: 20mm: Precipitable water is the amount of water that would fall to the surface if all the moisture in the atmopshere rained or hailed out.  Relative humidity is a measure of how saturated the air is at various levels in the atmosphere.  100% means saturated: most levels were at least 80% RH.

TTI index

Total Totals Index (TTI): 60: this is a forecasting index used to measure potential storm strength.  It is calculated using the difference in dew point and temperature between 850hPa (1500m) and 500hPa (5000m).  TTI’s in the MidWest of >60 can yield severe tornadic supercells.

Wind shear: this means change in speed and direction of winds with height.  April 12 saw little deep layer wind shear: winds were blowing at similar strength and direction throughout the atmosphere so little rotation or organisation into severe storms could develop.  Nevertheless, slow moving storms deposited a lot of rain locally and caused minor localised flooding.

WAA: warm air advection: introduction of warm air at the surface increases lapse rates and can increase likelihood of severe TS: 12 April saw little WAA and this inhibited the development of any organised severe weather.

Towards the evening warmer air moved in aloft and, along with the removal of surface heating as the sun set, rapidly reduced lapse rates and inhibited convection causing towering cumulus clouds to melt away leaving a clear night.

So, several ingredients were present to create a marginal storm risk but the absence of some other critical factors like WAA and wind shear kept a lid on the severity and distribution of thundery activity yesterday.  Hopefully, this brief round-up of key storm indices relevant for SE England should help in predicting more severe weather in our region in the future.

Tornado Titans posted this on the CAP and skew-t charts. v helpful. If this is all too much then watch this instead…!