Cities and surrounding rural areas look very different from space – and incoming solar radiation behaves very differently when it strikes a building as opposed to a field or a forest. Construction materials used for houses, roads and other urban developments have very different thermal properties to grass and other vegetation. This causes the urban heat island effect; a type of urban microclimate that often makes cities warmer than surrounding rural areas.
Light Absorption and Reflection
To understand the causes of the urban heat island effect, one first needs to understand something about the physics of solar radiation. All substances reflect or absorb solar rays to a varying degree. When light strikes a mirror, the vast majority of it is reflected; when it strikes tarmac it is mostly absorbed. The measure of an object’s reflective ability is called its albedo. In general, the brighter an object appears to the naked eye, the higher its albedo.
An easy way to think of this is in terms of colour. Visible light is broken down into a number of different wavelengths, with different wavelengths representing different colours. Red objects appear red because they reflect mostly red wavelengths and absorb others. Black objects absorb the full spectrum of colours while white objects reflect them all. There are of course wavelengths that aren’t visible to the naked eye, but in general white surfaces are good reflectors and black surfaces are good absorbers.
Urban albedo is generally higher than rural albedo because of the building materials used – this means that they absorb a greater proportion of incoming solar radiation. This effect is intensified by the lack of evaporative cooling in urban areas.
Plants are able to transpire water in order to release heat. Around 2320 kilojoules of energy can be lost per kilogram of water evaporated. This can significantly reduce the levels of radiation stored by plants. Moisture levels in cities are much lower – due to a number of factors, including impervious building materials that promote surface runoff and feedback from reduced precipitation levels that often result from the urban heat island effect – hence less energy is lost through evaporative cooling.
Radiation Heat Traps
Once heat is absorbed, cities do a good job of retaining it. This is because of the vertical nature of buildings. When heat is radiated from fields, for example, the surfaces that are radiating are facing the open sky and there are few obstructions to interfere.
In cities however a large percentage of surfaces are facing other surfaces and much radiation is reabsorbed. Even radiation from the ground can be intercepted by surrounding buildings. The obstructions created by construction also make it harder for air to flow, reduced heat loss by convection. Cities can act as very effective heat traps as a result.
Urban Heat Island Effect
The increased concentration of solar energy in cities serves to create a warm urban microclimate. A microclimate consists of local variations in wind, rain and temperature as a result of topographical features. The microclimate that surrounds many cities is described as the urban heat island effect.
In some cases, this effect can result in a difference in temperature of up to 10°C between the city and the surrounding green belt, while it often leads to reduced precipitation, increased pollution and a number of other potentially undesirable effects.
Urban areas are heat traps which absorb a great deal of solar radiation and serve to warm the air within them. This is caused by the physical and thermal properties of the materials used in construction, as well as topographical factors that serve to trap warmth and reduce winds. They are localised phenomenon that can account for large differences in weather conditions over the space of a few miles.
Voogt, JA, “Urban Heat Islands: Hotter Cities”, ActionBioscience.com accessed 28 April 2010
Santamouris, M & Asimakopoulos, D. (1996) Passive cooling of buildings, James & James, London