The thin envelope of
air that surrounds our planet is a mixture of gases, each with its
own physical properties. The mixture is far from evenly divided. Two
elements, nitrogen and oxygen, make up 99% of the volume of air. The
other 1% is composed of "trace" gases, the most prevalent of which
is the inert gaseous element argon. The rest of the trace gases,
although present in only minute amounts, are very important to life
on earth. Two in particular, carbon dioxide and ozone, can have a
large impact on atmospheric processes.
Another gas, water
vapour, also exists in small amounts. It varies in concentration
from being almost non-existent over desert regions to about 4% over
the oceans. Water vapour is important to weather production since it
exists in gaseous, liquid, and solid phases and absorbs radiant
energy from the earth.
Structure of the
The atmosphere is
divided vertically into four layers based on temperature: the
troposphere, stratosphere, mesosphere, and
thermosphere. Throughout the Cycles unit, we'll focus
primarily on the layer in which we live - the troposphere.
The word troposphere
comes from tropein, meaning to turn or change. All of the
earth's weather occurs in the troposphere.
The troposphere has
the following characteristics.
It extends from the
earth's surface to an average of 12 km (7 miles).
The pressure ranges
from 1000 to 200 millibars (29.92 in. to 5.92 in.).
generally decreases with increasing height up to the tropopause
(top of the troposphere); this is near 200 millibars or 36,000 ft.
averages 15°C (59°F) near the surface and -57°C (-71°F) at the
The layer ends at
the point where temperature no longer varies with height. This
area, known as the tropopause, marks the transition to the
Winds increase with
height up to the jet stream.
concentration decreases with height up to the tropopause.
The air is much
drier above the tropopause, in the stratosphere.
The sun's heat
that warms the earth's surface is transported upwards largely by
convection and is mixed by updrafts and downdrafts.
The troposphere is
lower density of molecules higher up would not give us enough
Interactions - Atmosphere and
In the Cycles overview, we learned
that water is an essential part of the earth's system. The oceans
cover nearly three-quarters of the earth's surface and play an
important role in exchanging and transporting heat and moisture in
- Most of the water vapour in the
atmosphere comes from the oceans.
- Most of the precipitation falling
over land finds its way back to oceans.
- About two-thirds returns to the
atmosphere via the water cycle.
You may have figured out by now that
the oceans and atmosphere interact extensively. Oceans not only act
as an abundant moisture source for the atmosphere but also as a heat
source and sink (storage).
The exchange of heat and moisture
has profound effects on atmospheric processes near and over the
oceans. Ocean currents play a significant role in
transferring this heat poleward. Major currents, such as the
northward flowing Gulf Stream, transport tremendous amounts of heat
poleward and contribute to the development of many types of weather
phenomena. They also warm the climate of nearby locations.
Conversely, cold southward flowing currents, such as the California
current, cool the climate of nearby locations.
Practically all of the
energy that reaches the earth comes from the sun. Intercepted first
by the atmosphere, a small part is directly absorbed, particularly
by certain gases such as ozone and water vapor. Some energy is also
reflected back to space by clouds and the earth's surface.
transferred between the earth's surface and the atmosphere via
conduction, convection, and radiation.
is the process by which heat
energy is transmitted through contact with neighbouring molecules.
Some solids, such as
metals, are good conductors of heat while others, such as wood, are
poor conductors. Air and water are relatively poor conductors.
Since air is a poor
conductor, most energy transfer by conduction occurs right at the
earth's surface. At night, the ground cools and the cold ground
conducts heat away from the adjacent air. During the day, solar
radiation heats the ground, which heats the air next to it by
transmits heat by
transporting groups of molecules from place to place within a
substance. Convection occurs in fluids such as water and air, which
In the atmosphere,
convection includes large- and small-scale rising and sinking of air
masses and smaller air parcels. These vertical motions effectively
distribute heat and moisture throughout the atmospheric column and
contribute to cloud and storm development (where rising motion
occurs) and dissipation (where sinking motion occurs).
To understand the
convection cells that distribute heat over the whole earth, let's
consider a simplified, smooth earth with no land/sea interactions
and a slow rotation. Under these conditions, the equator is warmed
by the sun more than the poles. The warm, light air at the equator
rises and spreads northward and southward, and the cool dense air at
the poles sinks and spreads toward the equator. As a result, two
convection cells are formed.
Meanwhile, the slow
rotation of the earth toward the east causes the air to be deflected
toward the right in the northern hemisphere and toward the left in
the southern hemisphere. This deflection of the wind by the earth's
rotation is known as the Coriolis effect.
