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The absolute zero temperature is the temperature
that the molecules do not move at all. |
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This temperature occurs at –273°C. |
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The Kelvin Scale (°K) is a new temperature scale
that has its “zero” temperature at this absolute temperature: |
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°K = °C
+ 273 |
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Why does air density decrease with height? |
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What is air pressure? |
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Why does air parcel expand with height? |
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The vertical structure of the atmosphere (i.e.,
density decreases with height) depends on a compromise between gravity,
which pulls molecules downward toward the surface of the Earth, and the
tendency of the molecules to move about freely. |
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One way
to measure how soon the air runs out in the atmosphere is to calculate the
height of an atmosphere of constant density that exerts the same pressure
as our atmosphere at Earth’s surface. |
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This height, which is about 10 km, is called the
scale height of the atmosphere. |
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Over this vertical distance, air pressure and
density decrease by 37% of its surface values. |
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If pressure at the surface is 1 atmosphere, then
it is 0.37 atmospheres at a height of 10 km, 0.14 (0.37x0.37) at 20 km,
0.05 (0.37x0.37x0.37) at 30 km, and so on. |
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Most (80%) of the mass of the atmosphere is
confined in the first 10 km from the surface. |
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The higher the temperature (T) è the more
energetic the air molecules è the larger the scale height |
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The larger the gravity (g) è air
molecules are closer to the surface è the smaller the scale height |
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The heavier the gas molecules weight (m) è the
smaller the scale height for that particular gas |
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H has a value of about 10km for the mixture of
gases in the atmosphere, but H has different values for individual gases. |
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The weight of air above a surface (due to Earth’s gravity) |
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The average air pressure at sea level is
equivalent to the pressure produced by a column of water about 10 meters
(or about 76 cm of mercury column). |
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This standard atmosphere pressure is often
expressed as 1013 mb (millibars), which means a pressure of about 1 kilogram per square centimeter. |
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In the
ocean, which has an essentially constant density, pressure increases
linearly with depth. |
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In
the atmosphere, both pressure and
density decrease exponentially with elevation. |
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Air
pressure decreases with elevation. |
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If a
helium balloon 1 m in diameter is released at sea level, it expands as it
floats upward because of the pressure decrease. The balloon would be 6.7 m
in diameter as a height of 40 km. |
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Pressure must remain proportional to density (if the temperature
does not change). |
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èBoyle’s law would permit the gases of the
atmosphere to expand indefinitely (provided their density and pressure
decrease at the same rate). |
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Air
molecules in the parcel (or the balloon) have to use their kinetic energy
to expand the parcel/balloon. |
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Therefore, the molecules lost energy and slow
down their motions |
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è The
temperature of the air parcel (or balloon) decreases with elevation. |
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This law states that (1) heat is a form of
energy that (2) its conversion into other forms of energy is such that
total energy is conserved. |
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A parcel of air expands and pushes its
surroundings back loses energy in the process. |
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That
energy comes from the random motion of the air molecules in the parcel. |
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After
the expansion, the molecules move less energetically è the
expansion causes the temperature of the air to decrease. |
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The atmosphere is heated from below by sunlight. |
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The heat is moved upward in the atmosphere in
three ways: |
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Conduction: requires physical contact between
the objects that exchange heat. Very slow for gases. |
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Convection: possible only in fluid and gases in
which the flow of heat from one region to another involves the movement of
matter from one region to the other. Very efficient. |
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Radiation: is possible across empty space |
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Warm,
low-density air rises convectively and displace cooler, dense air. |
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In this
way, the convection process redistribute heat from the surface to the rest
part of the atmosphere. |
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Sunlight
heats the atmosphere from below |
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Convection occurs and brings heat upward |
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Air
parcels expands as they move upward |
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(because air pressure decreases with height) |
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Thermal
energy in the parcel is used to |
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expand the air parcel |
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Air
parcel becomes cold |
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Temperature decreases with height in the |
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atmosphere |
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By considering the Boyle’s law (about random
motion of air molecules), the barometric law (about the gravitational pull
of air molecules), and the first law of thermodynamics (about the energy
conversion), it can be shown that air temperature decrease linearly with
height. |
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This linear decreasing rate is called the
“adiabatic lapse rate”, which is about 10°C per kilometer. |
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“Adiabatic” means no heat is added or subtracted
from a parcel as it rises, expands, and cools. |
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At the end of a sunny day, warm air near the
surface, cold air aloft. |
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In the early morning, cold air near the surface,
warm air aloft. |
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The later condition is called “inversion”, which
inhibits convection and can cause sever pollution in the morning. |
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Notes