Urban Heat Islands
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!
Why does this happen?
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.
How large does a settlement have to be to experience an
'Urban Heat Island?'
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
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.