Windswept clouds over Boulder

By Phil Plait | February 5, 2010 10:00 am

I love clouds, and Boulder is a never-ending and always-changing nebular cloudscape of them.

Last Saturday I saw this out my home office window:

It was gorgeous! It’s a lenticular (lens-shaped) orographic cloud; a cloud caused by moisture-laden air rising up and cooling as it passes over mountains. We see them here all the time just east of the Rockies, and when they get all lenticular it’s a very cool bonus.

Orographic clouds aren’t limited to the Earth you know; other planets have atmospheres with some moisture and tall mountains to overcome as well.

Some people think that science takes away the romance of nature. Those people are wrong. When I lie out in the Sun and muse about the pretty clouds over my home town, I can know that what I’m seeing happens on other planets spinning around the Sun, and I’ll just bet it’s happening somewhere on a planet orbiting some other distant sun, lost among the billions in our galaxy.

What could possibly be more romantic than that?


Comments (23)

  1. Chris

    I saw a lenticular cloud coming off Mount Greylock (MA, USA) a couple of months ago. They’re not common here and I had to pull over the car so i could just stare at it for a while.

  2. David W.

    I have seen them a few times here in the mountains around Los Angeles, going toward Las Vegas. I didn’t know what they were called, until I read about them on APOD (Astronomy Picture Of the Day). When I first saw some, I thought that’s what they were when they only seemed to form over the nearby mountains.

  3. SteveN

    I think that’s spot on. Science does not take the romance out of nature–it only increases the sense of awe and wonder one feels. The idea that the only way we can appreciate nature is by being ignorant of it is just silly. I’ve never understood how some people think understanding why a beautiful sunset involves red light scattering rather than blue light scattering would somehow take the joy out of seeing the processes play out.

  4. Are you familiar with the book “The Cloudspotter’s Guide”, a publication by The Cloud Appreciation Society? I highly recommend it.

  5. Mark Sletten

    While beautiful, and certainly thought-provoking (if you’re Phil Plait), lenticular clouds are harbingers of dread for aviators.

    Phil is right that these clouds form as a result of moist air lifted over an obstacle. The term ‘orographic’ means moving air is following the terrain. As air goes upward, the atmospheric pressure drops off resulting in adiabatic cooling and subsequent condensation, which forms the cloud.

    As a current of air passes over whatever obstacle is forcing it upwards, it will attempt to follow the terrain downward on the lee side. Lenticular clouds, unlike most clouds, do not move with the air current. That’s because the adiabatic process that forms the cloud also works in reverse. As the air current follows the terrain down the lee side of the obstacle, increasing atmospheric pressure results in adiabatic warming which evaporates the condensate. The cloud appears stationary, forming on the windward side and dissolving on the leeward, as the air moves up and down over the terrain.

    So why do lenticular clouds frighten aviators? Because they indicate strong air movement over terrain. (Lenticular clouds can sometimes form away from terrain. Air currents can be deflected upward by strong, widespread thermal updrafts, but the scale of thermal updraft activity required makes such occurrences very rare indeed.) The kind of air movement required to create lenticular clouds is almost always associated with a meteorological phenomenon called a ‘lee wave,’ or ‘mountain wave.’

    Mountain waves frequently result in strong — sometimes extreme — turbulence. If the air flowing over the terrain is moving fast enough, it may not be capable of smoothly flowing down the lee side of an obstacle. In that situation, the air flows over the top of the obstacle and begins to flow downward on the lee side, but not as fast as the terrain. If the space between the air current and the terrain grows large enough, the air current may curl down and back under itself — think of an ocean wave approaching the shore which breaks and curls.

    With enough distance between the lee side of one obstacle and the windward side of the next, a rotor can form. A rotor is a dense, horizontally rolling air current — like a sideways tornado. These are obviously air navigation hazards, not to mention an incredibly scary ride for those unlucky enough to fly through one. But the scariest thing about them is they are invisible; a pilot cannot maneuver the aircraft to avoid what he/she cannot see.

    Luckily, as we gain more understanding of meteorology, forecasters have gotten better at recognizing the conditions conducive to the formation of rotors. But even without an accurate forecast, a lenticular cloud is a strong indication of mountain wave activity.

    So, the next time you’re flying to or from an airport with surrounding mountainous terrain, look for those lenticular clouds. If you see them, you can be all but certain your flight is going to be exciting!

  6. Evil Timmy

    We get some spectacular lenticular clouds over Mt Rainier, near Seattle, WA. A few years back there was one that made the front page of the newspaper, as it was thousands of feet tall and nearly a mirror image of Rainier.

