For one fleeting Sunday in early September, Britain will borrow summer from the south — a southerly wind carrying the remnants of a continental heat dome that has scorched Spain and France all week will push temperatures in south-east England toward 28 degrees Celsius, far above the seasonal norm. It is a reminder that weather does not respect borders, and that the extremes anchored over Europe can briefly reach even the most temperate of islands. Yet the warmth will be a visitor, not a resident: by Tuesday, cooler and more familiar conditions will have reclaimed the calendar.