Localized heavy showers affected parts of south-eastern England during the afternoon and evening of 1st August 1998. South-east London, Surrey and north-western parts of Kent were worst affected by the storms, which produced, amongst other things, pea sized hail and a tornado in the Guildford area. The storms also produced some locally heavy downpours; weather observers across south-eastern England reported maximum rainfall totals between 45 and 68mm and flash floods more than half a metre deep were recorded in some places. Beckenham recorded the most rain from these storms - here 68mm rain fell, including a fall of 50mm in just one hour!
There was nothing particularly unusual about these storms (unless of course you were in some way affected by them in which case you'll have every reason to feel differently!) - heavy localized downpours are common in north-western Europe during the summer. Inevitably they will sometimes produce some hail and even tornadoes, but even this isn't as unusual as one might thing, after all the UK averages 33 (mostly weak) tornadoes per year. However, I did remember to save some interesting weather charts from this storm.
The surface synoptic chart on the left shows the synoptic situation at 1800 GMT on 1st August. Britain is sandwiched between an area of high pressure just west of Iereland and low pressure and its associated frontal system over central Europe, whilst the air over Britain was unstable enough to cause the storms in question. The satellite picture (centre) shows the view from space at 1800 GMT. The most striking feature is the mass of cloud over Germany, the Low Countries and France whilst over southern England there's also some cloud, which are the storms in question. The chart on the right hand side, meanwhile, shows lightning strikes on 1st August. As can be seen, there's a lot of them over southern England, most of them occuring during the afternoon and evening.
The diagram to the left is a tephigram (simply a graph showing air-temperature and dewpoint plotted against pressure - the information comes from a radiosonde [or weather balloon] ascent - used to help meteorologists make weather forecasts) for Herstmonceaux on the south coast of England plotted from data obtained during the early afternoon of the 1st August. The tephigram shows the state of the atmosphere over south-eastern England close to the time of the storms in the area where the storms occured. There's a few interesting things to point out from the tephigram.
The wind arrows on the right of the tephigram show that in the lower atmosphere (i.e. under 500mb) there wasn't much wind. These sort of storms are steered by winds at around the 700mb level, so because there wasn't much wind at this level the storms will have been moving very slowly which increases the chance of a large amount of rain falling at any one place affected by the storm and also means that any place affected by the storm will have been affected by the storm for a longer period than if the storm had been moving a bit quicker.
Of course the fact that the storms were slow moving increases the chance of flooding and flash flooding at any one place too, as does the fact that the tephigram shows that the atmosphere was saturated between 925 and 700mb - the more saturated the air in this region, the more rain can fall out of the storm cloud. The winds in the higher levels of the atmosphere (between 300 and 250 mb) were quite strong which means that the anvil at the top of the storm will have spread out which would make the storm's cloud area much larger than it's rainfall area - had these winds been lighter there would have been a smaller horizontal spread of higher level cloud associated with the storm. In other words the storm looked a lot bigger than it really was from the satellite image!
The temperature curve (the solid black line) shows that the freezing level in the storm was low - at around 925 mb - and research has shown that a low freezing level encourages hail to form.
The last thing to point out here is that the area between the temperature curve and the red line to its right is proportional to how active a storm will be. The greater this area (known as CAPE), the more active the storm. Also, a large CAPE area supports a strong updraft in the storm which in turn helps large hail to develop.
So, there you have it - the profile of the atmosphere over south-eastern England made during the early afternoon of 1st August 1998 shows that the atmosphere was ripe for some rather nasty thunderstorms to develop. If you want to find out more about thunderstorms you might like to visit the TORRO website and the Dutch Stormchasing website.
Climatological Obersvers Link Bulletin , August 1998.
The satellite picture was obtained from a satellite receiving station in Strasbourg and the lightning report image and synoptic chart were obtained from Wetterzentrale in Germany.
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Dan Suri, 29 April 2001