What keeps clouds afloat in the air

[Rothiemurchus]

Are clouds held in the air by archimedes principle or by rising heat or something else?

 

[mathman]

This may be oversimplifying, but water (mol.wt.=18) molecules are lighter than oxygen (32) or nitrogen (28) molecules.

 

[mgb_phys]

Except clouds aren't steam, they are small droplets of water/ice.
Strictly speaking clouds don't float ( they aren't boyant in air ), there are 2 mechanisms keeping them up;
1, Thermal motion of all the air molecules hitting the water droplets at random stops them settling out.
2, As droplets fall through the bottom of the cloud they hit warmer air, evaporate and as lighter than air water molecules float up through the cloud until they can condense back into water. This is what gives fluffy (can't spell cumulous ) clouds their sharp edges.

 

[Astronuc]

Strictly speaking clouds don't float ( they aren't boyant in air ),

Not quite. Clouds do float and the effect is buoyancy.

Thermal motion of all the air molecules hitting the water droplets at random stops them settling out

This is true.

As mathman said, moist air is lighter than dry air of the same molecular density. The cloud is very fine droplets dispersed in lighter air such that the mass of the cloud is suspended. And there are currents driven by variations in density and evaporation/condensation.

The most common ways to lift a parcel of air are: buoyancy, topographic lifting, and convergence. Buoyant lifting results from surface heating. This is a common manner of cloud formation in the summer. Buoyancy lifting is also called convection and occurs when local warm areas heat the air near the surface (fig 31a). The warm air is less dense than the surrounding air and rises. This rising air will eventually cool to its dew point and form a fair-weather cumulus cloud.

http://asd-www.larc.nasa.gov/edu_act/clouds.html

 

[lpfr]

Strictly speaking clouds don't float ( they aren't boyant in air ), there are 2 mechanisms keeping them up;

Not quite. Clouds do float and the effect is buoyancy.

You are both right but you are not talking the same language.
Clouds, as a whole, air plus droplets do float.
But the droplets or ice crystals in the cloud are not buoyant.

I just want to avoid confusion in inexperienced people.

 

[mgb_phys]

Good point - it's the usual physics point of view in the difference between an ideal simplified static system where water/ice droplets don't float in air, and the real world dynamic system where clouds as an object definitely do!

 

[BillJx]

Astronuc, that was a great explanation.
Let me try an analogy with dust. Dust can be kept aloft for awhile by brownian motion, but will eventually settle out in still air. A "cloud" of dust in a warm updraft could be carried upwards by the air around it, but we don't see stable clouds of dust because there's no mechanism connecting the dust with the lighter air.
In the case of clouds, the droplets of water maintain 100% relative humidity around themselves, so it's a stable system sort of like a lighter-than-air balloon without the balloon.

What I always find fascinating is the sharpness of the air interfaces - - the flat bottoms of clouds, the razor-straight frost line on a hillside, the horizontal layer of smoke in a bar etc. I don't really get why the layers are so sharply separated.

 

[lpfr]

This may be oversimplifying, but water (mol.wt.=18) molecules are lighter than oxygen (32) or nitrogen (28) molecules.

Right. But as moist air contains less than 1% of water vapor, the difference of density is less than .3% (a third of a percent). If there are droplets or ice, the global density of the cloud is higher (I have not an idea of how much).

 

[lpfr]

What I always find fascinating is the sharpness of the air interfaces - - the flat bottoms of clouds, the razor-straight frost line on a hillside, the horizontal layer of smoke in a bar etc. I don't really get why the layers are so sharply separated.

In a bars, it is due to air density. Hot air is lighter. And air expired from the lungs is hotter. But there must not be things to mix the air (fans, brawls, etc).
In the atmosphere and in the mountains it is due to the fact that air is colder upwards. This may seem in contradiction with the previous phrase. But you must take in account the "adiabatic gradient".
The flat bottoms of clouds are due to the same phenomenon. When the air lifts, it cools and, at some good height, it takes the good temperature for the moisture to condense and form droplets: a cloud. The height is the same for extended surfaces and all the nearby clouds form at the same height.

 

[pixel01]

The cloud is not formed in the ground air and goes up to its height. Clouds are formed right in that height. Because of the low temperature, water vapour condenses into droplets. The droplets of course are water so they tend downward. But when water vapour condenses, it gives off heat and this heat keeps the droplet+air mixture lighter or equal to around air. That is why clounds are floating.

 

[cesiumfrog]

the razor-straight frost line on a hillside

I've never observed that (though my area of Australia is likely less suitable), under what conditions (and where) do you frequently see it?

 

[Rothiemurchus]

-*Is it possible to make a cloud cool quickly so that it becomes one big block of ice and crashes to Earth? The opposite could happen - I could explode an atom bomb in a frozen lake and form a cloud! If the cloud to ice scenario is impossible is this because of entropy considerations?

 

[lpfr]

A cloud is a lot or air and just a little water (much less that 1%). It cannot become a big block of ice. And entropy is for nothing.
Ice to cloud is what happens in atom o nuclear explosions.

 

[Rothiemurchus]

There are water molecules and ice-crystals in clouds - why can't these come together to form a block of ice? It has never been seen to happen in nature but could it be made to happen by people? I suppose at the root of this question is this:how big can ice-crystals grow in a cloud and if there is a limit to their size, why is there a limit?

 

[lpfr]

You cannot make all the water or ice in a cloud come together to form a block.
Involuntarily, people can collect tens of kilograms of ice from clouds on the wings of planes until the plane crashes or until the pilot switch on the wing heating mechanism.
Ice crystals are limited to a few millimeters, but ice crystal aggregates, that is: "hail" can grow big, destructive and even fatal. Maybe the record book gives several tons, but I think that 1 kg hailstones are rare but possible.