The atmosphere consists of:
- a mixture of different gases; and
- tiny liquid and solid particles suspended in the gas
1. Mixture of gases
In clean, dry air (that is, a sample of air without any water
vapor, and no suspended liquid or solid particles), the proportions of
different types of molecules are:
of all Molecules
in the Sample
Well, it's what air is mostly made out of, and so when air moves
around—that is, "wind"!—it's mostly nitrogen
gas that we feel. Otherwise, not terribly important meteorologically
(though it's biologically important).
A distant second to nitrogen as a constituent of air; most of the
air sample that isn't nitrogen is oxygen.
Absorbs some ultraviolet radiation from the
sun, starting a process responsible for creating ozone in the
stratosphere. Otherwise not terribly important meteorologically. (Of
course, it's very important biologically.)
Zilch. It's a "couch potato" gas; it doesn't interact with other
substances chemically and doesn't absorb or emit radiation—doesn't
do much of anything. (It's not even biologically important.)
Affects the transfer of longwave infrared radiation through the
atmosphere, contributing significantly to the greenhouse
effect, which is responsible for keeping the earth's surface
warm enough to be habitable by living organisms such as us. CO2 concentrations in the atmosphere have been increasing since
the industrial revolution in the 19th century, at increasingly
faster rates. There is an international consensus among scientists
that higher concentrations of CO2
will enhance the greenhouse effect and cause global warming (which
is already happening), though to an uncertain degree and with
The proportions of the four gases above are essentially the same everywhere
in the atmosphere (except for small local differences in carbon dioxide due
to differences in photosynthetic activity by green plants). Another gas present
in clean, dry air:
average is 0.000001%;
typical maximum 0.0001%
(in ozone layer)
Absorbs destructive ultraviolet radiation from the sun, making it
possible for life to exist on earth's surface.
There are other gases present besides those in clean, dry air, of
course. Water vapor is by far the most important meteorologically.
*At very high altitudes; in very cold polar regions; and
often in deserts.
from almost 0%*
to about 4%**
- It is the source of water for cloud droplets and ice
crystals (and hence, ultimately, precipitation).
- It affects the transfer of longwave infrared radiation
through the atmosphere—in fact, it's the most significant contributor
to the greenhouse effect (but not to global warming, initially); and
- It absorbs or releases heat when it changes between
gas, liquid and solid phases or states. Water (that is, H2O,
regardless of phase or state) is the only substance that exists in all
three phases within the normal range of atmospheric temperatures and
**At the earth's surface over tropical oceans.
2. Tiny liquid and solid particles suspended in the air
- These particles average perhaps 10-20 microns (a micron is
one millionth of one meter, or one thousandth of a millimeter) across.
Individually these are too small to see, but together they can
scatter enough light so that their collective presence is visible as a
- Clouds are simply aggregates of very large
numbers of them. They reflect visible light very well—on the average,
clouds reflect back to space about 20% of the solar radiation striking
the earth (about 2/3 of all solar radiation reflected by the planet), helping keep the earth much cooler than it would be without them.
- Clouds absorb and emit all wavelengths of longwave infrared radiation very well. This makes them a significant contributor to the greenhouse effect.
- If enough of cloud droplets and/or ice crystals can collide and coalesce and become big
enough, the resulting precipitation particles (rain drop, snow flake,
hail stone, etc.) will fall out of the cloud and reach the earth's
surface. The typical size of a precipitation particle is about 100
times the diameter of a cloud droplet or ice crystal (though they can
certainly get much bigger than that), so it contains about 100x100x100
= 1,000,000 of them (since the volume is proportional to the diameter
cubed). Getting that many cloud droplets or crystals together is not
easy, which is why most clouds don't produce precipitation.
- Near the earth's surface, these typically are present in
concentrations ranging from 500 particles per cubic centimeter
over the open ocean, to 50,000 particles per cubic cm well inland,
though the variability is great. (A cubic centimeter is
approximately equal to the volume occupied by the end of your
finger including the fingernail.)
- They can serve as tiny, solid surface onto which water vapor
can condense to form cloud droplets or ice crystals—clouds would be
very rare without suspended solid particles of dust, salt, etc.
- They affect the transfer of radiation (especially visible
light, often by reflecting it back to space, which causes the earth's
surface to cool, as during the period following the eruption of Mt.
Pinatubo in the Philippines in the early 1990's; or by absorbing it, which warms the atmosphere but cools the surface.)
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