What cities do to the local climate

There is considerable concern about the possibility of world climate change due to the enhance greenhouse effect. Scientists have predicted global temperature rises of between 1 and 3.5° C by the year 2100. Yet temperature rises of about the same size have already occurred in some big cities, simply because the city is there! Particularly at night, cities are usually warmer than their rural surroundings, because of heat stored in bricks and concrete and trapped between close-packed buildings - the so-called urban heat island effect. Have a look at how temperatures across Melbourne are affected by land-use.

Reproduced with permission from 'The Age'. © 1992 The Age

Wind -Tunnels and Ventilation
City wind speeds are lighter, on average, and vary from place to place. Wind “tunnels” and “hot-spots” occur where winds are channelled down city streets or wash down the faces of tall buildings. By contrast, streets running perpendicular to the wind direction are sheltered, to the extent that pollutants may not disperse.

Urban warming affects the city's energy use, reducing the demand for winter space heating, but increasing the energy needed for cooling.

Run-Off
Paved surfaces mean that runoff from rainfall reaches a higher peak flow in urban areas, and reaches it much faster. At times, flash floods, caused by greater run off from the city's paved surfaces, result in big financial losses and can be life-threatening. As the city grows, the climate changes intensify.

Continue with the next topic: What city climates do to people.