Weather expert Chris Tibbs on what to expect when thunderstorms are forecast over the UK
Earlier this summer we saw considerable thunderstorm activity over the UK and Europe, resulting in flooding and some serious injuries. In the UK and North West Europe the occurrence of lightning strikes on yachts is thankfully rare; however, in the tropics – particularly near a large land mass – the chance of being struck does increase significantly.
In the UK we generally get thunderstorms for one of two reasons. The first is an active cold front. This is along the boundary between the warm moist air and the cold dry air of the advancing cold sector. The atmosphere along the front is unstable, with heavy showers forming. In an active front the cumulonimbus clouds continue to grow until they produce thunderstorms. They travel in the direction of the front and generally pass over quickly, although gale force gusts are likely.
This front was historically known as a squall line; the height of the cloud indicates how unstable the atmosphere is and they will have some severe downdrafts on the leading edge of the rain or hail.
What is a Spanish plume
The other main type of thunderstorm we experience is the result of air mass. In the UK we sometimes get what is known as a Spanish plume: this is a layer of warm air moving north from Spain across France. A temperature inversion traps this warm moist air, which is continually fed by evaporation from the land below, giving very warm moist air at low levels.
At the same time as this plume of warm air works its way north, cold dry air from the Atlantic at upper levels is moving east (often in the form of an upper air trough). Over time the inversion is eroded so that the warm moist air, having lost its cap, will rise to great heights triggering large cumulonimbus clouds and thunderstorms.
The atmosphere is so unstable that some of the updrafts in a big storm can reach around 60 knots although downdrafts are generally about half this speed. As the cloud top temperatures cool to below –20°C ice forms and the anvil top that is often seen is ice crystals blown from the top of the cloud.
We now have this vast overturning cloud that will produce severe downdrafts with hailstones and strong wind – these hailstones hurt and can damage varnish and wood. It is the downdrafts associated with the rain or hail that causes the difficulties when sailing.
As the rain and hail descends into warmer air some of it evaporates, which in turn cools the air so we have a cold column of air accelerating to the surface where it spreads out as a gust front. With large clouds we can easily get gale force gusts on the leading edge of the thunderstorm.
The western half of the UK experiences thunderstorms usually associated with active cold fronts and they are evenly distributed over the year, however central and eastern parts of the UK have more thunderstorms and these are largely confined to the summer months and linked with a warm moist air mass and the heating of the land – “three fine days and a thunderstorm” is how George II reportedly described the British summer.
But it is not only the UK that gets summer storms – around the Mediterranean in the late summer you will get some fairly active thunderstorms upsetting your sailing. As the cloud top temperature is low, precipitation often starts as hail although as the storm progresses the ice becomes water.
With a cold front we are already expecting strong winds and squalls, but the summer thunderstorms tend to be a bit more unexpected as we will have good weather – warm and sunny weather tends to make us less assiduous about getting forecasts. It is then more of a shock when the cloud darkens and we get a thunderstorm; although we normally see the towering cumulus sometimes the sky is obscured by cloud spreading out and the first real sign of the thunderstorm is the flash of lightning. Thunderstorms will be well forecast, however more as a general warning than for a specific place.
- Strong gust on or ahead of the precipitation – often to gale force.
- Visibility greatly reduced in precipitation.
- Hail or heavy rain.
- Sharp temperature drop.
- Java (Indonesia) is the most thundery place on earth. It has been estimated (globally) that there are 44,000 thunderstorms per day with about 100 lightning discharges each second.
- The mechanism of the Spanish plume is similar to what happens in the US where warm low level moist air from the Gulf of Mexico tracks north, while cold air at altitude moves in from the west. The capping inversion erodes giving some supercell thunderstorms and tornadoes.
- Count the seconds between lightning and thunder to get an estimate of how far away the storm is. The difference between the speed of light and that of sound means that a separation of about 6s is a nautical mile.