Post by Michael J. StricklandOn Sun, 3 Jul 2016 10:13:32 -0700 (PDT), James McGinn
...
Post by James McGinnThe most stupefying myth in all of meteorology is the myth that steam can
persist in our atmosphere.
Steam is water vapor above air saturation density. The steam coming
out of a kettle is opaque because it is well over saturation density
(>> 100% humidity) and pushes the air aside as it exits.
Partially true. It is not the steam that is over 100 % humidity, but the
*air* is (i.e., there is more water in the air that it can hold; the warmer
the air, the more water it can hold), *therefore* there is steam.
Post by Michael J. StricklandTechnically, its not persisting in the atmosphere since it has purged the
atmosphere from its vicinity.
Nonsense.
Post by Michael J. StricklandAs it spreads out, colliding and mixing with the heavier air molecules, it
loses kinetic energy (cools) due to the work it does against atmospheric
pressure. By the time it reaches saturation density (100% humidity) it has
become translucent (almost transparent) instead of opaque. If its density
increases beyond saturation due to a pressure pulse for instance or
further cooling, it forms water droplets (clouds).
I am not sure that is a completely correct description either.
Post by Michael J. StricklandAs water vapor (much lighter than air) rises and cools, it condenses
into water droplets (clouds).
You have omitted the most important part of the story. For a gas to
condense to drops that results in clouds, it does not suffice that its
temperature is reduced; there has to be something *solid* that it can
*condense on*. Condensation of water vapor to clouds is possible because
the air in our atmosphere is not clean: small dust particles (e.g., carried
away from deserts and ground soil, and from air pollution caused by humans),
which in this context are called *condensation nuclei*, are intermixed with
it, and the water vapor condenses *on them* (just like it condenses on a
pane of window glass; the scientific term of that is *phase boundary*).
<https://en.wikipedia.org/wiki/Rain>
Post by Michael J. StricklandAs water vapor condenses into a cloud, the volume reduction (vapor to
liquid) reduces the outward positive pressure of the cloud and allows
more moisture laden air to flow into the cloud. This effect causes the
cloud to grow in all directions.
How did you get that idea?
Post by Michael J. StricklandDroplets at the top of the cloud (coldest) fall downward with a very
small terminal velocity and usually evaporate from air frictional
heating before falling very far.
Instead, those "droplets" at the top of a cloud are actually condensation
nuclei surrounded by then-*frozen* *solid* water *ice*. They fall down
eventually because there comes a point when their mass becomes to great for
any upwardly directed airstream (as warm air rises, and the planet’s crust
is continuously warmed by its main star) to hold in suspense. Depending on
the temperature in and below the cloud they *melt* or even evaporate in the
process. [AFAIK, it is _not_ any “frictional heating” that determines what
kind of precipitation, if any, can be expected from falling out of a cloud,
but temperature and wind conditions in the lower part and below the cloud
(technically, there is snow/hail from *every* cloud; you can see that in
winter or in polar areas when and where the temperature of the air below the
cloud is not high enough to melt the snowflakes/ice pellets). When the
temperature below the cloud is just high enough for the falling
pellets/drops to evaporate before they reach the ground, you can see
/virga/, “falling streaks”, below the cloud. The friction between falling
and rising particles in the cloud, however, allows for a separation of
electrical charges and the cloud to be statically electrically charged,
which leads to lightnings as the form of resulting electric discharge inside
a large cloud, between clouds, or clouds and the ground.]
In weather modification, there is “cloud seeding”: clouds are seeded from
airplanes with drops of silver iodide as artificial condensation nuclei;
either to form clouds that can rain of where there would have been little to
none naturally (i.e., over arid regions and deserts), or to have existing
clouds rain off before they become to large (you want enough rain, but not
100 % cloud coverage for your plants; and you want rain, not a hailstorm,
down on your crops – as cumulus clouds grow *upwards* to develop towards
cumulonimbuses, they become more likely to be a source of a hailstorm due to
the much lower temperatures in the upper atmosphere).
<https://en.wikipedia.org/wiki/Cloud_seeding>
Post by Michael J. StricklandAs the cloud grows upward and the temperature at the top drops, colder
and larger drops form.
Not drops, but pellets of *water ice* frozen on dust particles. They are
not drops of (dirty) water until shortly before, or after they leave the
cloud. The temperature and air pressure are much too low there for liquid
water to exist (down to ca. 220 K/−53.15 °C and 0.5 bar/500 hPa near the
tropopause, according to the 1962 US Standard Atmosphere Graph; cf. NIST
standard conditions: 293.15 K/20 °C and 1.01325 bar/1013.25 hPa).
<https://en.wikipedia.org/wiki/Atmospheric_temperature>
<http://www.wolframalpha.com/input/?i=water+phase+diagram&dataset=>
Post by Michael J. StricklandThey are larger because the vapor molecules are moving slower and more
nucleate (stick together) per second.
No, they can get larger because as the water *freezes* on the condensation
nuclei, more *water* can *freeze* on the resulting ice pellets. It is the
same that happens when your tongue, wet with the water of the saliva, can
get stuck on a really cold and wet piece of soft-ice cream, or your hands,
wet with water of the skin, are stuck when touching the inside of a freezer
[you can get severe injuries this way; kids, do not try that at home, and
always wear thick insulating gloves when you are experimenting with dry ice
(solid carbon dioxide)].
I strongly recommend that you read the chapters on thermodynamics of the
Feynman Lectures on Physics, and introductory material on meteorology (such
as what I just gave to you) to clarify some of your misconceptions about
(water) vapor and clouds.
<http://www.feynmanlectures.caltech.edu/>
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