Telegraph: Global warming and the increasing size and density of cities could cause the number of nights when the temperature does not dip below 68F (20C) to rise dramatically, the Met Office found. In a process known as the urban heat island effect, cities can be up to 18F (10C) warmer at night than the surrounding countryside. This is because they absorb more heat in the day, which is released from roads, pavements and buildings at night; and because there is more heat being generated …