I did a search and saw the “747 v cloud” argument. But me and my borther argue different things to pass the time at work together. We’re both well versed in physics and aerodynamics (he’s a pilot) but this wuestion has us going back and forth.
Neither of us is wondering about clearly convective clouds like cumulus or cumulonimbis. There is an energy input from the ground or differing adiabatic lapse rates in both of those cases.
We are thinking more of long lasting stratus and altostratus clouds where no obvious lifting force is present. Clouds are visible because the water vapor has condensed to the point of forming water droplets. What is keeping those pesky things airborne?
With cirrus clouds (ice crystals) you can often see that the crystals falling as they form and fall from higher altitudes, a formation known as Mare’s Tails.
Well, I wasn’t sure if this was going to come up but…
Water vapor isnn’t what you wee in clouds. Water vapor consists of water molcules suspended in air. Clouds consist of water droplets, that is to say, congolmerations of water vastly bigger than individual molecules.
Joey G correctlly points out that water vapor weighs less than than the average weight of air and as such would percolate upward. But the question and OP is directed towards clouds in general and stratus in particular.
You’re right, I shouldn’t have said they are made of vapor. They are water droplets, formed around condensation nuclei. I had this beat into my head through college and still called it the wrong thing. Oh well.
Thanks for the props on thast typo. I tried to get an immediate re-post with some witty remark but was blocked by the time llimit.
I read Sci Am every month. But the effect of gravity is not negligible. In fact the Earth very nicely pulls down on each water droplet in the cloud.
I am thinking of fog and how gravity pulls those condensed water particles right down to the ground. My guess is that the bigger the fog particle the more effectively the force of gravity overcomes the viscosity of the surrounding air. The question of the OP is precisely why doesn’t this same mechanism tend to keep non-convective clouds from staying aloft.
Lest anyone misunderstand me, I know that water vapor is less dense than air but that water droplets are denser than air.
Ha! Someone at SciAm isn’t thinking straight. If you have a gallon of water, and if you pour it into a bunch of cups, the WEIGHT DECREASES? Oh really? Sounds like a Monty Python routine. What floats on water? Rocks. (very SMALL rocks.)
Another wrong answer found on www: clouds stay up because individual droplets settle so slowly. (Sure, but the average density of the cloud-stuff is still high, so mist-filled air weighs the same as the water which forms the mist, and it should pour downwards like a heavy fluid. This actually occurs if you collect the mist from an ultrasonic humidifier. It acts like heavy CO2 gas, but without the CO2. It’s why fog hugs the ground.)
The reason that clouds stay up there is very well known: they’re hot.
If a blob of water vapor (invisible H2O gas) should rise upwards, the pressure drops, and because the pressure drops, the temperature drops, and at low enough temperatures the vapor starts condensing on any handy nuclei. But remember, it takes thermal energy to evaporate something and when that vapor condenses, you get the thermal energy back again.
Condensing droplets get hot.
So does the air between them.
Imagine a hot air balloon which is lifting a giant tank of hot water, and is weighted for neutral bouyancy (so it’s neither rising nor falling.) Now imagine that the water in the tank is sprayed as an extremely fine mist into the hot air within the balloon. Individually the water droplets settle only very slowly. But those droplets are surrounded by hot air. Now remove the balloon, and the blob of hot mist continues to float along, neither rising nor falling. You’ve just built a cloud.
Also note that the heating caused by the condensing droplets is the “engine” which drives thunderstorms and hurricanes. The updraft in the center of a storm is a sort of runaway feedback effect: it sucks in moist air which then warms and rises fast because droplets condense when decreasing pressure tries to lower the temperature. This sucks in more air at the bottom, and you get a violent “chimney” effect, like a fire which is “burning” moist air and leaving behind raindrops.
I think there’s an added effect which appears when raindrops descend out of the warm cloud which had been keeping the water aloft. Without that large extra mass weighing it down, the remaining warm air goes blasting upwards.
For extra credit calculate the density of air which contains 0.1% of H2O gas by volume, and compare it to the density of dry air (both at STP.) Does the moist air rise?
Let the H2O gas condense into liquid droplets which remain suspended by viscous drag. Now calculate the increased density of the droplet-filled air and compare it to the density of dry air at STP. Does the cloud fall downwards?
Next, calculate how much you must increase the temperature of the droplet-filled air in order to make it match the density of dry air (and you’ll know how warm a non-rising cloud should be.)
Finally, let the droplets fall as rain, then calculate the resulting density of the warmed air. How many kilojoules/km^3 of work could this hot air perform if it was allowed to rise by, say, 5km?
Classic trick pulled by educators who want their students to do the drudge work!