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I snapped this photo with my phone flat against the window of a bus, facing south while traveling west across a bridge in New Taipei City. I'd say it was at about 24.995N, 121.438E. It was about 14:00 local time, 28-05-2017, a cool day, roughly 20C outdoors but I don't know the temperature in the mountains. The overall bluish/greenish hue is the result of the tinting of the bus' window.

Is there a name for this "pouring clouds" effect?

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  • $\begingroup$ My only (rather simplistic) answer is... fog! Hear me out... fog is typically dense due to it being colder... so it can potentially pour downwards from higher sources (not all too different from CO2 sinking in cool videos like this). Can't think of any special name it'd have, just a cool effect :-) $\endgroup$ – JeopardyTempest May 28 '17 at 8:01
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    $\begingroup$ Combining both answers to date: hko.gov.hk/education/article_e.htm?title=ele_00484 . $\endgroup$ – David Hammen May 28 '17 at 16:55
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    $\begingroup$ @DavidHammen: if you were to write you comment as an answer I'd up vote it. $\endgroup$ – Fred May 28 '17 at 17:39
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    $\begingroup$ You see these very frequently in the San Francisco peninsula when there's dense fog on the coast that pours over the coastal hills. $\endgroup$ – BillDOe May 30 '17 at 19:46
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    $\begingroup$ @uhoh these appear to be banner clouds - www-das.uwyo.edu/~geerts/cwx/notes/chap08/banner.html $\endgroup$ – gansub Apr 9 '18 at 3:36
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The term used by the American Meteorological Society is orographic fog. A perhaps more descriptive name is "orographic waterfall cloud".

Prevailing winds can force air to flow upwards over rising terrain. The rising air cools more or less adiabatically, and this can in turn result in clouds. The nature of the orographic clouds (orographic: "related to mountains") that result vary greatly.

In arid regions, orographic clouds can form lenticular clouds that look amazingly like UFOs (three links). Unstable conditions can result in wave clouds, where an obstruction causes a cloud after cloud after cloud downwind of the obstruction. Even icebergs can cause wave clouds. Another form of orographic cloud is a banner cloud, where a cloud flies like a banner downwind of a mountain:

Banner cloud flying behind the Matterhorn
Source: https://commons.wikimedia.org/wiki/File:Banner_clouds.jpg . Copyright Zacharie Grossen, license CC BY-SA 3.0

This is getting closer to an orographic fog. Climbers on the Matterhorn oftentimes experience that cloud, firsthand. Even closer to home is Table Mountain near Cape Town, South Africa.

Table Mountain, with a table cloth cloud descending from it
Source: https://commons.wikimedia.org/wiki/File:Cape_Town,_Table_Mountain,_Table_Cloth.jpg. Copyright KodachromeFan, license CC BY-SA 3.0

This is an orographic fog. (Table Mountain can also create some rather spectacular lenticular clouds.) Another example of an orographic fog is detailed in the article "Waterfall-like Orographic Clouds".

The conditions needed to form an orographic fog include

  • Prevailing winds that force air to blow upwards over an obstruction, with the orographic fog flowing downwind from the obstruction.
  • The flowing air must be sufficiently humid. Without this condition, you might see a very cool looking lenticular cloud, but you won't see a banner cloud or an orographic fog.
  • The flowing air must be sufficiently cool. Humid air at the same pressure and temperature of arid air is less dense than is that arid air. The flowing humid air has to be fairly cool for the flow to be downward downwind of the obstruction.
  • The flow has to be fairly slow (low Reynolds number) and uniform (low Froude number). Otherwise, a wave cloud or banner cloud will result.
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    $\begingroup$ Note to potential editors: I linked to multiple images via hypertext links rather than embedded images. The linked images are copyrighted; please do not insert those images without permission of the copyright holder. $\endgroup$ – David Hammen May 29 '17 at 4:15
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    $\begingroup$ Note also: I could have gotten Reynolds number and Froude number reversed. Fluid mechanics! I wouldn't be surprised if Weber's number also comes into play. $\endgroup$ – David Hammen May 29 '17 at 4:27
  • $\begingroup$ OK they have both a descriptive name and some interesting science behind them, thanks! I may try to figure out where those clouds are forming and see if I can get up close some time. $\endgroup$ – uhoh May 29 '17 at 4:58
  • $\begingroup$ I certainly learned something, great answer :) $\endgroup$ – JeopardyTempest Jan 10 at 5:05
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Effect like this may be caused by temperature inversion. When warm air/cloud runs into barrier like a hill or mountain it raises (convection) and temperature drops (adiabatic change). Then you see an effect like this "pouring cloud". When it comes down it may cause smog when city is polluted.

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  • $\begingroup$ Thanks. So you think the wind was bringing very moist air from behind the mountains - roughly from the south? $\endgroup$ – uhoh May 28 '17 at 12:37
  • $\begingroup$ Yes, humidity will probably rise slightly. $\endgroup$ – aglango May 28 '17 at 13:01
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The term you are looking for is orographic clouds. specifically the föhn wall type created by föhn winds.

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    $\begingroup$ I'm not sure about this. The stuff I've photographed just pours down the side of the mountains, almost stuck to them. It doesn't really look like a cloud as much as really think, heavy fog, as suggested in comments above. Of course there may not be a clear distinction between those two in all cases, I don't know really. High fog vs low clouds... The graphic in your link shows an altitude cut-off (red dashed line) i.stack.imgur.com/fV4GI.jpg $\endgroup$ – uhoh May 28 '17 at 17:14
  • $\begingroup$ Then again @DavidHammen 's link above shows a photo that looks almost identical, in a similar climate also near sea level, so perhaps this is the right term after all! $\endgroup$ – uhoh May 28 '17 at 17:15
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    $\begingroup$ @uhoh its not an altitude mark as much as it is change in height, its not a set height be varies with temp and pressure. The shape of the cloud varies a lot based on the local wind patterns. In your photo notice how it pours over then disappears as it drops below the condensation level. $\endgroup$ – John May 28 '17 at 18:23

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