Properties of Air: Temperature, Pressure, Density
To understand wind and weather, we must first know about air.
Young, color blind John Dalton was fascinated by weather. Beginning in 1787, he
maintained a daily weather diary for the next 58 years! But no one could answer his
most basic weather questions. Where does water go when it evaporates? Where does
rain come from? Why are oxygen and nitrogen so thoroughly mixed in the atmosphere-shouldn’t the heavier oxygen settle to the bottom like water settles below oil?
With no answers available, Dalton set out to find the answers himself. He decided that
the only way to understand the workings of weather was to start by learning what air
consists of and what governs the behavior of all gases. With little background in
chemistry, he repeated experiments that had been performed by others but with new
insight provided by his knowledge of weather. In the process he not only discovered
some of the gas and vapor laws, but gave humankind its first glimmerings into the nature
of atoms.
These and other basic discoveries about air, atoms, and molecules made by scientists
who followed in Dalton’s footsteps served as a stepping stone for understanding and
predicting some of the most complex aspects of weather. And thus, the invisible air gave
blind humanity eyes to see.
Experiments and Demos: 1: Magdeburg Hemispheres, 2: Inverted Barometer (Cup upside down), 3: Clear
tubing to inhale air and raise water, 4: Helium Balloon
The air we so often take for granted is a miraculous substance. Heat it and trap it in a
balloon, and it will lift you above the highest mountains. Let the sun heat it, and it will
expand and rise to produce the winds. Cool it enough and the water vapor in it will
condense and metamorphose into snowflakes or raindrops to slake the Earth’s thirst.
The seemingly vacuous air has enough character to generate cataclysmic storms and
support enormous hailstones. Air is also our shield, our blanket, and our window to the
world. It protects us by absorbing the sun’s lethal ultraviolet radiation, yet allows its
visible light through to illuminate us, and warms us by absorbing outgoing long wave
infrared radiation emitted by the surface. Air is all this and more, for it contains the very
breath of life.
AIR PROPERTIES
Trapping the invisible air or expelling it from glass tubes such as thermometers and
barometers enabled us to measure its pressure, temperature, volume, mass, and
density. Relations between air’s properties include,
1. Pressure increases when air is confined in a decreasing volume.
2. Air expands when it is heated if it is not confined in a rigid container.
3. Air temperature increases without adding heat when it is compressed.
4. Any gas will condense to a liquid or deposit to a solid if it is cooled enough.
Volume, V of Air
Air occupies volume even though it is invisible. As you fill a container with water you can
feel the air rushing out if the opening is narrow. To measure the volume, fill the container
with water and then pour it into a measuring cup. And, as the water pours out of the
narrow opening, the air will bubble back in.
Mass, m of Air
Mass is the fundamental property related to the amount of matter. Mass is related to
weight but whereas weight depends on gravity, g, mass is independent of gravity. If the
scale is calibrated in kilograms, weight equals mass when the acceleration of gravity (g)
has its average value at sea level on Earth (g  9.8 m/s2).
Weight, W = mg of Air
Air has weight even though it floats freely in the atmosphere. Weigh air the same way
you weigh a baby. First get on a scale with the baby. Then get on the scale without the
baby. The difference is the weight of the baby. To weigh a volume of air in a container,
first weigh the container with the air in it. Then pump the air out of the container and
weigh it again. The difference is the weight of air. Weight is actually a force.
Density, r = m/V of Air
Density is equal to mass divided by Volume. It is related to how closely packed matter is.
The more neutrons and protons that are squeezed into a container, the denser it will be.
The density of water (1 g/cm3 = 1000 kg/m3) is often used as a standard of reference.
Specific gravity is equal to the density of a material divided by the density of water.
Materials with 2 times the density of water have a specific gravity = 2.
Density, r
r 
A block of gold weighs incredibly more than an equal size block of
Styrofoam. This indicates that gold has a much greater density.
m
Density units are kilograms per cubic meter (kg m-3) or grams per
V
cubic centimeter. The density of fresh water at 15C =1000 kg m-3 or
1 g cm-3. Water is then used as a standard of comparison. Gold is 19.3 times denser
than water, typical rock (granite) is about 2.6 times denser than water. But water is about
800 times denser than air near sea level.
