last updated 24th August 05
by 4ecotips.com

Desperate city uses metro seepage to cool surfaces

Although Japan is currently experiencing more than its usual share of rain, once temperate Tokyo is slogging its way through another sweltering summer and is desperate to cool its streets. So now the capital has turned to the neglected custom of uchimizu - sprinkling water on the ground to lower air temperature - but with a high-tech twist.

This latest attempt to bring down summer temperatures that have been hovering in the 40s C involves pumping up the water that seeps into the metro system and spraying it from the kerbside onto the road surface. A water-retentive coating stops the water from draining away, and evaporation does the rest.

At the test site, directly outside Japan's parliament building in central Tokyo, a solar and wind-powered pump forces the subway flood water into high-pressure sprinklers that spray it over a 350m stretch of road. Recently, the researchers managed to cool the road surface - which often reaches 60 °C during the summer - by 10 °C, and the air above the road by 1 °C.

Japan longs to return to the cooler summers that were the norm decades ago. Outpacing global warming by a factor of four, average temperatures in Tokyo alone have risen 3 °C in the past 100 years.

The experiments were designed to reverse the cause of the country's urban woes: the heat island effect, in which temperatures in cities rise higher than those in the surrounding countryside. In Tokyo, the main culprit is the rapid loss of trees and other vegetation to development.

Other human activities driving the heat island effect include heavy use of vehicles and of air conditioning units, which set up a vicious cycle by churning out more heat as they cool buildings. To tackle this latter problem, the government has suggested a scheme called “Cool Biz”, encouraging the suited ranks of salarymen to abandon their jackets and ties and set air conditioning thermostats higher.

Tokyo also hopes to counter urban warming with heat-busting greenery on all new high-rises.