Today (15 July) is known as St Swithin's Day in the UK. According to folklore, if it rains on St Swithin's Day it will rain for the next 40 days in succession.* The British are, of course, fixated by weather, as evidenced by last month's heatwave. Whenever temperatures soar and the sun comes out, the nation goes into weather overdrive. "Mad dogs and Englishmen go out in the midday sun" wrote Noel Coward, not without some justification. Annette Farr looks at the opportunities open to water companies from the heat of a summer.

It was a mass of hot, humid air feeding in from Europe - where some explosive thunderstorms led to flooding in large parts of the Czech Republic, Austria, Germany, Romania and Poland - which fuelled June's heatwave in the UK.

As it became hotter, an A level 3 health warning was issued by the NHS with advice on how to cope with the heat, including the drinking of plenty of water. In the stifling heat of the London Tube, passengers were advised by London Transport to carry a bottle of water with them as they ventured underground.

Such is the importance of drinking water during a heatwave that Danone, producer of Evian and Volvic bottled waters, operates a 'Weatherman' service to forewarn retailers of increasing temperatures. According to Danone, taking 14ºC as a base, when the sun is shining and temperatures reach 20ºC, sales will typically rise by 36% and when it reaches 25ºC sales will increase by 78%. If the temperature tops 30ºC bottled water sales will typically increase by 125%.

Figures just supplied by Danone Waters UK show that June saw a record-breaking number of dispatches of Evian to the UK. Overall June sales were up 25.8% compared to June 2008 and grocery sales for the first week of July were up in excess of 50% compared to the same week last year.

Records were also broken at Highland Spring. Sally Stanley, the company's marketing director, disclosed that during June, the brand sold 30m litres of bottled, which equates to 13% of its annual sales in one month. Turnover in June reached GBP8.25m (US$13.5m), the first ever month in the brand's history to break the GBP8m barrier.

As far as Stanley is concerned, there is an undoubted connection with temperature. "There's an unequivocal correlation in the rise of bottled water sales in heatwaves," she says, "especially when temperatures reach a peak when nothing but pure hydration in the form of plain water will do." Conversely, she notes during the floods of July 2007 there was a dip in sales.

UK temperature levels, however, were as nothing compared to India's where, prior to the late arrival of the seasonal monsoon, temperatures of 49ºC were recorded in the Bundelkhand district of northern Uttar Pradesh state. The heatwave claimed over 100 lives, schools were closed and, although no official figures have yet been released, sales of soft drinks were reported to have soared.

It is being mooted by meteorologists that the delayed monsoon in India, along with other indications such as droughts in Australia, Indonesia and the Philippines, and the seas around South America warming up, mean that an El Nino is developing in the tropical area of the Pacific.

A strong El Nino results in severe climatic change: it was only a little over ten years ago that the particularly strong 1997-1998 El Nino became headline news. Then, air temperatures rose by 1.5ºC, wreaking havoc with harvests and crops worldwide, while subsequently creating a knock-on effect for producers of soft drinks.

At the G8 summit last week, world leaders agreed to the goal of keeping the world's average temperature from rising more than 2ºC, and to cut gas emissions by 80% by 2050. As the world warms up (by 2030 forecasters are suggesting that temperatures in London could reach 40ºC and cities will be acting like giant night storage heaters), people will want to drink more and more water, be it from the tap or bottle.

Climate change will bring its own challenges to bottled water producers who are, after all, stewards of the land from which the water is drawn. Protection of the land and meeting consumer demand will be a delicate balance. But the UK's recent heatwave has shown that, when the sun is out and temperatures rise, a bottle of water from a refrigerated source reigns supreme as the ultimate hydrating thirst slaker.

The decline in bottled water sales in the UK over recent times has been as much to do with disappointing summer weather as it is to environmental concerns. Indeed, with the fillip given by record breaking sales, Sally Stanley believes that "the category will be nudging back to growth by the end of the year".

And let's not forget that since no ingredients are used in production, natural mineral and spring bottled water has the least carbon footprint of all beverages.

* The legend dates back to the Saxon burial of St. Swithin, Bishop of Winchester who asked that he be buried humbly outside, but when attempts were made to remove his remains to a shrine inside Winchester Cathedral, the heavens opened and it rained for 40 days and 40 nights , an indication, it is said, of the saint's displeasure at being moved. Hence the traditional rhyme

St Swithin's day if thou dost rain
For forty days it will remain
St Swithun's day if thou be fair
For forty days 'twill rain na mair