Chapter 10: The Physics and Chemistry of Water and Air
A couple of chapters from now we discuss climate, but we’ll make a mess of that unless
we first discuss weather (see definitions in Unit 3 introd). So we start here with weather.
We will begin with a complete review of what I would normally take to be ‘prerequisite’
material concerning basic properties and processes that involve the atmosphere and
hydrosphere of Earth. You’ll need this before we can understand much of anything about
Let's begin this section with a brief description of the atmosphere as it exists already
made; later, we'll consider how it was made.
Scientists describe the present atmosphere
as comprised of several layers, each with its
own physical, chemical, and temperature characteristics (Fig.1). From the bottom up,
these layers are: (1) the troposphere, from surface level to about 12 km; (2) the stratosphere, up to about 50 km; (3) the mesosphere, up to about 80 km; and (4) the
thermosphere up to 500-700 km [The thermosphere is sometimes called the ionosphere,
but strictly speaking the ionosphere is any electrified region of the upper atmosphere. At
heights of 80 km, the gas of the atmosphere is so 'thin' that free electrons can exist for
short periods of time before they are captured by a nearby positive ion. The existence of
charged particles at this altitude and above, signals the beginning of an ionosphere
The lowest layer, the troposphere, contains about 80% of the mass of the atmosphere.
It's thickest near the equator (close to 18 km) and thinnest near the poles (roughly 8 km).
A sharp rise in the thickness of the troposphere occurs at mid-latitudes, and this exerts a
fundamental control on global weather
patterns (we’ll return to that point below).
The troposphere is the main layer of
atmospheric circulation (Fig. 2).
The tropopause, the boundary between
troposphere and stratosphere, is a surface
of equal pressure. This boundary is marked
Fig.3 by jet streams, which are narrow zones of
swift high-altitude winds (Fig. 3).
The stratosphere is the zone above the weather, and temperature gradually climbs with
height. This is also the zone that contains the ozone-enriched layer.
The mesosphere is the layer where incoming asteroids or meteoroids first begin to burn,
and probably because of its lack of ozone, the temperature drops within it ( down to about
minus 85 C before it starts to rise again in the therm).phere
About 100 km above Earth's surface lays the outer
atmosphere, or thermosphere (sometimes called the
ionosphere). Air here is very thin and highly responsive to
solar radiation. The varying velocity of solar winds
especially affects it. During solar flares, the velocity of solar
winds is greatly enhanced, and temperature in the
thermosphere can rise to 1225 C as a result oo solar
bombardment, and then fall as low as 225 C. These dramatic
temperature changes occur simply because there are very
few molecules present to absorb and distribute heat. By the
way, when we see the so-called Northern Lights, they are
being produced in the thermosphere by ionization processes
as an extraordinarily strong flux of electrons streams in from
the Sun (Fig. 4).
Except possibly in the earliest stages of Earth evolution, when surface temperatures were
very high, the only gaseous species that have been able to escape from the Earth's gravity field are H 2hydrogen) and He (helium); Ne (neon), with an atomic weight of 20, does
not escape, and the lightest of the major atmospheric gases have similar molecular
weights NH – 37 (ammonia); H O – 182(water); CH – 16 (met4ane). Thus, the quantity
of volatile or potentially volatile material in the whole Earth is assumed not to have
changed since the earliest times, except for loss of hydrogen and helium.
2.1 Origin of the Atmosphere
We can safely assume that Mars, Earth and Venus
originally had virtually identical atmosphere
compositions. If Earth's CO (c2rbon dioxide) that
is tied up in limestone were evolved to the
atmosphere, we'd again all be identical. At some
time, it's likely that all three planets also had liquid
water, inherited from comet and asteroid impacts,
on the planet's surface.
Consider Venus first (Fig. 5). In the early days of
differentiation, solar radiation readily penetrated
the clouds of Venus to warm the planet's surface.
As solar output increased and the surface heated,
and as more CO from volcanoes entered the
atmosphere (thus increasing atmospheric pressure),
liquid surface water was probably evaporated.
Eventually the atmosphere became so thick that
radiation could not be reflected back into space
from the surface; the greenhouse simply got
stronger and the atmosphere got thicker. Finally,
equilibrium was reached where the atmosphere
(and particularly its outer shield of hydrogen
sulfide droplets) was so thick that it reflected most
of the in-coming solar radiation, thus keeping the
whole planet at about 737 K constantly.
