Weather wise

LAST SUMMER the smog got so bad in Paris that the city authorities had to ban half the city's cars on alternate days. This year, Athens begins an experiment in staggering the hours at which schools, banks and shops open, in an attempt to reduce rush-hour smog peaks. And last week in Mexico City the pollution was so bad that 1.7 million cars were banned on Saturday.

In each of these places, very special factors combine to push levels of air pollution far above that expected for a city of comparable size with comparable numbers of vehicles. In Paris, the air quality has been dealt a hammer blow by the low taxes imposed on diesel fuel - a policy designed to favour French car manufacturers, which have invested heavily in diesel technology.

In the Greek capital, a city ringed by smog-trapping mountains and with a humid and hot summer climate, tens of thousands of old vehicles are a legacy of the Greek government's policy of taxing cleaner new cars at sky-high rates.

In Mexico City, like the one-time smog capital of the world, Los Angeles, the geography is again largely to blame - a mountainous barrier to dispersion of pollution is exacerbated by a warm, dry and sunny climate.

Smog is usually far less of a problem in British cities. The dominant blustery winds from the west usually blow the polluting gases away before the sun can work its photochemical mischief on the sulphur and nitrogen compounds emitted by exhausts.

But in very hot summers air pollution can hit danger levels in central London, and the culprit is largely the capital's diesel-powered buses, taxis and trucks. It isn't until these vehicles are driven off the road and replaced by cleaner alternatives such as LPG that we can hope to see real improvement in air quality.