Bob Maddox
Bob is a retired teacher living in the Alpujarra for most of the year with his wife
Belinda. His interests include painting, photography and of course writing. He researches
his articles meticulously and always manages to include his own brand of humour.
Whatever the subject they are ‘a good read’. He has been a keen supporter of the
Moor Times and has contributed many interesting pieces since its conception in September
2009.
Articles by Bob Maddox
Beauty and the Beast
On the afternoon of March 5th, 1966, a Boeing 707 of BOAC’s Flight 911 was approaching Mt Fuji, Japan. It was an unusually clear day and Captain Dobson decided to alter course to treat his passengers to a closer view of the Japanese icon. Minutes later, in sparkling clear air on the leeward (downwind) side of the mountain, flight 911 began to come apart in midair, breaking into three pieces and killing all 124 on board.
What Captain Dobson had unwittingly flown into was a series of Standing Waves in
the atmosphere on the leeward side of Mt Fuji. It was Dobson’s misfortune that the
air was unusually clear that day, for had it been otherwise, he might have been warned
of the presence of the beast by two of the most beautiful of cloud formations which
regularly visit us in the skies of La Alpujarra – Lenticular Clouds and Rotor Clouds.
When a wind blows against a mountain, the effect is a little like what happens when
water in a fast flowing river encounters a rock. Just downstream from the rock,
the water is forced up into a series of waves, which stand still in one place as
the water flows through them – hence the name, Standing Waves. You may recall from
our first article that air can be stable or unstable, according to its temperature,
humidity and the state of the air around it. Once lifted, unstable air becomes buoyant
as condensation releases heat and so it tends to continue on its upward way like
a hot-
Let’s think about what will happen if a stream of stable air blows southwards from
Granada. This air will find its way blocked by the Sierra Nevada and will be forced
upwards, cooling as it climbs. Once it clears the mountains, the air will find itself
cooler than the air around it and will begin to sink back to its original level.
As it flows down the leeward side towards the Alpujarra, it will begin to warm up
again, become buoyant and begin to slow down. However, its momentum will carry it
beyond its original level. Here, it finds itself warmer than its surroundings and
so starts to rise again, gathering speed until it passes its original altitude again,
before slowing and eventually starting to sink once more. In this way, the air is
forced into a series of waves as it rises and falls, gradually returning to its original
level many kilometers downwind of the mountain. Think of the water in the river.
Another way to visualize this is to think what happens to a weight suspended from a spring. Push the weight upwards, release it and down it goes, passing its original level before bouncing back up higher than its starting point again. So the weight bounces up and down in a series of smaller and smaller oscillations, until it finally returns to its starting point again. It’s the same with our stable air.
As the air rises to the crest of a standing wave, it cools and then warms again as
it descends into the next trough, before rising once more. These changes in temperature
create ideal conditions for a swing between the condensation and evaporation of any
water the air contains – and so for a swing between long peaks of cloud separated
by troughs of clear air downstream from the mountain. Because the wind has been
shaped into standing waves, these clouds remain perfectly still as the air flows
through them and they take on the shape of the cross-
Because of their shape, these are known as Lenticular Clouds and their haunting lens and saucer shapes are particularly beautiful when lit by the long, low red light of a sunset, when they are often the subject of mistaken UFO sightings!
But it wasn’t a lenticular that brought down flight 911, but a beast which often lurks just beneath it in the atmosphere; a Rotor – a vicious horizontal spiral wind generated by rapidly spinning air in the main body of the wave. A rotor can extend anywhere from ground level to the mountain top and can yield ferocious winds and turbulence. As air spins within the rotor, condensation can generate a cloud which picks out the shape of the monster – a Rotor Cloud. You’ll see these as long streamers of cumulus cloud, running parallel to the mountain range.
So, the next time you see a series of lenticulars hanging motionless in the lee of
Sierra Nevada, try to picture the sweeping up and down motion of the air as it moves
away from the mountain in a series of huge standing atmospheric waves, capped by
stationary clouds. Follow these clouds away from the mountain and you should see
them decrease in size as the air finally begins to settle to its original level and
the waves subside. And don’t forget -
It’s perhaps one of nature’s great ironies that the tranquil beauty of lenticular and rotor clouds are simply the outward signs of an atmosphere in turmoil and struggling to regain its composure. Never forget that the Alpujarra, set within its great mountain bowl, is a marvellous setting for the cloudwatcher. Feeling down? Look Up and feel your spirits rise!
© 2010 Bob Maddox
