The Atmosphere
The earth’s atmosphere surrounds it like a gaseous blanket, kept in place by the gravitational field. The
depth of the atmosphere varies from time to time, being greater in the summer than in the winter and
greater at the lower latitudes than at the higher latitudes. Heat radiating from the sun causes
atmospheric gases to expand into space and it is the interaction between this force and gravity that sets
the limits of the atmosphere.
Composition
The atmosphere is a mixture of various gases. The three of greatest importance are Nitrogen (78.09%),
Oxygen (20.95%) and Carbon Dioxide (0.03%). The remainder is made up of rare gases. The composition
of the atmosphere is remarkably constant up to approximately 300,000 feet although at high altitudes
the distance between gas molecules becomes progressively greater and collisions between particles
becomes rarer. There is a variable amount of water
vapour in the atmosphere up to about 30,000 ft. and in the lower altitudes there are also solid pollutants
which provide nuclei for condensation.
Divisions
We live in the troposphere which means “the area of change”. In the troposphere the temperature
decreases with increasing altitude at a rate of 1.98°C or 3°F/1000 ft. The troposphere extends up to 60,000
ft. over the equator but only to about 30,000 ft. over the poles. At that altitude it becomes the tropopause
where the air temperature is fairly constant between - 50 and -55°C. Above the tropopause, which is about
30,000 ft. deep, is the stratosphere which extends to
about 50 miles (80 kms.). There is no weather in the stratosphere and indeed there is little weather above
35,000 ft. One of the joys of flying is that on the dullest day one can break out into bright sunlight if
you climb high enough.
Ozone
Within the stratosphere lies the ozonosphere at 18-30 miles (30-50 km.) above the earth. The temperature
here becomes warmer (about 35°C) due to heat released when ozone is converted to oxygen by solar
radiation. Only in the last decade has the importance of the ozone layer become apparent to us all. In the
stratosphere oxygen absorbs ultraviolet (UV) radiation of 2,000 Å. And 3 molecules of oxygen are transformed into 2 molecules of ozone. If unchecked
this reaction would produce a huge amount of ozone, however ozone in turn absorbs ultraviolet light from
2,100 to 2,900 Å. and is converted back to oxygen. This balanced reaction results in the almost total
absorption of harmful ultraviolet radiation. Recently it has been found that the ozone layer is being
destroyed, particularly over the poles, by earth’s pollutants. This may give rise to the increasing
penetration of UV light causing an increase in skin cancer, cataracts and other health problems.
Ozone is a blue, unstable, toxic gas. The concentration at ground level is 0.03 parts per million
(ppm) by volume but this increases rapidly above 40,000 ft. to become a maximum of 10 parts per
million by volume at 100,000 ft. Modern supersonic aircraft fly at altitudes where this can be a problem.
In the human, acute exposure for two hours to between 0.6 and 0.8 ppm. reduces the diffusing
capacity of the lungs and slightly reduces vital capacity and forced expiratory volume. Fortunately
these effects are not permanent unless there is continual exposure. Ozone impairs night vision in
man and in human cell cultures can induce chromatic breakages identical to those produced by x-rays.
Fortunately ozone is thermally unstable and is decomposed promptly to oxygen at 400°C. This
temperature is reached by the Concorde’s air conditioning compressor circuit during climb and
cruise, neutralizing what could otherwise be a significant problem.
Atmospheric Pressure
Atmospheric pressure is the weight of the gases surrounding the earth. It is a function of height,
density and the force of gravity. At ground level it is recorded by meteorologists as 101.32 kilopascals but
many people prefer the older 14.7 lbs. per sq. inch or, as used in most medical calculations, 760 mmHg.
Atmospheric pressure decreases with altitude and at 18,000 ft. it is halved (380 mmHg) and at 33,000 ft.
quartered. It should be noted that the changes are small and gradual compared to the changes observed
going down in water. Here atmospheric pressure doubles at 33 ft! As will be noted later this is a point
of importance when dealing with fliers who are also scuba divers.
Figure 5 - Properties of the
Standard Atmosphere
The properties of the standard atmosphere showing the variation in the height of the tropopause.
Cosmic Radiation
The atmosphere is affected by both galactic and solar ionizing radiation. The former is a predictable, low
density flux of high energy particles from outside the solar system. Most of these are deflected by the
earth’s magnetic field although the protection isgreater in the equatorial regions than at the poles.
There is also some protection by the solar interplanetary
magnetic field and by the stratospheric absorption of low energy particles. Measurements of
this type of radiation have been taken from high altitude aircraft. Fortunately the annual dose is
relatively low in constantly exposed air crew. Solar
radiation is of lower energy but its production may be intense and is generally unpredictable although it
appears to reach a peak about every 11 years. The earth is well shielded by its atmosphere but the dose
may be significant in prolonged high altitude or space flight. It’s most commonly observed effect is
interference with radio and other forms of communication equipment at the time of solar flares.
Space flight will lead to more precise measurements of its long term effects.
|