Liquids and solids tend to have high density because molecules are packed closely
together. Most solids have slightly higher density than liquids because the molecules are
packed tighter and more efficiently. Gases the have much lower density than liquids or
solids because the molecules move freely and are far apart.
Materials sink if they are denser than the liquids or gases in which they are immersed
and rise buoyantly if they are less dense than the liquids or gases in which they are
immersed .
Packing of Atoms in Salt (Sodium
Chloride) Na = gray, Cl = green
Air Pressure and Moving Molecules
Pressure is Force (e. g., weight) divided by
Area. When you support a book on your open
palm you do not feel much pressure on your
skin. But if the book is balanced on a needle on
your palm, the pressure is so great (because the
needle’s area is tiny) that it will pierce your skin.
Pressure of a gas is caused by the impacts of
moving molecules (dots). The purple lines are
pistons that are free to fall but will rise when
molecules hit them hard and often enough. The
chamber to the right has more molecules than
the chamber to the left, so the piston is forced
higher by more frequent collisions. At higher
temperature molecules move faster and push
the piston higher.
Piston and Air Molecules
Pressure Units
Mean Sea Level Pressure is 101325 Pa (Pascals). This is about 14.7 pounds per square
inch or 104 kg per square meter (m2). Thus 1 m2 of atmosphere weighs 10 metric tons.
Other pressure units are expressed in terms of the height of a column of mercury that
the atmosphere can support. Mean Sea Level Pressure is 29.93 inches or 760 mm.
mm
787
750
713
Weight of Hg
Force of air
pressure
The mercury (Hg) barometer is a tube filled with
Hg and inverted in a pool of Hg. The Hg column
can only rise until its weight balances the force of
atmospheric pressure. At sea level, this occurs at
a height of almost 30 inches (29.93”) or 760 mm
on average*. If the tube is longer, the space
above the Hg is a vacuum. If air leaks into the
barometer, the Hg will sink to the level of the pool.
Hg is 13.6  denser than water, so atmospheric
pressure can support a water column 13.6 times
higher than a Hg column  34 feet or 10.3 m. This
is one reason we do not use water barometers.
*Blood pressure is also
measured in mm of Hg.
Healthy
values
of
diastolic
(maximum)
and systolic (minimum)
arterial pressure are
about 120 and 70 mm of
Hg. Thus, the heart is a
pump that generates
about 20% of normal
atmospheric pressure.
Temperature, T
Temperature is a measure of the degree of warmth or cold. But what measure? Even
though the feeling of temperature seems obvious, temperature is really a subtle
concept, related to the speed (kinetic energy = ½mv2) of molecules. Temperature
scales were developed that depended on indirect properties such as the expansion of
materials when heated. For example, mercury expands faster than glass when
heated so that as temperature increases the mercury rises from the bulb up into the
tube. Fixed points such as the melting point of ice and the boiling point of water at
normal sea level pressure were used to establish the temperature scales.
The temperature scales that we are used to (Centigrade or Celsius and Fahrenheit)
are artificial in the sense that they are not directly proportional to energy. Only Kelvin,
or Absolute temperature is directly proportional to energy. (Theoretically at 0 K all
molecular motions stop and the molecules would fall on a pile on the floor.)
Since Kelvin Temperature is proportional to energy, Most scientific laws related
to energy use Absolute Temperature.
The relations between the Fahrenheit, Centigrade and Kelvin temperature scales are
illustrated on the next slide and in the program below.
TEMPERATURE
SCALES
KELVIN
(ABSOLUTE)
CENTIGRADE
(CELSIUS)
Temperature
Scales
FAHRENHEIT
Temperature
Scales
Water Boils
at Sea Level
373
100
212
273
0
32
Ice Melts
-459
Molecules
are Still
0
K = C  273
-273
C =
F  32
1 .8
F = 1 . 8  C  32
Temperature and Moving Molecules
All molecules of a gas or air move rapidly
and frequently collide. Collisions ensure that
light molecules (e. g. Hydrogen, H2) move
faster than heavy molecules. Molecules also
move faster as T increases, exerting more
pressure.
Run Program COLLIDE
Atoms in a molecule vibrate faster
as T rises. If T gets high enough
the vibrations will be so rapid that
the molecule will rip apart. When
this happens, solids liquefy (melt),
and liquids evaporate to gases.
