Sunday, 11 August 2013

both sides now

Bows and flows of angel hair
And ice cream castles in the air
And feather canyons everywhere
I’ve looked at clouds that way.

I’ve looked at clouds from both sides now
From up and down, and still somehow
It’s cloud illusions I recall
I really don’t know clouds at all.

Joni Mitchell, Both Sides Now.
When it comes to looking at clouds, I suspect that most people will follow Joni Mitchell in imagining all manner of fanciful resemblances, but clouds are more interesting as an indicator of atmospheric processes. In this regard, the first point to bear in mind is that with occasional local exceptions, clouds form in unstable atmospheric conditions.

This may seem like a statement of the screamingly obvious, except that in meteorology, stability has a very specific meaning. It describes the behaviour of discrete air ‘parcels’ as they rise or sink through the surrounding air. A stable parcel will tend to return to its original position once the force that moved it in the first place has been removed, while an unstable parcel will remain where it has been pushed.

I had intended to provide a more detailed explanation of these processes, but when I consulted my copy of Atmosphere, Weather and Climate by Roger Barry and Richard Chorley, a standard university textbook on the subject, I quickly realized that a simple explanation was beyond my powers. I understand the science—I ought to, given that I edited the book—but it is impossible to provide any kind of explanation that doesn’t invoke adiabatic lapse rates, partial vapour pressures, supersaturation and latent heat of condensation, the mere mention of which I suspect will cause most readers’ eyes to glaze over.

Given that the reason for this post is to showcase a series of photographs of clouds as aesthetic compositions, no further explanation will therefore be provided, but I will include the following astounding snippet of information: a mere 4 percent, on average, of all the water vapour in the atmosphere is to be found in clouds.

Clouds are classified on the basis of shape and altitude. Although stratiform clouds are most often seen along warm fronts, this photo shows a rare type of stratiform cloud, the lenticular or wave cloud. These clouds form on the lee side of mountains, which set up a kind of undulating air flow by forcing horizontally moving air masses to rise. Whenever the movement of such an air mass is turbulent (the usual condition), no clouds will be formed on the lee side of the upland area, but when the flow is laminar, then the peak of each undulation will be marked by a thin lens of cloud with limited vertical extent.

This photo, taken looking west from the roof of my house in Fanling, illustrates laminar flow rather well. Everything may look chaotic, but there is no obvious turbulence, so the airflow can be seen clearly.

This photo is of the same area of sky three days later. It shows cirrocumulus (high-level) clouds being elongated horizontally by exceptionally strong winds, with a few smutches of altocumulus (mid-level) in the foreground.

If I had to come up with a word to describe this picture, it would be ‘biblical’. It reminds me of the type of sky painted by pre-modern artists when depicting dramatic moments from the Old Testament, such as the adoration of the golden calf and the subsequent destruction of the Ten Commandments by Moses, or Shadrach, Meshach and Abednego being cast into, and subsequently rescued from, the fiery furnace.

Finally, in a nod to the old Cumbrian saying about red sky at night being a shepherd’s delight, I include a picture of a large cumulus cloud being illuminated by a rapidly westering sun, although I do have to admit that there are no shepherds in Hong Kong, where this photo was taken.


  1. The first picture looks so much like the sandy desert if you can imagine it in another colour, can't believe it's actual clouds.
    It's amazing how the clouds can look so different over 3 days in the same space too. It does look as if they clouds have been stretched.
    Biblical is a good word for the next photo. Scary isn't it? In London we are unfortunate enough to see darkish clouds regular, at least 10 months of the year :(
    The final picture. There have been times when our skies have gone a deep red whilst the sun is going down. It just looks so wonderful.

    I must admit that I'm so use to seeing certain patterns and slow movement in the sky, if I see anything that looks too different when abroad it does make me think 'what's going on'. There was a time when I felt that the clouds were moving way to fast for my liking but wherever I was in the world at the time (can't remember) they said it was normal.

    Thanks for explaining about the clouds. Very interesting Dennis.

    1. Clouds can move quickly in this country too Rum. I was outside my local yesterday, and I noticed a big rain cloud approaching from the southwest. The cloud itself wasn’t moving that quickly, but it must have been extremely turbulent inside the cloud, because it was changing shape even as I watched it.

      …And, sure enough, it started raining a few minutes later.

  2. Dennis, I love looking at clouds AND I love that song! Used to enjoy singing it now and then.

    1. An interesting song Jean, although I have to admit that it isn’t really to my taste.


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