Cloudscapes Part 2
Beauty and the Beast
by Bob Maddox
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-air balloon, often forming towering thunderclouds. Stable air, on the other hand tends to find its way back to its original altitude.
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-section of the crest of the wave – a lens shape. As the air warms again in the following trough, the cloud evaporates, leaving the air clear, until it climbs to the crest of the next wave and forms a cloud…and so on.
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 - those long streamers floating underneath are anything but as gentle and innocent as they appear.
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 marvelous setting for the cloudwatcher. Feeling down? Look Up and feel your spirits rise!
© Bob Maddox 2010
Beauty and the Beast
by Bob Maddox
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 marvelous setting for the cloudwatcher. Feeling down? Look Up and feel your spirits rise!
© Bob Maddox 2010
Lenticular clouds lit by the dying rays of the sun resemble UFO's as they float in the Alpujarran sky. Click to enlarge.
Cumulus cloud forming in unstable air. Contrast the shape with the stable lenticular clouds and standing waves caused by stable air. Click to enlarge.
Rotor clouds forming over Sierra de Gador. Clouds like these are often a sign of the deadly spiral winds which destroyed Flight 911.
Click to enlarge.
That banana under the almond tree again. Enlarge at your own risk!