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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
breathing, but this is covered
elsewhere. Thus, the actual pressure
at a given place depends on its
height, and the temperature and
density at that point (see Density
Altitude, below).
Station pressure is the atmospheric
pressure at a particular place. Several
of these are taken, converted to sea
level pressure and marked on a map,
with the ones that are equal
connected up. The lines that join the
dots are called isobars (iso is Greek for
same), and will be 4 mb apart. The
closer the isobars are together, the
more the millibars drop per mile and
the more severe the pressure gradient
will be, so you get stronger winds
(air moves from high to low
pressure). Isobars are like contours
on a map, and make common
patterns, two of which are the low or
high, the common names for cyclone
and anti-cyclone, respectively (this has
nothing to do with the cyclones that
always seem to do severe damage to
trailer parks. Another name for a low
is a depression).
Three arrows (see below) are the
traditional way of displaying the
wind direction. The numbers in the
middle of each figure are typical
pressures (the exact position of a
system will be marked by an X).
The arrows across the isobars are
wind directions after Coriolis Force
has taken effect (see below).
The patterns are round, being circles
of isobars, and difference between
them is simple; a high pressure area
has most pressure in the middle,
whilst a low has the least. In other
words, the air in a high is descending
(and diverging), and that in a low is
ascending (and converging), so the
weather is settled in the former and
tends to instability in the latter, since
upwards movement implies clouds
forming. You might often see a
secondary low, which is a smaller one
inside a larger system, in which the
weather is more intense as it feeds
on its bigger brother, although the
winds will be lighter between them.
However, just because cloud is
mostly absent in a high, don’t expect
clear skies, as the descending air
might trap haze or smoke, leading to
a phenomenon called anticyclonic gloom
near industrial areas. Sometimes,
areas of layer cloud may also get
caught if they are below an inversion.
As an example of the influence a
high pressure area can have, a strong
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