Gas, Liquid, Solid - Why Sweating Cools You
Of all the gases in Earth’s atmosphere, only water can change between gas, liquid, and
solid. But Mars gets cold enough in winter near its poles (T  - 225F = -143C) that carbon
dioxide (CO2) can change from gas to solid (dry ice), and Pluto is so cold (T  - 414F = 230C) that even nitrogen (N2), condenses and freezes.
When water vapor condenses and falls as precipitation, atmospheric mass and pressure
decreases. But since water vapor constitutes only about 1% of the air, the reduction of
pressure is small. On Mars the situation is different. So much CO2 freezes during the winter
that 25% of its thin atmosphere is deposited on the Polar Ice Cap.
Water molecules are so strongly attracted to each other (by hydrogen bonds) that it requires
much energy (called Latent Heat) to free them, and melt from ice to water and evaporate
from water to vapor. The energy water molecules gain when they evaporate comes from the
surroundings, which includes your skin when it is wet. This strong attraction gives water a
much higher boiling point than the atmosphere’s other gases and, separates water from oil.
On cold nights, farmers spray water on
their fruit to protect them from freezing.
Ironically, when water freezes is keeps the
temperature from falling as much because
the energy it loses when turning to ice
heats the surroundings. So a layer of ice
coats the fruits in morning but so long as
the temperature remains above about 2C, the fruits will not freeze.
** Boiling Point (C) at Mean Sea
Level Pressure = 101325 Pa
Gas
Boiling Pt**
H2
-253
He
-269
N2
-196
O2
-183
CO2
-78.5
H2O
100
The Theatre of Gas Molecules and the Electrolysis of Water
One of the early experiments with electricity was the
electrolysis of water. When the poles of a battery are
inserted in a triple tube filled with water and a bit of
salt to conduct electricity, the water (H2O) splits into
hydrogen and oxygen. Hydrogen accumulates at the
cathode (negative pole) and oxygen accumulates at
the anode (positive pole). This experiment showed
scientists that water is not an element but a
compound consisting of 2 parts H2 and 1 part O2
since the volume of hydrogen is twice the volume of
oxygen. This fundamental finding indicates that
At any pressure, volume, and temperature the
number of molecules of any gas is the same, no
matter how light or heavy.
This property is much like the seating capacity of a
theatre. Each person, no matter how large or small,
gets one seat so that the number of people in a filled
theatre is always the same. It also means that each
seat in the molecule world is large enough to hold
the largest molecule (person).
Hoffman Electrolysis
Apparatus
T, p, r Relations in Air
Boyle’s Law: When T is held constant as pressure, p, increases, Volume, V
decreases (and density, r increases) so that pV (and p/r) are constant.
Charles’ Law: When p is held constant, as T, increases, volume, V increases
(and density, r decreases) so that the ratios, V/T (and is r  T) are constant.
Kelvin first determined Absolute 0 Temperature by extrapolating to 0 Volume.
When the restrictions about holding either T or p constant are removed and
Boyle’s Law and Charles’ Law are combined, we obtain the complete relation
between temperature, T, density, r, and pressure, p in any gas or mixture of
gases such as air, called the Ideal Gas Equation.
Universal Gas Constant (8314)
RUN GAS LAWS
Temperature (K)
Density
rR T
*
Pressure (Pa)
Mean Molecular Weight
of Dry Air (28.97)
p=
(1  . 61 w )
Md
Water Vapor Mass Fraction
Illustrating Charles’ Law
Vertical Structure of Atmospheric Pressure: Hydrostatics
The sky doesn’t fall because it is
weighed down by gravity. Pressure
decreases with height in the oceans
or atmosphere because there is less
water or air weighing down from
above. When the decrease of
pressure with height exactly offsets
the weight above, the result is
hydrostatic balance.
Because air is compressible, density
decreases as pressure decreases.
But as density decreases, pressure
decreases less rapidly with height,
like a pile of balls or a long vertical
spring compressed at bottom by its
own weight.
The result is that Atmospheric
pressure decreases by half (50%)
for every height increase of
roughly 5 km. Thus the decrease of
atmospheric pressure with height is
approximately exponential.
Run Compress
The atmosphere thins
rapidly with height as
thunderstorms tower up
through the troposphere
http://spaceflight.nasa.gov/gallery/images/shuttle/sts-100/hires/s100e5498.jpg