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tropopause with it.
So, underneath the tropopause is the
troposphere, and above it is the
stratosphere, where the temperature
remains relatively constant with
height – it decreases with height in the
troposphere, which is where weather
happens (temperature stops
decreasing at the tropopause). The
troposphere contains more than
80% of the mass of the atmosphere.
Although the air gets thinner the
higher you get, the proportions of
the gases making it up stay the same,
because of the constant mixing. If
the air wasn't continually being
stirred up, the heavier gases would
sink to the lower levels.
The atmosphere will be wetter or
drier, warmer or colder, or denser or
lighter in different areas. The key
words here are therefore humidity,
temperature, pressure, density and
radiation, as it behaves like any other
gas, and obeys all the physical laws,
such as expanding when heated, etc.
Temperature, pressure and humidity
78 JAR Private Pilot Studies
all affect density, which ultimately
affects aircraft performance.
The climate of any area is its average
weather. Weather is what happens
when the atmosphere is affected by
heat, pressure, wind and moisture,
but heat has arguably the most
effect, since changes in weather
occur when temperature changes.
Heat arises from the Sun's rays
passing through the atmosphere and
being converted to longer wave
radiation when they hit the ground.
The darker the area that is hit, the
more absorption takes place, and the
more heat is generated. Thus, any
heat in the Earth comes from the
Earth itself, and only indirectly from
the Sun. In other words, moisture in
the atmosphere acts like the glass in
a greenhouse, that lets short wave
radiation in, and long waves out.
The Seasons
We get seasons because the Earth is
not vertical in space – it is actually
inclined at an angle of 23 ½ ° so that
different areas are pointed towards
the Sun in their turn, and do it for
longer periods, hence Summer.
Pressure Patterns
A column of air above any point has
weight, which is commonly
measured in terms of millibars or
inches of mercury, and called the
atmospheric pressure. This value won’t
change with temperature, but the air
density will, as it is the weight of the
air contained inside a cubic foot. As
it gets hotter, the air in the imaginary
cube will expand, and overrun the
boundaries, leaving less air inside.
To make sure that everyone works
on the same page, a couple of typical
scientists went to a typical place on
the South coast of England many
years ago and measured the
temperature and pressure, which
turned out to be 1013.2 millibars
(29.92" of mercury) and 15°
Centigrade. This was adopted as the
standard atmosphere, and now everyone
who makes altimeters, or whatever,
calibrates them with it so you don’t
fly at the wrong levels. The pressure
actually works out to be around 15
lbs per square inch, which equates to
20 tons on the average person. In
short, ISA is a standard that provides
universal values of temperature, pressure,
density and lapse rate, by which others can
be compared.
In the standard atmosphere, ½ sea
level pressure is obtained at 18,000',
1/3 at 27,500' and ¼ at 33,700'.
Another quality of a column of air is
that it gets cooler with height, as
mentioned above, so the standard
atmosphere is also taken as
decreasing at 1.98°C per 1,000 feet,
called the standard lapse rate, which is
really an average, used for
convenience. The sea level pressure on
which it is based relates 1 inch of
mercury to 1,000 feet of altitude, so
you would expect to see an altimeter
read 1,000 feet less if you set it to
28.92. 1 millibar is equal to 30 feet.
If you were faced with a question
that asks you to compare an actual
temperature at a height with the ISA
standard, first of all, find out what
the ISA temperature should be. For
example, at 12,000 feet, it would be
15° minus the height times the lapse
rate, in this case 12 x 1.98, so 15-
23.76=-8.76, rounded to -9° for
convenience. If the actual
Weather 79
temperature were -7°, it would
therefore be warmer than ISA, +2°.
Finally, the air gets thinner with
height - at 18,000 feet, it is 50% of
its density at sea level. This also
means less oxygen, and difficulty in
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