If Roland Emmerich ever got his hands on a Potter film, you could kiss goodbye to the magic. The box-office logic is impeccable. The Day After Tomorrow is a big-weather disaster movie designed to put the fear of God into audiences with Olympic stamina and peanut-sized brains. According to Emmerich, the world is on the verge of a new Ice Age, but will anyone listen to the top weatherman, Dennis Quaid? How we howled.

There was an interesting BBC documentary recently about what might happen if the Gulf Stream suddenly stopped. The documentary offered persuasive evidence of a big freeze, but it stopped short of hailstones as big as footballs, minus 65C (-85F) temperatures and tidal waves the size of Manhattan. Curiously, there was no mad rush to pluck six-year-old boys with cancer from their hospital beds, and no pressing desire to find a nun with a guitar. Surely fatal oversights by the BBC.These are the basic survival tools in a Hollywood disaster, and Emmerich pulls on them as if they were lifelines to a brave new post-Ice Age world.

One must never underestimate big weather, particularly when it has a personal grudge against most of the cast. When Quaid isn’t clambering through White House meetings like Ranulph Fiennes, he is blasted by to bits by wind machines. His son, Jake Gyllenhaal, has most of the fun and the hottest tips. If you are being chased by packs of wolves in frozen Manhattan, head for the Public Library with a cute girl (Emmy Rossum) and a box of matches.