The sunsets were purple and red and yellow and on fire, and the clouds would catch the colors everywhere

August 30, 2005 4:44 PM

What makes clouds form where they form, and how?

I don't fly often. Flying up to and down from cruising altitude a few times over the last week, I noticed that clouds seemed to form in certain shapes at certain layers. Those layers seemed well defined at specific altitudes — like a blanket of clouds at, say, 12000 ft, and another layer at 24000 ft. What causes certain shapes at certain altitudes, and "clustering" at specific elevations? Or are my observations completely off the mark?

posted by Rothko to Science & Nature (6 answers total)

The venerable AskJeeves provided this link when I tried your question.

Apparently, Earth's topography can account for the cloud elevation in certain areas. Also, (according to the article linked) a phenomenon known as Forced lifting is caused by air flowing from different directions comes together and is forced to rise.

Finally (and verbatim), Even the earth's surface itself can cause cloud to form! As air moves over the surface of the earth, friction can cause turbulence, result in the upwards motion of air.
posted by purephase at 7:50 PM on August 30, 2005

I found lots of good info by sifting through the results of this Google search.

(Not intended as snarky. I didn't know the answer, so I did a search, and this is what I found. The links on that page can explain it way better than I could.)
posted by Dr. Wu at 7:53 PM on August 30, 2005

Response by poster: I'm not asking how clouds form generally, which I can find with Google just fine. I'm asking how clouds form where they form, and in the shapes they form.
posted by Rothko at 8:24 PM on August 30, 2005

Perhaps adjusting the search like so will find the answers you're looking for?
posted by Dr. Wu at 8:47 PM on August 30, 2005

I haven't got the time to give a more detailed answer, but basically it boils down to the local pressure and temparature as a function of height. Patches of moist, warm air rise from the earths surface. As you gain height, the mean temparature of the surrounding air decreases nearly linearly. At a certain elevation, water vapour starts to condensate and droplets (=clouds) form. This evaporation induces a cooling of the air, so that the rising velocity is reduced. Note that there are also 'inversions' in the temparature profile (regions where temparature rises with elevation), effectively being some sort of ceiling in the atmosphere.

If you want detailed reference, check this out.
posted by swordfishtrombones at 9:15 PM on August 30, 2005

Swordfishtrombones is correct. Clouds form for the same reason that your open can of Coke gets water condensation. The only difference with clouds is that the air is colder because of the higher elevation instead of the fact that it happens to be right next to the can you just took out of the fridge.

If you're really a weather geek you can find out what the dewpoint is and estimate the height of the cloudbase. You just figure out how much the air has to cool before dew will form and plan on the temperature dropping about 4.4 degrees per 1000 feet (the standard 'lapse rate'). If the current temp is only 9 degrees away from the dewpoint, for example, you can estimate that the clouds you see are forming at around 2000 feet.
posted by richmondparker at 6:14 AM on September 2, 2005