Radiation is the transfer of heat energy without the
involvement of a physical substance in the transmission. Radiation
can transmit heat through a vacuum.
Energy travels from
the sun to the earth by means of electromagnetic waves. The shorter
the wavelength, the higher the energy associated with it. This is
demonstrated in the animation below. As the drill's revolutions per
minute (RPMs) increase, the number of waves generated on the string
increases, as does the oscillation rate. The same principle applies
to electromagnetic waves from the sun, where shorter wavelength
radiation has higher energy than longer wavelength
Most of the sun's
radiant energy is concentrated in the visible and near-visible
portions of the spectrum. Shorter-than-visible wavelengths account
for a small percentage of the total but are extremely important
because they have much higher energy. These are known as
In the homosphere each
gas exerts a partial pressure, the product of the total atmospheric
pressure and the concentration of the gas. Thus as oxygen represents
about 21% of the composite gases, the partial pressure of oxygen is
about 21% of the atmospheric pressure at any altitude within the
Interpolating from the
pressure gradient graph above, oxygen partial pressure at selected
altitudes is shown below. The decreasing partial pressure of oxygen
as an aircraft climbs past 10 000 – 12 000 feet has critical effects
on aircrew; the maximum exposure time for a fit person, without
inspiring supplemental oxygen, is shown in the right hand column.
Exposure beyond these times leads to unconsciousness.
The average density of
dry air in temperate climates is about 1.225 kg/m³ at mean sea
level, decreasing with altitude.
There are several gas laws
and equations which relate the temperature, pressure, density and
volume of a gas. However the equation most pertinent to aeronautical
needs is the equation of state:
r = P/RT
r (the Greek letter
rho) = density in kg/m³
P = the static air
pressure in hectopascals
R = the gas constant
T = the temperature in Kelvin units =
°C + 273
We can calculate the
ISA standard sea level air density, knowing that standard sea level
pressure = 1013 hPa and temperature = 15 °C or 288 K
i.e. Air density = 1013 / (2.87 × 288) = 1.225
However if the air temperature happened to be 30 °C or
303 K at the same pressure then density would = 1013 / (2.87 × 303)
= 1.165 kg/m³ or a 5% reduction.
By restating the equation of state: P
= RrT it can be seen that if density remains constant, pressure
increases if temperature increases.
The ICAO International Standard
Civil Aviation Organisation's International Standard Atmosphere [
ISA ] provides a fixed standard atmospheric model used for many
purposes among which are the uniform assessment of aircraft
performance and the calibration of some aircraft instruments. The
model is akin to the average condition in mid-latitudes but contains
the following assumptions:
- dry air is assumed throughout the
- the mean sea level pressure =
- the msl temperature = 15 °C [288
- the tropopause is at 36 090 feet
[11 km] and the pressure at the tropopause = 226.3 hPa
- the temperature lapse rate to 36
090 feet = 6.5 °C per km or nearly 2 °C per 1000 feet
- the temperature between 36 090
and 65 600 feet [20 km] remains constant at –56.5 °C.
The table below shows a few values
derived from the ISA. Those pressure levels noted with a flight
level designator are standard pressure levels used for aviation
weather purposes, particularly thickness charts.
Not immediately apparent from the ISA table
is that the pressure lapse rate is about one hPa per 30 feet up to
the 850 hPa level, then slowing to 40 feet per hPa at the 650 hPa
level, 50 feet at the 450 hPa level, 75 feet at the 300 hPa level
and so on, however, this provides a useful rule of thumb:
Rule of Thumb
altitude change of 30 feet per hPa can be assumed for operations
below 10 000 feet."
sea level pressure and altimeter setting
pressure is the
actual atmospheric pressure at the elevation of the observing
pressure corrected to the official airfield elevation. An altimeter
set to the particular airfield QFE reads zero when an aircraft is on
the ground (strictly the height of the altimeter above the ground).
In the circuit, the height indicated is the height above official
pressure 'reduced' to mean sea level, assuming ISA temperature
profile from the station/airfield to MSL. An altimeter set to the
airfield QNH reads the elevation of the airfield when on the
The pressure chart
shows the distribution of atmospheric pressure. Pressure systems -
depressions (LOW pressure regions) and anticyclones (HIGH pressure)
are marked and Isobars are drawn on the chart to link areas
with the same pressure. Isobar lines are drawn at 4mB interval (4
HPa) and weather frontal systems are marked using standard symbols.