Clouds
"The yellow fog that rubs its back upon
the window-panes,
The yellow smoke that rubs its muzzle on the window-panes
Licked its tongue into the corners of the evening,
Lingered upon the pools that stand in drains,
Let fall upon its back the soot that falls from chimneys,
Slipped by the terrace, made a sudden leap,
And seeing that it was a soft October night,
Curled once about the house, and fell asleep."
A philosophical soul I happen to know once mentioned that
clouds have sharp edges and clear features only when they are far away,
a reversal of how things seem to get fuzzy with distance. But to get to
the point...
If
a cloud is made out of water then why doesn't it fall out of the sky?
This seems to me to be a perfectly reasonable question. If fog is just
a cloud that happens to be at low elevation then I've spent many
mornings waiting for the school bus lost in a cloud. And these
clouds never just collapsed to the ground so I'll concede that empirically clouds have a right to
hover in the air. And perhaps they are
made out of small droplets of condensed water. Of course such
small droplets will fall quite slowly, as we notice with mist, but fall
they must because they are subject to gravity... so in fact I've
arrived at two refinements to my original question:
Supposing water vapor--a gas--exists at some density, perhaps a density
that increases with time in one spot...
- How
does that water vapor undergo a phase transition back to liquid water
droplets?
And once the water has condensed...
- What
force counteracts the (slow) descent of these droplets due to gravity?
Now the standard party-line has two easy answers: "Small particles act
as nucleation sites" and "updrafts, for example from solar heating of
the earth's surface." But I say "That's too easy!" I say "That's
not enough!" I say "I'd like to understand that a little better!"
I say "Nonsense; let's build a cloud and find out for ourselves!"
To continue with our construction of a
cloud, we go as follows...
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