The thermometer can keep rising for hours after solar noon. In many European cities, the roughest sightseeing window is 15:00 to 18:00, exactly when many visitors are still walking between landmarks. That is how a pleasant Rome, Seville, Athens, or Lisbon break turns into an expensive search for shade. The fix is not to cancel summer travel. Build the day around heat exposure, then keep one indoor backup within a short walk or transit ride. Think of it as a cash-flow plan for your energy: spend your best hours on outdoor sights, preserve a reserve for the hottest block, and…