June usually takes it place in the meteorological calendar as the first ‘summer’ month (even though summer does not officially begin until after the summer solstice around June 21st).

Although we may think of June as a warm month it is not often that it is especially settled. Even in the warmest Junes however long spells of dry, fine weather can be broken by the outbreak of thunderstorms, which may lead to some quite severe weather in places.

Quite often the thundery weather is initiated by what you might hear being discussed as a ‘spanish plume’ event. During the summer months high level ‘plumes’ of warm air that originate over the Iberian peninsula can move north, across the Bay of Biscay and through Western France and eventually up into the British Isles. These plumes generally contain high dew points and also notably high temperatures in the first couple of thousand metres of the atmosphere.

Whilst we may pass the whole summer without seeing a single event of this nature during other summers they seem to occur regularly, when the upper atmospheric patterning is right.

Only last year (2005) on the 18th-19th of June, for example, such a plume moved north across Biscay and western France and up across much of England. The event peaked on the 19th with temperatures at 850 hectopascals (hpa), an atmospheric layer that is commonly used in forecasting, at or over 19C, right up in NW England. These values are unusually high for the British Isles, even during a ‘plume’ event. At the surface the air was very warm and moist having originated in the tropics several days earlier and been carried north east a few days earlier under the plume. When such combinations occur in summer the consequences can be quite severe as the air mass can become highly unstable, resulting in a widespread thundery breakdown.

In such situations it often only needs a trigger event to initiate some quite nasty storms and in this case that trigger was provided by a convergence zone over Wales and the west Midlands. This is an area formed by the meeting of two winds (see our weather glossary for more details of this event). Large cumulonimbus clouds developed and thunderstorms, often with hail occurred shortly afterwards. Prior to the storms temperatures had risen rapidly as the strong sun easily heated the air. Wyton (Beds) reached 33.7C by mid afternoon with values above 30C quite widespread. However the thunderstorms brought rapid drops in temperature of up to ten degrees centigrade, along with gusty winds and large marble sized hail (locally golf ball size in some locations in fact). As the storms moved north east some of the most violent weather was noted in Yorkshire by the late afternoon.

Villages in the North Yorks Moor area such as Sutton-under-Whitestonecliff, Thirlby and the town of Helmsley were especially affected by flooding. At

Hawnby, for example, almost 60mm of rain fell in just one hour. There was widespread flooding due to the rapid run off of the rainfall and a wall of up to 2 metres of water swept through some villages in the Ryedale Valley, the forceof it being enough to sweep furniture out through homes and remove livestock and vehicles with considerable ease along with the bridge into the village of Hawnby (N Yorks).

Such events serve to remind us that along with the very best summer weather can come some of the worst that we see. If we have more than our ‘three fine day’ quota of English summer weather the resulting thunderstorms can pack quite a punch!