    Here’s a few examples:

    And a whole lenticular gallery:

  7. Richard

    Wait. You “love” clouds! Clouds are the mortal enemy of the astronomer. Next thing you know, you’ll say you love astrology! Blah! :-)

  8. Raine

    Thats what there called… I see that all the time in Lakewood, southeastish of Boulder…

  9. Mark, great explanation of what is going on. To add, the wave also can “bounce” downwind so that sometimes there is more than one lens shaped cloud downwind of the first main one. And even if there aren’t more clouds downwind, the bumps and upward moving air probably are. Glider pilots really like poking around (carefully) the waves. I know a couple of guys who found wave over Mt Diablo near San Francisco and ended up flying around the bay area up to 18,000 ft. The air traffic controllers that day had fun talking to someone new.

  10. DaveS

    Yes, very romantic. Let’s hope you do a bit better than that a week from Sunday.

  11. That’s got nothing on this.

    Also, I think if I’m ever in Boulder, I should keep an eye out for that fence, which appears to be a popular incidental subject for you. ūüėõ

  12. Paul

    You should give Mile High Glider club a call when you see “Lennies” in the sky and maybe they can take you on a wave flight. Here in Boulder wave flights typically get into the 20K ft range and sometimes into the 30s. A great way to get a different perspective on clouds.

  13. DrFlimmer

    No snow in Boulder? That seems unexpected to me. Do you want to have some from our snow here in Germany? Although much is melting right now, the temperatures will fall again on Sunday below 0¬įC, so there is plenty of snow left for you!
    Maybe Vancouver wants some?

    Btw: Nice pic!

  14. Liam

    Damn that evil GOVERMENT!! This is just THE EVOLUTION (or would be if such a thing existed) of CHEMTRAIL tech!!! A chem-BLANKET of MIND controlling toxins! We’re all doooooooomed!!!!!!!!!!!!!!!!!!!

  15. Chip

    Phil writes: “I love clouds, and Boulder is a never-ending and always-changing nebular cloudscape of them…”

    Then you probably like the mysterious and beautiful piece titled “Nuages” by Debussy. It is the first of Debussy’s “Three Nocturnes” for orchestra. In fact, the whole suite is perfect music for star gazing as well.

  16. Dionigi

    Anyone can see it is a flying saucer with ion drive exhaust around it. Why does everyone have to go for the non obvious?

  17. JB of Brisbane

    @Liam – No, it’s the latest HAARP experiment, along with the rainbows in spray from sprinklers. :-)

  18. Ron 1

    5. Mark Sletten Says: “If you see them, you can be all but certain your flight is going to be exciting!”

    While there is risk associated with mountain waves (ie turbulence) I’d hate to underestimate their potential for fun. In the right aircraft and with appropriate skill, ‘surfing the wave’ is a real hoot.

    While working in an Air Traffic unit in Whitehorse Yukon, I remember days when I’d receive PIREPS (PIlot weather REPortS) from pilots who’d reported that they’d taken a real beating while flying to the lee of Mt Logan or other coastal mountains. While these reports indicated a hazard (usually indicated by lee wave clouds), to some pilots these reports were simply incentive to go play. By play, I mean they’d take a high performance twin (ie. King Air or Cheyenne III) and file a flight plan for a ‘training’ flight into the affected air and they’d spend their ‘training’ time surfing.

    As well, as we frequently see along the Alberta Canadian Rockies, lenticular clouds indicate a potential ‘subsidence break’ or Chinook Arch along the mountains. What this means is that while low cloud or fog might blanket the prairies, there could be a narrow band of clear skies immediately to the lee of the mountains through the area of bad weather – and therefore a path for VFR flight. This clear sky area is a result of airmass subsidence – ie. descending air. As a saturated airmass (100 percent relative humidity) descends it is compressed and heated and once heated the airmass is no longer saturated (RH less than 100 percent) and the cloud dissipates and the sky clears.

    Point being … Those lenticular clouds mean more to aviators than just hazard. While being pretty, they are also an open book to what the atmosphere is doing at that particular location and when properly understood they can offer opportunity as well as hazard.

  19. Crux Australis

    Aren’t all cloudscapes nebular?

  20. Rod

    I concur about science and the “romance of nature.” Throughout my life, science has only enhanced the romance, the wonder, the nights of dreaming under star-studded skies….

  21. Dequack

    Well said Ron1. There is more…

    There is one group who has found a use for lenticular clouds-“lennies”-glider pilots. Flying in a good sailplane with a skilled pilot on the front side of these clouds is how altitude and duration records are set, never mind the glory of flying in strong lift. Avoiding the rotor is part of the game.
    I recommend the experience if you are serious about feeling the awe and wonder the universe provides. My most perfect flights have been in southern Alberta in the lee wave east of the Rockies. Pilots travel from all over the world to fly there, deliberately!

    Lennies can be a pilots friend- like surfing the atmosphere.

  22. Shane Castle

    Dunno how that’ll come through. I’ve got some nicer ones on negatives somewhere…




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