Q: Why do clouds hold their form?

Physicist: The short answer is: they don’t!

Clouds are just a bunch of moisture in the air (contain your shock).  What’s a little surprising is that the transparent, non-cloudy air around them typically has almost the same amount of moisture.  So when you see clouds in the sky, you’re not seeing a few wet blobs surrounded by dry air, you’re seeing a lot of humid air, some fraction of which has tipped past the dew point and begun to condense into visible vapor.

A patch of air can only hold so much water vapor.  Hotter or denser air can hold more, and colder or thinner air can hold less.  The “dew point” is where the air can no longer hold water vapor, which instead begins to condense out and become visible.

Evaporated water condenses when the humidity of the air it’s in increases too much, or when the temperature drops too much.  That’s why you can see your breath when it’s cold outside.  The air in your lungs is warm (because you’re a mammal) and it has plenty of opportunity to pick up moisture (because lungs are soggy and gross).  Mixing with cold air means that the temperature of your exhalation crashes and drops through the dew point, making it visible.  Mixing with lots of nearby drier air dilutes your breath, dropping the humidity in any particular parcel of air, and raises the breath/outside-air mixture back above the dew point.

Your breath is warm, so in your (humid) lungs it’s above the dew point. When it cools it drops below the dew point and becomes visible.  Then as it mixes with drier air, it pops back above the dew point and disappears.

Clouds are governed by the same physics.  If you spend some time staring at clouds (and why wouldn’t you?), you’ll find that they continuously grow and shrink, appearing and disappearing.  A given cloud maintains its rough shape and position because it takes time for conditions in air to change; if the air in some region is on one side of the dew point, then it’ll probably stay there for the next few minutes.  But if you speed up time, you find that clouds don’t hold their shape any better than steam coming out of a kettle.

The big difference between clouds-in-the-sky and breath-on-a-cold-day is what causes the air to pass through the dew point.  The dew point depends on both the humidity and temperature.  For breath, the local humidity fluctuates a lot (as anyone with close-talking friends can tell you).  For clouds, the humidity stays relatively constant, but as the air changes temperature (mostly through the expansion or contraction from changing altitude) different regions pass through the dew point and become clouds.  If you’re in the middle of the ocean or Kansas (or any other flat, featureless landscape), there’s no particular reason for any given location in the sky to have a cloud.  It’s just the luck of the draw (humidly speaking).

But when the conditions change predictably, the clouds appear and disappear predictably and you get “lenticular clouds”.  Here are some beautiful examples.  Lenticular clouds are a great way to see that clouds aren’t “objects” that move through the sky, they’re regions that, for whatever reason, are below the dew point.

Normally air moves across the land or sea in a “laminar”, smooth way, with nary a tumble or nor turbulence.  Across grasslands clouds tend to vary slowly and randomly, but when there’s an obstruction, like mountains, suddenly the air is forced to flow in a particular pattern.  If that pattern involves abrupt changes of altitude, then the air experiences abrupt changes in pressure and temperature which leads to abrupt changes in cloudness.  The same amount of moisture is present in the air at the foot of the mountain as there is at the top (give or take), but the top of the mountain is where you’ll see clouds.  In fact, if the conditions are just right, this is a clever/cheap way to get some insight into what the (normally) invisible air currents are.

So clouds roughly hold their shape (for a little while) because it takes time for the humidity or temperature to change, or for the cloud to be twisted up by local air currents. But since they’re not “blobs-of-water” so much as “patches-of-air-with-slightly-different-conditions” they change continuously and are free to pop in and out as they cross the dew point.

As for how they manage to be pretty and invariably end up looking like something familiar (cotton, marshmallow men, other clouds, negative Rorschach blots, etc.), that’s more psychological than climatological.

The clouds passing by overhead video is from here.

The lenticular cloud video is from here.