David Randall: August is the cruellest month... so why does it still surprise us?

If the sun is blazing down on you as you read this, remember: the eighth month has for generations been a con artist, that makes long-term promises of heat but tends to deliver rain

The scene is a smart terraced house on an evening in February. A frost is already beginning to creep on to the grass outside, but inside, all is centrally heated. He and she are at home, the children have been put to bed, and so, on the kitchen table, the summer holiday brochures and last Sunday's travel pages are spread.

A holiday in Britain, they think, this year. None of that faffing about with security lines at the airport, just a canter down to Devon, and a fortnight at that enchanting cottage by the cliffs. But when to go? August, probably. This is the month when the road ahead shimmers in the afternoon heat, panting dogs slurp greedily at their water-bowls, and hollyhocks in the cottage gardens begin to fold over in the midday sun. The couple half close their eyes, call childhood reading to mind, and see a beach strewn with starfish and a day so hot that the Famous Five have run out of lemonade, and Rupert Bear has had to take off his scarf. That settles it. The reservation is made. August it is.

Fast forward six months. It is the ninth day of the holiday, but the sun has hardly been seen. The family are breakfasting, each one clad in sweaters hastily bought on an expedition to Plymouth's shops. It was their fourth such trip, a measure of how soon they ran through the tourist information centre's wet weather suggestions. It is dry at the moment, but, as a little voice at the end of the table utters the first "Bored!" of the day, a familiar sound is heard. It is the spattering of rain. A lowering sky had crept up on them while they mueslied, and now, as heavier drops race each other down the windows, another sodden day looms at their seaside letting.

If months were people, August would be a con man. April might be an optimist and December a party animal, up for a bout of hedonism before January the accountant presents his bill; but August is a huckster – a sweet-talking, smarmy fraudster who makes us all think, with those chocolate-box calendars, and images of sunny seasides and ripening cornfields, that we're in for 31 days of deep summer. And then, when we've handed over our cash and bought into the promise with our holiday booking, what we actually get is a monthful of rain and drear. It is the British climate's biggest annual confidence trick – and we fall for it every time.

You don't need to go far this year to find its victims. A month's rain fell in 36 hours in Scotland; by mid-month rainfall records for the whole month had been set in Northern Ireland; roads were dammed by mudslides, and, in County Limerick, waters rose so fast that cattle drowned. In gloomy Cornwall, rumours of a break in the clouds sent holidaymakers scurrying across the peninsula in the hope of catching a little sun. There was flash flooding in Cambridge and Norfolk, a mini-tornado in Suffolk and Plymouth, among other places, had twice the August average rainfall.

Today, the sun may be making a rare guest appearance, but the first half of the month was one of the wettest on record. In the first 17 days, some 95.5mm of rain fell (about normal for a bad December). This is 10mm more than the long-term monthly average, and makes it one of the 10 wettest Augusts since 1914. So far this month, the Met Office has issued 73 severe weather warnings, a total more associated with the very depths of winter.

Nor is this entirely out of character. You may need to read the following three times to absorb its shocking impact, but it is a provable, statistical fact that, for large swathes of Britain (most of the North-east, Lancashire and the Cheshire Plain, the Scottish Borders, Shropshire, parts of East Anglia and the East Midlands), August is the wettest month, not just of the summer, but of the entire year.

It's a measure of just how bad the month is that its 2008 efforts pale beside some other Augusts, notably 1912 when, without the benefit of global warming, 192.9mm of rain fell – fully 80mm more than in its December, and 21 times the year's April total. Mere statistics cannot begin to do justice to the misery inflicted by this triple gold medal winner among Augusts, being the wettest, coldest and dullest of the 20th century. The average temperature was a mere 12.9C, and London had only one day warmer than 21C, while Birmingham and Manchester could not even pass 19C.

Rainfall was 231 per cent of the average, with East Anglia getting the worst. On the 26th, heavy rain began and continued for 30 hours. Norwich was cut off for two days, and 40 bridges throughout the region were destroyed by floods that reached 15ft in places. Crops were devastated (the corn turned black in Buckinghamshire); other cereals rotted before they could ripen, and potatoes and hops were hit by a succession of viruses. And, five years later, just to press home the message, came the second wettest August of the century which announced itself with 53 hours of continuous rain in Kent.

Downpours of tropical dimensions are a particular speciality. In 1956, some parts of the North-west had their wettest month in the whole century, and the bank holiday was so bad that it was one of a run of such Mondays that prompted the still mystifying decision in 1965 to move the holiday from the first week of the month to the last.

In 1975, on the 14th, more than twice the average rainfall for the entire month descended on Hampstead in less than three hours, 100 homes were flooded within five minutes and one man died. Four years ago, Boscastle in Cornwall saw severe flash flooding and destruction, but the worst August torrent was at Lynton and Lynmouth in Devon in 1952. On Exmoor, nearly three times the monthly average rainfall in 22 hours sent a surge which carried before it boulders that acted like battering rams. Nearly 100 buildings were destroyed and 34 lives lost.

Sometimes, of course, August throws in a period of sustained hot, dry weather that bolsters our touching belief in it as a month of benign, sunny warmth. For a few years at the beginning of the 20th century, a number of glorious Edwardian late summers (33C in Bedfordshire in 1906, and 36C at Epsom, Surrey in 1911) may well have been the events that set in the national imagination the idea of August as a festival of sunshine. The drought year of 1976 was extraordinary, and, just five years ago, 2003 delivered an August heat-wave so extreme that 1,000 deaths were attributed to it.

But you don't need to wait long for it to revert to type, with, for instance, an August hailstorm. In 1879, one shattered 3,000 windows in Kew Gardens' Temperate House, and, in 1956, at Tunbridge Wells, an afternoon deluge left the town centre a foot deep in hailstones, with drifts up to four feet. And it is entirely due to August that a famous insurance company was founded. On 9 August 1843, severe storms developed over the heart and east of England. Hailstones up to an inch across fell, flattening crops from Oxford to Norwich, and laying a metre and a half deep. As a result, the General Hail Storm Insurance Company was formed. We know it today as the Norwich Union (or did, until it rebranded itself as Aviva).

As far as August weather goes, the only surprise, really, is that we're ever surprised. But we are, year after year. We do seem to have in our heads a misguided little almanac which says that December will be snowy, April windy and showery, and August blazing hot. Formed in childhood by calendars, and cosy little books of clichéd pictures, this comfortingly certain image of the months persists – whatever the weather.

Since there seems no hope of shifting that, might we perhaps shift something else? Much of the fixation on August for vacationing derives not only from a rosy myth but also from the fact of England's school holidays.

A thought: the last few weeks of the school year are frittered away on activities confected to see the little snowflakes through to the happy day when they can be released into the community for six weeks. Might we not, as many states in the US do, and much closer to home in Scotland, give our families drier, sunnier, longer days, boost domestic tourism takings, and increase productive school time, by starting the holidays in late June and ending them as August begins to gather its opening storm clouds?