Have you ever driven from the suburbs straight into a metropolis and suddenly felt the air around you get hotter? Even if you have not, if you have lived both in the suburbs or the country, and the city for a quite a while, you must have noticed that the city is significantly hotter than country or the suburbs. This is the urban heat island effect. You can realize why this effect is called ‘island’ if you look at the picture below.
There are a couple of reasons why this happens:
- Buildings are quite efficient in absorbing (if the buildings are dark-colored) and reflecting (if the buildings are light-colored) heat. Due to reflections and the dense building networks in cities, the heat is bounced back and forth between buildings. Thus the heat is effectively trapped.
- Dense rows of buildings block winds and this reduces heat lost by convection
- Urban drainage systems do not allow much water to stay on the ground. So, the amount of evaporation is less. This also contributes in increasing urban temperatures.
- Waste heat from industries, air conditioner etc. also plays their part in rising urban temperatures
- The fact that urban areas have lower vegetation cover does not help
So, what is the solution for this burning island? Well, there are quite a few solutions, but I will mention just one very important one. That is, the increasing of green space.
Plants lose heat by the process of transpiration, that is losing water through their leaves and other parts. Because water has a very high specific heat, which means a lot of heat is needed to increase the temperature of a unit mass of water, its temperature does not rise much even in a hot day. Thus, when this water is released into the atmosphere, the temperature there also remains low. Thus, we see water acting as a kind of heat buffer.
In fact, researchers at Manchester University have found out that just by adding 10% of green cover, the maximum surface temperature of Greater Manchester can be kept to the 1961-1990 baseline for up to the 2080’s. (Carter and Handley, 2006)
Here are two pictures of the city where I live, Dhaka. The images have been taken from Google Earth. You can obviously tell where it is more comfortable.
As you can see, if we do not want Dhaka to be a completely unlivable city, we really need to start working now. And increasing green cover is one of the most important thing to start working on.
Do you live in a city? What does your city look like and how much of the urban heat island effect do you feel there? Is there a lot of green cover there? Comment below and we can have a little discussion.
C3 Headlines Available At: http://www.c3headlines.com/global-warming-urban-heat-island-bias/
Carter J. and Handley J. (2006) Adaptation Strategies for Climate Change in the Urban Environment, draft final report to the National Steering Group Available At: http://www.sed.manchester.ac.uk/research/cure/research/asccue/publications.htm