THIS August was a good summer month. With winds mainly from a south-westerly quadrant, temperatures over the month were above normal.

Just warmer than last year's, it was the mildest August since 1997. It was also fairly sunny and rainfall generally in the region was around average.

There were never any really hot days in the North-East, nor any cold ones either, but many were warm or very warm. These tended to come in short spells of three or four days, some of which ended with a thunderstorm or two, in keeping with the saying about the traditional British summer.

I heard thunder on four days. On two of these days, the 15th and the 19th, the storms were very widespread and not many will have missed them. The 19th was unusual in that there were several individual cells passing by during the day, each giving a few rumbles.

Those on the 15th, at least to the east of the A19 where they arrived just after dark, were very spectacular and lasted an hour or more. Lightning was frequent, lighting up the sky every few seconds, accompanied by almost continuous thunder. Such storms are quite a rare event in Northern Britain, more typical of those you get on the Continent, and, three or four times a year in South-East England, where I used to live. They are about the only thing I miss from that part of the world.

As far as I'm aware, associated rainfall amounts were only great in one or two spots. Rainfall on the 7th and 8th was torrential at times over a wide area, with local flooding. This, however, was caused by a developing wave depression running north-eastwards.

Why is it on some days showers don't form, or only give the odd splash of rain, while, as we've seen this month, they can develop into vicious beasts? In the extreme, they can give damaging hail and even spawn devastating tornadoes, though, fortunately, very rarely in this country.

Clouds form when air rises and cools sufficiently for the moisture in it to condense out into water droplets. There are many factors that contribute to their development. For example, in low pressure areas the air is tending to ascend anyway, so this will help.

The temperature structure of the atmospheres is crucial. The bigger the contrast in temperatures between the ground and the air aloft, the more rapidly the thermals are going to rise, so resulting showers are likely to be more vigorous. On the other hand there may be layers of warm air that stop the thermals rising any further, putting a cap on the expansion of the clouds.

Perhaps one of the most fascinating ingredients for the growth of violent storms is that the winds must shift in direction, clockwise, with height. At high altitudes, it must be almost at right angles to that at low levels. You would think that this would cause the cloud to be pulled apart, but it is necessary for the continual growth and decay of the cells within the storm.

A typical situation for such widespread, intense downpours is for a depression to be moving north-east across the country from the Bay of Biscay, ahead of a cold front moving south-east. Hot, humid south-easterly winds are drawn in from the Continent, while there are south-westerlies aloft. Thunderstorms may break out ahead of the front, which then brings further outbreaks of rain, before cooler, fresher conditions arrive from the west or north-west. It was essentially variations on this theme that occurred four or five times during the past month, with periods of westerlies in between.

August temperatures and rainfall

at Carlton in Cleveland

Mean max 20.9C, 69.5F, (+0.9C, +1.5F)

Mean min 12.2C, 54F, (+0.8C, +1.5F)

Highest max 27.0C, 80.5F, 15th

Lowest min 6.5C, 43.5F, 28th

Total rainfall 61.5mm, 2.4in, (-1mm,

-0.05in)

Wettest day 13mm, 0.5in, 7th

No of rain days, with 0.2mm (0.01in) or more 16

(Figures in brackets show the difference from the 19-year mean, 1984-2001)