This weekend, moviegoers who arrive in time to see the trailers and commercials before the main feature will be treated to a 60-second compilation of breathtakingly fast clips from 28 forthcoming films. Images from Mamma Mia!, The Incredible Hulk, the new Indiana Jones movie and the new chapter in The Chronicles of Narnia, among others, flash on screen and are gone as quickly.
This sequence of clips has just one purpose: to get those same British audiences back into cinemas this summer.

The same motive lies behind the free distribution of a million copies of Focus, a 16-page glossy magazine that previews the summer's forthcoming films. It will arrive at households in targeted areas within easy distance of cinemas, and will also be available at cinemas, railways stations and health clubs.

Never before has the notion of going to see summer movies been so heavily promoted in Britain. We still regard it as an American idea: a device dreamed up to give kids from across the pond something to do during their interminably long summer vacation.

And because American kids were the target audience, the movies that opened in summer reflected their tastes: action-dominated adventures, heavy on spectacle and shorter on plot. Typically family-friendly, they tend to be large-scale and loud; they may be funny, but more often than not they offer a thrill-ride a minute.

Yet times have changed, and summer movies now earn the profits that drive the studio system year-round - far more than those prestigious winter films that are staples of awards shows.

Because of the money to be made from these movies, we in Britain now have a four-month season of our own. This year it starts next Friday, May 2, with the release of the superhero thriller Iron Man and the family comedy-adventure Nim's Island.

"Here in Britain we used to have a summer movie season that included just July and August - the school holidays," says Mark Batey, chief executive of the Film Distributors' Association. "Now it's expanded enormously." Indeed. It has expanded to match the summer movie season in the US, which itself is longer than it was. Summer in cinemas used to start on America's Memorial Day weekend at the end of May, but has now crept forward to the start of that month.

When you see the statistics, you see why. Last year, 160 films were released in this four-month period, and it will be the same this summer: some 40 films each month will see the light of day between now and the end of August. And in 2007, there were 66.5 million cinema admissions in that four-month period, almost 14 million more than the figure four years previously. In the 1990s, summer movie admissions averaged out at some 40 million a year.

Little surprise, then, that 27 of the top 50 grossing films in Britain were released in these summer months. At the top of the tree is the second Pirates of the Caribbean movie, Dead Man's Chest, closely followed by The Full Monty (1997) and Star Wars Episode I: The Phantom Menace (1999), all of which grossed more than £50 million here.

One difference these days with America summer movies is that the gap between their release in the US and the rest of the world has diminished sharply. For instance, Indiana Jones and the Kingdom of the Crystal Skull, the fourth in the series, opens in almost every global territory between May 21 and 23.

The film industry is coy about the reasons for this, but in effect, it's the threat of piracy. Advance screenings of all these titles are left as late as possible, and their release compressed to as few days as possible across the world to foil attempts to pirate these hugely popular films.

This year's summer list looks rather promising. It's not entirely sequel-free (what summer ever is?), but at least, unlike 2007, there's no glut of "three-quels" - that is, films which were the third in their series. Some of us recall the redundant Shrek the Third with a shudder.

True, this year's line-up offers sequels from the Indiana Jones and Chronicles of Narnia series - but, given that these don't occur every year, they should attract genuine interest. Iron Man, based on a Marvel Comic character, will be slugging it out with a Marvel stable-mate, The Incredible Hulk. This last title marks a re-start for the Hulk, after the commercial failure of Ang Lee's sophisticated attempt at a superhero movie five years ago.

Then there are the titles tailored for grown-ups. Does Sex and the City, making its transition from TV, count as a summer film? Its release date, in late May, confirms that it does. So does the excellent but sombre Gone Baby Gone, directed by Ben Affleck and starring his brother, Casey, about a young girl's abduction. Summer movies have expanded in ways other than the months in which they open.