What is the point called when water can not rise above or visually seem to have a ceiling in the atmosphere? What makes that point ideal?
3 Answers
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Clouds have ceilings and bases because the atmosphere is stratified. Atmosphere is stratified due to earth's rotation and variance of composition, humidity, density, temperature, and pressure from surface of the earth to the edge of the orbital space.
Clouds form under specific conditions of composition, humidity, density, temperature, and pressure. These conditions are often persisted within specific layers in the atmosphere. Complex fluid flow of air is also a contributing factor for layering within the atmosphere between air masses for varying density.
The answer is probably best summed up as the result of the physics of fluid dynamics.
answered Jun 4, 2018 at 13:59
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$\begingroup$ Yeah that's more concise than what I was writing anyway. $\endgroup$– AshCommented Jun 4, 2018 at 15:08
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I know this question has an answer confirmed, but I'll add my own comments for posterity's sake. There are different types of clouds. Buoyancy-driven (cumulus-type) clouds are the types that have the most definitive bases and ceilings.
The cloud base is approximately the Lifted Condensation Level, which is the height that the air near the surface would have to rise to to saturate.
If the parcel of air can get to the Level of Free Convection, then the parcel will continue to rise and accelerate until the parcel encounters more stable air. It then starts to decelerate.
The level where the parcel stops moving is called the Equalibrium Level.
There are other types of clouds with other formation mechanisms. Cirrus clouds, for example, occur when the clouds in the above process go so high that they reach freezing temperatures and are moved into other regions. Stratus clouds form in the above process when the parcel never reaches step 2. Fog may form from either advection or the air diabatically cools to reach saturation. Kelvin-Helmholtz clouds form due to inertial instability. Contrails form from the hot moist exhaust mixing with the cool, relatively dry atmosphere.
answered Jul 3, 2020 at 18:38
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$\begingroup$ Explain that "buoyancy driven" thing, pls. Is there a packet of air forming a cloud and floating in mid air ? What about stationary clouds from an exited airflow, e.g. lenticularis over mountains ? Aren't they "sharper" defined, usually, than a puffy cumulus humilis ? $\endgroup$– user20217Commented Jul 3, 2020 at 18:43
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$\begingroup$ The packet of air is the cloud, so to say. It is just an arbitrary box of air. If it is less dense than the air around it, it will rise. This could be due to warming or moistening. As it rises, it cools (see Lapse Rate). When it cools to the dewpoint, it becomes a cloud. $\endgroup$ Commented Jul 3, 2020 at 21:13
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$\begingroup$ @a_donda There are a large variety of clouds. Lenticularis, for example, does not need to be buoyant, but still has an LCL. That would be a topographic. It just never continues to rise as long as the environment is stable. (ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/oth/org.rxml) Stratospheric clouds are another 'other' cloud which don't even have water in them. $\endgroup$ Commented Jul 3, 2020 at 21:24
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$\begingroup$ I am irritated by the "buoyancy" term, because I understood it as a static force, while the (simplified adiabatic, irl there's mixture) rise of a package of air is a dynamic thing, want to say the parameters (relative moisture, dry/wet gradient of the package as well as the sorrounding airmass) that control the rise change during the process. Btw, unrelated, we had a hefty inversion ~500m ASL, warm air above, a few fractus clouds with a lot hair underneath. Impressive sight, can post a photo if wanted. $\endgroup$– user20217Commented Jul 3, 2020 at 22:14
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In a sense, the full structure of a cumulus cloud extends all the way to the ground. The cloud is formed from the convection of air rising off the ground. The rising column becomes visible as a cloud when the atmospheric temperature becomes cold enough to condense water vapor, and the level where that occurs is the base we see. From Wikipedia:
Cumulus clouds form via atmospheric convection as air warmed by the surface begins to rise. As the air rises, the temperature drops (following the lapse rate), causing the relative humidity (RH) to rise. If convection reaches a certain level the RH reaches one hundred percent, and the "wet-adiabatic" phase begins. At this point a positive feedback ensues: since the RH is above 100%, water vapor condenses, releasing latent heat, warming the air and spurring further convection.
In this phase, water vapor condenses on various nuclei present in the air, forming the cumulus cloud. This creates the characteristic flat-bottomed puffy shape associated with cumulus clouds.[1][2] The height of the cloud (from its bottom to its top) depends on the temperature profile of the atmosphere and the presence of any inversions.[3]
Cited References:
1. "Cumulus clouds". Weather. 16 October 2005. Retrieved 16 October 2012.
2. Stommel, Harry (June 1947). "Entrainment of Air Into a Cumulus Cloud". Journal of Meteorology. 4 (3): 91–94.
3. Mossop, S. C.; Hallett, J. (November 1974). "Ice Crystal Concentration in Cumulus Clouds: Influence of the Drop Spectrum". Science Magazine. 186 (4164): 632–634.
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$\begingroup$ Convection can also be limited to layers in between inversions. $\endgroup$– user20217Commented Jul 3, 2020 at 16:39