I know that certain fluids don’t mix well, and that there are various reasons for this.
For example, oil and water don’t mix. I won’t pretend to understand why, but I’ll bet the terms “cohesion”, “viscosity” and “surface tension” are probably involved. That’s okay, oil and water don’t bother me.
What bothers me is when fluids which are very similar resist mixing. For example, we had a thread a few weeks ago called “Warm Spots” while swimming in the ocean? which discussed various pockets of warm water at the beach which ought to get mixed in with the other water much more quickly than they do. For some reason, these pockets seem to last quite a while.
What I’m really curious about is the atmosphere. I imagine that there are different layers of the atmosphere, and logically, the lower ones would be denser (heavier) than the higher ones. It seems that a cumulus cloud (laden with water vapor, it would be relatively heavy, no?) sits at the very bottom of one layer, right on top of a layer which is even more dense that the cloud.
I’ve always been fascinated be the bottom of a cumulus cloud. How is it so flat and smooth? Why doesn’t the cloud above mix with the air below? The wind is not still, why isn’t the bottom of the cloud churning a bit? What keeps the cloud together? Does a vapor have surface tension?
Lemme phrase it this way: I understand that a denser fluid will sink and fall below the less dense one. For example, the carbon dioxide from a piece or dry ice, or from an open soda bottle, will collect at the bottom of what container it is in, separate from the lighter air above it. But only a very small movement is needed to stir it up and mix the two. Why doesn’t a cumulus cloud mix the the air below it?
Have these questions been sufficient to show you what it is that I’m trying to figure out? I tried looking this stuff up in Google, but I don’t even know the best words to search for. So I’m hoping a fellow Doper can offer a link or two that explains this stuff.
I am too lazy to do any research or cite anything, but here goes:
The clouds only appear flat and smooth because they are really far away; the boundary is not terribly well-defined.
Water vapor becomes visible at certain temperatures and pressures; the barrier you see could be a pressure or temperature change rather than a well-defined section of moisture.
OK, I just tried typing out about 5 different ways of explaining it, but they all seemed weird and wordy, so I’m gonna break it down like so:
-Moisture needs certain properties in the air - pressure and temperature being key- in order to change from water vapor in the air (invisible) to little droplets (visible as clouds). This is condensation
-It’s easier for moisture to condense when it’s cooler and the air has a lower atmospheric pressure.
-As you get farther from the ground, the pressure and temperature drop. Sometimes the level where the pressure and temp. are right is quite distinct, seeming to start abruptly.
-under the cumulus cloud is a rising column of air. The rising column simply goes from being invisible to visible once it crosses that level where the conditions are right.
I’m really sorry if that sounded weird and confusing, but my last weather class was about 3 years ago. I know that I have a cache of links somewhere for this. Hopefully i can at least point you in the right direction!
I’ll agree that the bottom of a cumulus cloud is “not terribly well-defined” if you’ll agree with me that the top of a cumulus cloud is not well-defined at all. In other words, something is definitely going on at the bottom, and I want to know what it is.
Why is it so abrupt? And why don’t they start mixing when the wind blows?
Sometimes I see a cumulus cloud moving across the sky. There is clearly some turbulence in the bottom, but it always looks to me sort of like a sheet being pulled over an uneven surface, like whatever got mixed in immediately settled out.
Thanks for the responses. Gotta go to bed now. See y’all in the morning.