Today I am in Lindau. Lindau is an island city on Lake Constance, about 10 miles east of Friedrichshaven from which Weather Eye has reached the world in recent days.
Lindau has a mediaeval aura with fragments of the ancient walls surviving, and a magnificent harbour looking southward towards the Alps in Switzerland. And the waters of Lake Constance, like those of any lake, may seem blue or black, or a variety of other colours in between, often depending on the weather.
To some extent the colour of any body of open water is determined by the sky. The water reflects the changing patterns of blue and white and grey above, and this alone can impart a wide variety of colour to it. Sometimes, too, reflections of mountains and greenery around the shoreline will impart a much darker colour to the water of a lake than might otherwise be the case.
But there are reasons for the frequent preponderance of blue. Water slowly absorbs any light that passes through it, absorbing the longer wavelengths of red and orange light more efficiently than the short blue wavelengths - in effect filtering the sunlight to leave it with a distinctly bluish tinge.
Moreover, as the sunlight passes down into the depths, the tiny particles of liquid water obstruct some of the surviving blue light-waves and "scatter" them back in the direction of the surface - there to emerge again to be seen by an observer.
So the sunlight entering the water is first filtered until it is blue, and then some of this blue light is scattered back in the direction of someone watching from above. And so the surface of a lake, just like the sea, often appears in various shades of blue.
But sometimes, too, the colour of a lake will be affected by impurities suspended in the liquid, and the extent to which they affect the different wavelengths making up the sunlight impinging on the surface of the water.
Very pure waters - and those of Lake Constance, by and large, enjoy this desirable distinction - often deliver a distinctly bluish tinge, in the way described above. Increasing proportions of iron salts or humic acids in a lake, however, result in scattered light that varies from light yellow to a darkish shade of brown.
And sometimes, if the water is rich in large particles of peat washed down by rivers flowing through surrounding bogs, the sunlight may be completely absorbed as it tries to penetrate the depths; no light remains to be scattered upwards and the lake takes on a sinister - or perhaps romantic - black appearance.