ALT Extreme Heat: dangerous even after the sun goes down. Large cities often experience higher temperatures than surrounding areas because of all the heat-absorptive surfaces, such as dark pavement and buildings. This is known as an urban heat island. The upper floors of brick buildings are particularly susceptible to the dangers of extreme heat without air conditioning, because they retain heat after the sun goes down. The strong influence of the urban heat island on nighttime temperatures limits the ability of people to cool down and recover the heat of the next day.