Properties of air/Atmospheric Structure - Lecture 3

A useful mental picture of Air
Numerous molecules in a small volume of air ;
like spheres moving in great speeds ;
constantly colliding with one another;
constantly in random motions .

* Molecular collisions would thoroughly mix the different types of gas ;
This idea accounts for the observation :
same relative concentrations from the ground to 80 km.
exception: H2O, O3, N2O
because they have significant local sources & sinks in atm

A volume of air as a whole also can have a measurable movement ;
referred to as WIND

To characterize a sample of air we define three
Thermodynamic properties :
temperature (T), pressure (p), density (r)
T : a measure of the average molecular motion
p : the force that the molecules press against a surface of unit area.
r : the mass of the molecules in a unit volume

Concept of "Unit" .
necessary for quantifying any property of an object;
different "systems of units" ;
must consistently use the same system of units in scientific analysis.

MKS system, ( metric system ; base of 10 )
Basic units for 4 FUNDAMENTAL properties :
m= meter, unit of length
kg= kilogram, unit of mass
s= second, unit of time
K= Kelvin, unit of temperature

Units of DERIVED properties
m/s , unit of velocity
kg/m3, unit of density
N=kg m/s2, Newton, unit of force
Pa= N/m2, Pascal, unit of pressure;

Different units of the same thing
Velocity : mph, knot (1 m/s ~ 2 mph ~ 2 knots)
pressure : Pa, mb, "atm", "in"
Temperature : F, Fahrenheit, C Celsius , K Kelvin
How to convert among C, F and K ?
100 F = (100-32)*5/9 C
30 C = (273 + 30) K

"Law of Nature"
A statement about the nature of certain type of phenomena observed in nature ;
e.g. any two objects would pull toward one another.

no exception or else the "law" would be abandoned ;
scientists would then try to replace it with a new "Law" .

Ideal Gas Law
A gas is called "ideal" if its values of T, p, r are interrelated
by the following equation
p = R r T

R = constant value called gas constant ;
"Ideal Gas Law" provides a good approximate description
of all atmospheric gases on earth.
Each type of gas has a unique value of R.

Notion of approximation in scientific studies
is both necessary & valuable.
e.g. "Air" as a mixture of many types of gas below 150 km also collectively behaves almost like an ideal gas .

How is the air & its related properties distributed ?
Most in a relatively thin layer ;
thin compared to the earth's radius (1:500)
Why ?
Air molecules are strongly attracted downward by a FORCE
associated with the heavy solid earth; gravity

Atmospheric T,p,r vary most greatly in the vertical direction;
influence of gravity
Each has a representative (time average) vertical structure

The atmosphere has a layer-structure :
(1) based on temperature :
troposphere ( ground to 12 km )
stratosphere ( 12 to 50 km )
mesosphere (50 to 90 km)
thermosphere ( above 90 km)
separated by : tropopause (12 km), stratopause (50 km), mesopause (90 km)
T vs Z sketch

(2) based on composition :
homosphere ( between 0 and 80 km),
heteorosphere (between 80 and 500 km),
exosphere (above 500 km)
(3) based on electrical properties :
ionosphere (above 80 km ),
neutral layer below

A measure of the vertical variation of temperature in a layer: lapse rate, G
G= - D T / D z

D z = height at top of layer - height at bottom of layer
D T = temperature at top of layer - temperature at bottom of layer
Values of lapse rate
Troposphere : 6 C/km
Stratosphere : -2 C/km
Mesosphere : 2 C/km

Tropopause height in low latitude (15 km) is much higher than in high latitude (9 km) :

Pressure at the surface level is approximately equal to the weight of all the gas above;
pressure always is smaller at greater height ;
amount of gas above a certain height is always less than that above a lower height ;
pressure distribution may then be used as an alternative indication of height.

i.e. Pressure can be used as a vertical coordinate
Useful to make weather maps on a pressure surface instead of at a constant elevation.
A great advantage to do so in practice ;
meteorological data are collected at predetermined pressure levels
All weather maps, with the exception of surface map, that meteorologists use everyday are maps on pressure surfaces

Warrant to make separate consideration for water in the sky
Clouds & related matters are specially important weather elements;
A cloud contains billions of condensed/frozen water vapor ;
A cloud is visible because the water droplets & ice crystals alter the sunlight that shines on it;
Clouds themselves have different shapes , sizes & life-span;
involving a unique set of physical processes

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