Next, look at Mars. There's plenty of evidence of surface water at some (early) stage.
There still is surface and subsurface ice, and recent evidence of liquid water coming to
(but not being stable ) surface (see: http://www.nasa.gov/mission_pages/mars/news/mgs-2006120).html
It's been suggested that Mars' proximity to the asteroid belt left it exposed to a somewhat
higher flux of meteorite impact than Earth, and the result was a gradual erosion of Mars'
atmosphere. More likely, volcanism decreased as the (smaller) planet cooled. With the
greenhouse gone (and the surface temperature dropped), liquid water became impossible.
Solar radiation broke down the water molecules, hydrogen escaped (being small, Mars
has a low escape velocity) and oxygen was combined with the surface soils (thus the
reddish colour). Today, the atmosphere on Mars is very thin, and surface temperatures
change dramatically during a single day. Comets and carbonaceous chondrites contributed the elements of the volatiles of Earth,
Mars and Venus in the days of accretion and the following periods of heavy
bombardment. So, the atmosphere compositions should have started out being the same
for all three planets! Once volcanism became an active process on all three planets, the
atmosphere and hydrosphere were maintained by degassing of the interiors.
2.2 Dynamics of the Atmosphere
Most weather-related phenomena originate in the troposphere. Although there is little
mixing between the troposphere and the other divisions, the troposphere itself is
thoroughly mixed and extremely dynamic.
Much of our concern with storms has to do with what water is doing in the troposphere.
Water is a most unusual material since it can exist in three physical states at the Earth's
surface. Whenever a compound goes through a change of state, energy is either absorbed
by or released by the process. A couple of points to remember:
In going from a more ordered state (say, ice) to a less ordered state (say, liquid
water), energy is absorbed.
The amount of heat energy released or absorbed per gram of matter during a
change of state is known as latent heat (look at the top part of Fig. 12.6).
o For example the latent heat of condensation (from less ordered to more
ordered, thus released energy) is 2260 joules per gram, or 540 calories per
gram (so...how many joules per calorie?).
o The latent heat of evaporation is 2260 joules per gram (=540 calories/g).
Another way of saying that is: "Turning liquid water, heated to 100 C,
into water vapor requires the input of 540 calories per gram of water, with
no change in temperature during the change of state". Why do you feel cool on a hot day when you throw water on yourself? Because some of the
2260 j/g needed for evaporation of that water is absorbed from your skin,
so your body temperature drops slightly.
The reason the troposphere is so dynamic is that air moves in convection cells in
density gradients, and
All these factors interact, of course.
Density is always
[mass/volume]. The air's density
Fig.7 depends on its temperature, its
pressure and how much water
vapor is in the air (by the way,
check out Fig. 7 to see how much
variation there is in the amount
of water vapor that air may
contain). Consider dry air first.
At sea level, if the air is
completely dry and at 0 C, the
density is 1.275 kg/m . If we
increase temperature, thus speed
up the molecules of oxygen, nitrogen and other things in a 'box', the pressure on the box
increases. If the heated air is surrounded by nothing but air (i.e. no 'box'), it will push the
surrounding air aside, thus the air's density decreases as the air is heated. Pressure has
the opposite effect to temperature, increasing the pressure increases the density; as you
go higher, the air's pressure decreases. So, the air's density is lowest
at a high elevation
on a hot day
when the atmospheric pressure is low.
[Example: a hot day in mile-high Denver just as a storm is moving in; i.e. a low-pressure
The air's density is highest
at low elevations
when temperature is low
and when pressure is high
[Example: a sunny, cold, winter day in Alaska]
You can convince yourself of the relationship between density and air temperature in
another way. If you've ever used a manual pump to pump up a tire, you've noticed that
the pump gets hot as you compress the air and force it into the tire (so the heat from the
warm air is transferred to the pump). If you've put in too much and let some air out, it
feels quite cool until it expands. These are two adiabatic processes (where the name comes from Greek meaning "no passage"). Adiabatic processes are so named because
they are processes that occur without addition or subtraction of heat from any external
Warm air is less dense than cold air and therefore rises, creating convection cells in
the process. Because of the temperature gradient in the troposphere, air is continually
rising or falling. However, because air pressure decreases with increasing altitude, the
rising air expands, and because there is no heat source within the troposphere itself, the
rising air expands adiabatically and so its temperature falls. In the case of sinking air, the
reverse happens, and it’s still an adiabatic process (no energy from any external source is
involved in that convection of air).
