Tuesday, May 02, 2006

Cloud droplet phenomena


This image shows how light scattering by small cloud droplets produces multiple effects that are actually all part of the same phenomena. The scene was taken by Leigh Hilbert in Washington State in January ’06. The shadow of the descending aircraft is surrounded by a bright glory (1, 2,) centred just behind the wing where Leigh was seated. Much further from the aircraft shadow is a circular cloud bow (1,2), a form of fogbow (1, 2), produced also by scattering by cloud water droplets. The classical light paths producing it are those of the rainbow (1,2,3) but diffraction by the small droplets produces something much broader and almost lacking in colour. Inside the main cloudbow is a supernumerary arc that, characteristically for cloudbows and fogbows, has more colour saturation than the primary. The more distant clouds at the image top have produced a narrower cloudbow indicating that their droplets were larger.

7 comments:

Claudia Hinz said...

Good photo and jazzy explanation!

To our halo meeting in the last year we had a discussion about difference between fog bow and cloud bow, because in the mountains comes the fog from overlying clouds. What would you say?

Cordial greetings
Claudia

Les Cowley said...

Hello Claudia! The two names seem to refer more to _where_ the bow is seen. If the droplet sizes are similar then so is the appearance? Are cloud droplets much different?

Claudia Hinz said...

Thank for your answer. I mean this case like the picture from Carolin Baumann. This bow orginated in a Stratocumulus Cloud which hanged on the mountain peak. Of course ... on this side of the mount ... was fog ...

I've somewhere read, that the fog bow vertical and the cloud bow horizontal. But we've many observations where the bow is (spacially) slanting???

Les Cowley said...

Yes, the geometry of viewing must mean that most fogbows seen on a mountainside will be from a fairly low sun and their centres will therefore be not far below the horizon. Compared with that, most cloudbows from aircraft are seen under a high sun and looking down on a horizontal droplet layer. But of course, fogbows or cloudbows do not really exist in space - they are just a collection of rays entering your eye or camera lens. They look the same regardless of the tilt or otherwise of the droplet layer producing them.

Yuji Ayatsuka said...

In my impression,
if a screen (yes, it is a virtual screen) of the bow looks flat,
it is often called as a fogbow, otherwise a cloud bow.

Günther Können said...

I would call it a fog bow when the cloud/fog are so nearby that the drop distribution is more or less monodisperse - hence a good bow is visible. A cloud bow refers to the reverse situation, hence drops so remote that the variance in the drop size spectrum dominates. Then the bow is less distinct. Occasionally, in the latter situation (eg in altocumulus) the cloud bow becomes only apparent because of its polarization -- as I have noticed many times.

Günther Können said...

Further to my 6-May comment: it is indeed true that the distance from the observer is coupled to the size distribution of the bow-making drops. Because of turbulence and entrainment, there exists in fog/clouds a gradient in drop size from their center to their edge. If one is close to the bow-generating drops, that effect does not play a role and a well-formed fogbow may appear. However, if one is far away from the drops, a bow – e.g. in alto/strato cumulus etc – may extend over different individual cloud elements, which adds an extra variance in the drop size distribution. This latter contribution may even be the dominating factor in the variance of dropsize distribution if integrated over all clouds. Then, a bow that extends over several cloud elements, will suffer from that effect by showing irregularities in its shape, as one can frequently observe in cloudbows seen from high-flying planes, or cloudbows seen from the ground in altocumulus.

The effect of the decreasing variance in the fogbow-making drops when a cloud/fog deck approaches the observer is easily observed by a descending airline passenger or, (what I saw a few times at La Palma Observatory) when the cloud/fog approaches the observer on the rim of a volcano. The change in appearance of the cloud/fog bow, or better: the transition from cloudbow to fogbow, is clearly visible in such situations. When the clouds are far away, the fog/cloud bow is a bit irregularly formed, with usually no supernumeraries (and a poor glory). However, when the cloud comes near by all these features become distinct, as the glory/fogbow occupies only one cloud and ultimately only a small part of it. In such a (small) part of a cloud, the size distribution is much more monodisperse.