The Rhine Valley is awash with sunshine. As you, poor reader, prepare to dodge the showery torrents promised for the holiday weekend, I drowse in the summer sunshine, with here and there a butterfly, a bee, a flower or two of brilliant red, and overhead a deep blue, cloudless sky. As I write in a shady corner of my German garden, I quaff a long, cool glass of beer, mine to admire and contemplate before each icy cold, delicious sip.
My glass, as happens in these circumstances, is coated on the outside with a myriad of tiny drops of water, a phenomenon that we know as condensation. It forms because the beer has come from my refrigerator, and is therefore much colder than the air surrounding it.
And air, as we also know, always contains some moisture in the invisible form of water vapour and, if its temperature is gradually reduced, sooner or later the air reaches what we call the dew-point, the temperature at which the water vapour begins to condense into tiny, visible drops of liquid water.
The air temperature as I write is 25 degrees or thereabouts, and the dew point is probably somewhere in the region of 13. My recently refrigerated beer is at, say, five degrees, and the air in immediate contact with the cold glass has therefore been cooled below its dew point; its excess moisture has condensed on to the outside surface of the glass. My beer, and of course the glass in which it is contained, will stay cold for some considerable time. It has, we say, a "high specific heat", which, paradoxically, means that it takes a lot of sun to bring about a noticeable rise in temperature.
As time goes by, however - unless I drink it very quickly - the beer will become tepid and its temperature will exceed the 13-degree dew point of the surrounding air. No further condensation will then take place; not only that, the air around the glass, no longer chilled by contact with a cold surface, will have spare capacity for moisture and will reabsorb any residual droplets by evaporation. The glass will become clear again.
Long after I have finished lazing in the garden and retired indoors, the sun will set. Deprived of the incoming solar radiation, the surface of the Rhine Valley will cool and in due course its temperature - like that of my beer-glass as it is at present - will fall below the dew point of the air above it. The same phenomenon will occur; little droplets of water will condense on to the cold surface underneath. This time, it will be dew. .