If this is sounding complicated,
Fig.8 sometimes it helps to simply express
the same thoughts in other words.
Here goes! 'Parcels' of air rise in the
atmosphere if they become less dense
than the surrounding air. The principal
way to make air less dense is to heat it!
Normally, we heat air close to Earth's
surface. As we heat a 'parcel' of air
(let's say we're doing that over some
area of bare rocks that are warmed by
Sun’s energy), the heating causes
expansion of the air parcel, it becomes
less dense, and rises. As the warm air rises to higher elevation, it is rising into a volume
of the atmosphere with less pressure (look at Fig. 8; because there are fewer air
molecules at higher elevations, there MUST be lower air pressure at higher elevations),
and that results in additional expansion of the rising 'parcel'. BUT...that expansion
requires the use or expenditure of heat energy, and no source of heat exists at higher
elevations! The result MUST be that the rising parcel of air begins to cool. As a result of
that, its density will slowly increase to match that of the surrounding air, and the parcel
stops rising! Those processes that we've see happen to the parcel of air are, together,
called adiabatic processes. OK? Just remember that the upward loss of heat is a dry
process, completely independent of any amount of water that might be carried by the air
- that's a whole other 'kettle of fish'!
Now.... let's put some water in the air! Latent heating is the other process that can
destabilize the atmosphere - and destabilization is what storms are all about! As we
already know, latent heating occurs as a wet process, driven by water vapor, which
weighs roughly a third less than the mixture of gases that form the atmosphere (check it
out with a table of atomic weights if you don't believe me! In fact, here, let me prove it for
Humid air is lighter or less dense than dry air. I know that's not the way it feels, but
it's true! To appreciate that fact, remember the basis behind Avogadro's Number: "Equal volumes of all gases at the same temperature and pressure contain the same number of
molecules". What this means for us is that if some lighter gas molecules went into a
"box" already filled with a heavier gas, and the volume, temperature and pressure stayed
the same, some of the heavier molecules in the box would have to leave. If we didn't
allow gas in the box to escape, adding more molecules of another gas would increase the
pressure. So, adding the lighter molecules decreases the density of the gas in the box
because they replace some heavier molecules. [Nitrogen has an atomic weight = ~28;
Oxygen = ~32 and Water = ~ 18. In the free atmosphere, adding water vapor to the air
replaces some heavier nitrogen or oxygen molecules with lighter water molecules.]).
Evaporation adds water vapor to the atmosphere at low elevations and causes a net
decrease in the density of the air, which can then rise to higher elevations. As before (for
dry air), the moist air rises, expands, and at some point cools to the temperature at which
it becomes fully saturated with water vapor. Then condensation begins. Condensation
releases latent heat (see Fig. 6 again), which partially opposes the cooling of the rising
air parcel due to its expansion. The release of this heat in the air parcels also makes them
more buoyant (or less dense) and allows them to rise much higher in the troposphere
(sometimes up to 10-15 km). Eventually, the cumulative loss of most of the water vapor
stops the release of latent heat that had kept them rising.
Actually, compared to the differences made by temperature and air pressure, humidity
has a very small effect on the air's density.
2.3 Convection and Rotation
The air in the atmosphere does not simply rise from a higher temperature at Earth's
surface into a cooler upper troposphere with no overall direction. We all know that more
of the Sun's heat is received per unit of land surface near the equator than near the poles
(Fig. 9). This uneven heating gives direction to convection currents. The heated air near
the equator rises, and near the top of the troposphere it spreads outward in the direction of
both poles. As the upper air travels northward and southward, it gradually cools, becomes
heavier, and sinks. On reaching Earth's surface, this cool, descending air flows back
toward the equator, warms up, and rises, completing the convection cycle. That's the way
things should go without continents and without complications like rotation (Fig. 10).
Thetharth's rotation messes up the simple pattern. The Coriolis effect, named after the
19 century French mathematician who first analyzed it, causes any body that moves freely with respect to the rotating Earth to
veer to the right in the Northern
Fig.10 Hemisphere and to the left in the Southern
Hemisphere. This is true regardless of the
direction in which the body may be moving
[Fig. 11 is a simple-minded illustration of
the Coriolis effect on the aiming versus