The reader will perhaps be sceptical that there could be enough water in the system to produce such a dramatic result. There always is! Here is a picture taken through a supposedly sealed double glass window pane in my house.

The low afternoon sun is swinging round from the left of the picture and shining on the glass to the right. The limit of the shadow cast by the brickwork is visible as a dark line up the centre. Water is being distilled from its absorption sites on the two inner glass surfaces that are being warmed by the sun, into the cold shadow, where it is confined to a steadily narrowing volume, from which it condenses as fine droplets, so fine that they give a smoky brown transmission colour. The total amount of water is tiny, just that which can be absorbed invisibly on the glass surfaces within the once sealed double glass pane when it is at a uniform temperature.

I present one more example, which shows that the phenomenon also has economic consequences. In well ordered countries, a menu with prices outside the restaurant is required for a licence to sell food.

The cunning chef knows that if he frames the menu behind glass with a metal frame, condensation will occur every evening at the edge of the glass, which cools first and condenses water released from the paper, which is fastened to the warm wall behind. The mouth watering descriptions of the food will be clearly visible while the prices will be obscured by condensation. The coffee and desert may well be entirely covered by fungi and bacteria, on the menu at least, but the customer is already hooked.



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