Credit: DANNY LEHMAN/CORBIS

Ever thought about escaping the heat of the city by getting down to the lakeside? According to Arón Jazcilevich and colleagues (Climatic Change 44, 515–536; 2000), having a lake on your doorstep may not only provide a waterside retreat but help keep the city's climate appreciably cooler.

Mexico City, it seems, was such an example. Earlier this century, the relatively small city (86 km2) was bordered by Lake Texcoco (120 km2), and the ‘urban heat island’ effect — where city temperatures exceed those in surrounding rural areas — was only 1.5 °C. Urban expansion has dramatically altered the balance, so that the sprawling city (pictured), at 1,200 km2, now dominates the Basin of Mexico. The lake has been reduced to a mere 10 km2 or so in area, and the heat of the city can exceed that in neighbouring countryside by 8–10 °C. This commonly observed effect is usually explained by the surface geometry and thermal properties of all that extra concrete and asphalt.

But what part might the lake play? Jazcilevich et al. developed a model to reproduce urban climate under present-day conditions and those of 1921, using the respective city and lake areas. Then, taking the situation in 1921, and reducing lake area to today's extent, they found that the expected temperature field was almost identical to that of today. From this the authors infer that the increase in city temperatures is largely due to the reduction in lake area and the evaporative cooling it provided, as well as the massive urbanization.

Perhaps the finding has come too late for Mexico City. As urban areas continue to swallow up land around the world, however, town planners would be wise to keep a little bit of Venice in their designs for the comfort and health of inhabitants.