Today I'm going to talk about a phenomenon known as the urban heat island effect . Put simply , cities tend to be noticeably warmer than the surrounding countryside , particularly at night . In some large cities , the difference can be as much as seven degrees . There are several reasons for this , and the first concerns the materials we build with . Concrete , brick and asphalt absorb heat during the day and release it slowly after sunset . Secondly , cities have far fewer trees , which normally cool the air by releasing water vapour . Tall buildings also play a role , because they trap warm air and reduce wind speeds at street level . Finally , heat is produced directly by human activity , from vehicles , factories and air conditioning units . Ironically , air conditioning makes the problem worse , since it pumps hot air back into the streets . These higher temperatures are more than an inconvenience ; they have serious consequences for public health . During heatwaves , elderly people and those with heart conditions face a significantly higher risk of illness . Warmer cities also consume more energy , which in turn leads to higher greenhouse gas emissions . So what can urban planners do to reduce the effect ? One widely adopted strategy is the installation of so-called green roofs , which are covered with plants . Research suggests that a green roof can lower the surface temperature of a building by up to thirty degrees . Another approach is to use reflective materials , such as light-coloured paving and specially coated roofs . Expanding parks and planting trees along streets has also proved effective in many European cities . However , these measures require long-term investment , and their benefits are not always immediately visible . In the next lecture , we'll look at a case study of how one city tackled the problem . For now , I'd like you to read the two articles listed on the course website .