On Wednesday the town of Darmstadt, where I live, was the hottest town in Europe. At a sweltering 39 Celsius in brilliant sunshine, even the sun-loving Germans found the heat oppressive. Moreover, it is a climatic feature of these parts that when the temperature is high, the air stagnates in the deep, wide valley of the Rhine, and evaporation from the river provides a plentiful supply of moisture. The result is a very high humidity which, combined with heat, makes the atmosphere almost unbearably debilitating.
"O for a beaker full of the warm South," yearned Keats in Ode to a Nightingale. At present any cup of such a kind flows over; one's wish is O for a beaker full of the cool sea breeze of the kind that blows in from the Blasket Sound in Kerry. But here, of course, just north of Heidelberg and south of Frankfurt, we are as far from the sea as it is possible to get in western Europe.
There are, however, other more insidious symptoms of the current heatwave. As one drives along the autobahn, every second overpass displays a banner with the ominous display Ozonealert. It tells us the torrid air contains more ozone than is healthy, and implied is the advice we should take the precautions that are recognised as sensible at such a time.
Normally there are only the tiniest quantities of this substance in the air we breathe since, as we know, some 97 per cent of the world's ozone occurs in the high atmosphere where it acts as a shield against harmful ultraviolet radiation from the sun.
But sometimes concentrations near the ground increase locally as a result of photochemical action by sunlight on the exhaust gases from motor vehicles and other pollutants. Conditions are particularly favourable when the temperature is very high and when light winds restrict the turbulent mixing of a stagnant airmass. Recently in the Rhine valley, all the necessary ingredients have been there.
This "tropospheric" ozone, as it is called, makes breathing difficult for those with respiratory problems, and may contribute to accelerated ageing of the lungs. It diminishes the yield of crops, and it corrodes a wide range of metals, rubbers and certain types of fabric.
Given the trappings of our modern civilisation, little can be done in the short term to avoid occasional instances like this when tropospheric ozone levels are high enough to cause concern. But the reason for the Ozonealert is so that susceptible members of the public can at least take precautions, by avoiding excessive physical activity, for example, or just by simply staying in the house.