BIG CITY DWELLERS love the buzz of the urban jungle while country inhabitants prefer the peace and greenery of more open landscapes. But there’s more than merely a lifestyle choice separating the two habitats – big cities can actually create their own mini-climates that are different from those of the surrounding countryside.
Climatologists have a name for this phenomenon – they call it ‘microclimate’, in other words, the prevailing weather conditions that apply only to a specific small area. But in the case of city microclimates, there’s a more evocative name: ‘urban heat islands’. Cities can literally be hotspots in a cooler rural landscape. The temperature differences can be massive: London, for example, can be approximately a staggering 10°C warmer than the surrounding countryside. And there’s more: research at NASA suggests that some cities are so hot that they can even create their own weather – including violent storms with thunder and lightning.
So how does this happen? Think about it – all those concrete roads and buildings, all those brick walls, all those pavements, soak up the sun’s heat during the day, then release it slowly after sundown like giant electric night storage heaters. Narrow streets and high buildings can trap heat too, giving it less space to escape; pavements cover bare ground and there is relatively little vegetation to absorb water so that when it rains, the water has nowhere to go and there is a greater risk of localized flooding. No wonder cities are warmer and more humid. In the countryside, by contrast, there are open spaces and trees, streams and rivers, which help heat to dissipate as moisture evaporates from leaves and water surfaces, and rainfall can soak into the ground.
Another surprising manmade microclimate can be your own garden. In cool temperate climates, gardeners have long understood how erecting walls and fences can make their plots warmer and more sheltered, allowing them to raise those spe-cimens that might otherwise not survive in that particular environment. Training tender fruit such as peaches or apricots against a sunny brick wall is an old trick – the brick absorbs the sun’s heat and radiates it out to keep the tree warm and help the fruit to ripen. Fences can help to break the passage of prevailing winds to give additional shelter.
Nature can create her own heat islands too, in dense forestation. The canopy of a thick forest forms an almost unbroken surface that soaks up the sun’s heat during the day, in a similar way to the surface of the ground. This warm canopy conserves the heat but shades the forest below so that the lower you go, the cooler it is: the difference may be as much as 5°C during summer. In addition the trees block the free flow of air through the forest. So within the forest it may not only be cooler than the surrounding countryside, but more humid too.