Günther Können

Cloud streets

When you know about them, you will notice them: cloud streets. They occur in coastal regions if the wind blows from sea. They consist of individual cumulus clouds, which are organized in parallel lines or streets. The distance between two streets is typical 1.5 km. On the Frisian island Terschelling, where I spend a few weeks any year, I usually spot such streets a few times during each stay. They occur there during south-western wind, that is when the wind direction is parallel with the island. Over Terschelling, about 4 km wide, there are usually three of these streets; between them the sky is clear. If you are unlucky enough, such a street keeps you many hours out of the sun. Over the sea there are no clouds at all.

If you recognize a clouds street, it worth while to search for its starting point, which is of course stream up. This starting point displays a beautiful dynamics: continuously a new ‘first cloud’ is formed, which is then transported land inward by the wind. These ‘first clouds’ seem to emerge out of nothing. Downstream one can observe that the street dies out where it happens to arrive over sea.

Photo: Cloud street, photographed on 30 June 2008 on the Dutch island Terschelling by G.P. Können

A clouds street is formed in an unstable and moist air mass from the sea, in which the air temperature is lower than the seawater temperature. When it has arrived over land, it may pass a spot that has heated up by solar radiation to a higher temperature than the air. This forces the air to ascend, during which cloud formation occurs. Next to a cloud street the air is descending and the sky is clear; next to that region the air ascends again and a second street is formed. From satellites the streets resemble smoke plumes which are blown land inward by the wind. Near the west coast of the Netherlands there may appear a large number of parallel streets. They are completely stationary, as their position drifts, together with that of the hottest spot, slowly north or southward along the coast. This does not happen on an island like Terschelling, where the elongated shape and small width of the island firmly pins the position of the streets down.

‘If you are ignorant of something, you will fail to see it’ – see the first line of this article.

The existence of cloud streets is only ‘recognized’ for about 40 years, and this only happened when they showed up so clearly in images taken from weather satellites that they could not be overlooked any more. Every time when I look at the cloud streets, I am surprised that we, human beings, can be so blind for the obvious. I have no doubt that much more clear effects are hidden in Nature, just waiting for a first keen observer to be discovered!

Geometric effects and illusions during sunset

Sunset from the beach is everlasting spectacular. Before the sun disappears, we perceive a huge sun disk: its size seems to be three times larger than normal. Indeed, ‘seems’, as the swollen sun is not real but ‘between our ears’: it is just an optical illusion. Don’t you believe? Take a picture then of the setting sun and a second one when the sun is high in the sky – and you will see that these suns are equally big*. This simple truth doesn’t need to spoil the magic of sunsets: we experience the sun the way we do, and that happens to be an enlarged version.

Left: sunset, photographed by the Englishman Les Cowley. The sun disk is flattened because the light rays from its bottom are stronger refracted by the atmosphere than the rays from its top. Right: same picture, rotated by a quarter of a turn. As the vertical‑stretching illusion acts now in the other direction, the flattening looks stronger than on the non-rotated picture.

Apart from enlarged, the setting sun is also flattened. This is indeed real, and is related with refraction by the atmosphere of the incoming sun rays. But also in this effect optical illusion is present: objects near the horizon become vertically stretched in our brains, and this subjective effect partly compensates the real flattening. So the flattening seems less than it actually is. But if we keep our head inclined, the real and the subjective effects act in the same direction and the sun looks cigar-shaped.

This can even be seen on pictures of the setting sun, by rotating the image by a quarter of a turn – see Fig. 1 Once aware, vertical stretching is apparent in many objects. Distant persons on the beach appear as thin lines; the same happens with people standing on a remote hill. Sheep on a ditch seem to have unnatural tall legs, if one watches them from below. But the best opportunity to see the effect is indeed during sunset on the beach. All the more reason to watch it regularly.

Finally another noticeable effect during sunset: seen from a line of dunes (~10-15 m high) the sun disappears about one minute later behind the horizon than from the beach. Sitting on the dunes one notices indeed that the sunset-watchers on the beach start to return home while from your position the sun has not completely set. *In 1995 I made a presentation about this kind of effects for a number of artists. They just could not believe that the swollen sun was ‘behind their ears’!  

Wide angle picture of the 2006 eclipse

WPS member Günther Können observed the eclipse near Side in Turkey, almost on the central path. He took some wide angle pictures during the totality with a 16 mm lens. The horizontal field of view is 135º. Venus is visible in the 4 o’clock position. Mercury, about half way between the Sun and Venus, was discernable with the naked eye, but is not visible in this picture.

Eclipse watchers sometimes wonder how dark the sky is during totality. Günther knows the answer. Primary scattered sunlight disappears at totality; what remains is light that leaks via the horizon from the region outside the lunar shadow, which is than scattered by the air molecules. So there is only secondary scattered light. Still, the sky remains surprisingly bright: its intensity is about 4000 times less than at day, which implies a limiting magnitude of about +3. Hence the sky is about as bright as during twilight with the sun about 6 degrees below the horizon, which is about 45 min after sunset. This is much brighter than the night sky during Full Moon. The solar corona is about as bright as a Full Moon, hence a million times less than the sun. So coronal light scattered by the air does not contribute to the brightness of the sky.

A scientific friend of his, Prof. Michael Vollmer from the University of Brandenburg in Germany, stayed a few kilometres from Günther’s location, and took measurements of the air temperature (in the shadow) and luminance during the eclipse. The graph shows that the temperature dropped by 5.5 ºC, from ca. 22 to 16.5 ºC. Subjectively, the drop looks much stronger, as our temperature sensation is mainly determined by direct radiation. Vollmer also measured the temperatures at 1 cm depth in the soil, which shows a drop of at least 10 degrees, from 30+ to less than 20 ºC.

The total luminance, as measured by Vollmer, includes the direct solar radiation. So decrease in total luminance is much more than the factor 4000 drop in brightness of the sky alone. Vollmer saw a decrease from 112.000 lux to about 5 lux at totality!