hat is an Urban Heat Island?

Have you ever noticed when leaving a city that the countryside seems much colder? In fact, it has been known for more than 100 years that cities are usually warmer than rural areas. This phenomenon called an 'Urban Heat Island'. Therefore, many cities have their own climate. Add this to the absence of wind because of high buildings and cities can become rather uncomfortable in summer!

It happens because a significant amount of the sun's energy is used to evaporate water from vegetation and soil. It takes a large amount of energy to raise water temperature by 1 degree. Cities have less open soil and vegetation than rural areas and as a result more solar energy is directed into heating the buildings and roads. This causes a temperature rise in cities.

Do Urban Heat Islands only appear in the daytime?

No. During the night the stored energy in buildings and roads is slowly released into the air which keeps urban areas warmer in the night as well as in the day.

Even small settlements can have their own urban heat islands. In my research (see an Investigation into the Urban Heat Island of Bournemouth) a small hamlet outside of Bournemouth called All Saints had its own heat island although they are far smaller than those belonging to cities.

How large are Urban Heat Islands?

They can be almost undetectable. Other times there can be a difference of 10 degrees centigrade or more between cities and the countryside. In my research the urban heat island of Bournemouth on a hot sunny day was around 3 degrees centigrade. At night the difference can be the same.