Air pressure and altitude meter
Air exerts a pressure on the surface of the earth under the influence of gravity. The
amount of pressure depends on the quantity of air above the respective level. The
pressure reduces as the altitude increases. As air can be compressed, the change in
pressure is not linear to the increase in altitude. When standing at an altitude of 3000
meters (9840 ft) above sea level you already have approximately one third of the earth's
atmosphere beneath you, at an altitude of 5500 ((18040 ft) meters the air pressure has
reduced by half and at 11 000 meters (36080 ft) the pressure is again reduced by half, i.e.
it is just a quarter of the original value.
Höhe = altitude
Höhe nimmt zu – increase in altitude
Druck nimmt ab – drop in pressure
Druck – pressure
Höhe in km – altitude in km
Fig. 3 Decrease of pressure in the earth's atmosphere
The air pressure does not drop uniformly with the increase in altitude
The amount of oxygen in the air also reduces parallel to the drop in air pressure.
Therefore, at an altitude of 5500 meters (18040 ft), humans only intake half the amount of
oxygen than they would at sea level. In mountainous areas it is therefore necessary to
compensate this oxygen shortage by breathing more quickly. The reduced amount of
oxygen in the air at high altitudes leads to a definite reduction in a person’s physical and
mental performance, particularly if he or she is not well acclimatized. The altitude at which
such a reduction occurs, varies from person to person. Some mountaineers walk
excellently in lower regions, suffer a severe setback at altitudes above 3000 meters (9840
ft) and completely give up at heights in excess of 4000 meters (13120 ft).
The weight of the air can be measured using a barometer. Torricelli (Italian physicist of the
17th century) was the first to demonstrate that air, under normal conditions and at sea
level, has the same weight as a mercury column of the same cross section, measuring 760
millimeters in length. It was therefore a matter of course to consider the length of the
mercury column as a measure for air pressure. As it is illogical, from a physical point of
view, to specify pressure as a measurement of length, the millibar or hectopascal were
introduced by the field of meteorology some time ago, as units for measuring the air
pressure. With some delay, these units are now gradually being found in barometers and
altimeters available in shops today. In a few years time we can expect the length of a
mercury column (abbreviated with mm Hg or Torr) to be replaced with millibars
(abbreviation mb or mbar) or hectopascal (abbreviation hPa). Nevertheless, it is still
important to understand the relationship between these commonly used pressure units.
This is: 760 mm Hg or Torr = 1013.25 mb (mbar) = 1013.25 hPa
The altimeter is a practical device used by many mountaineers, which also indicates
changes in barometric pressure caused by the weather. If the user remains in one place
for a while, the tendency of the air pressure provides important information on the
development of the weather (see section on weather rules P. 94). For this purpose it is
important to note the following:
-A drop in air pressure indicates a rise in altitude,
-A rise in air pressure indicates a decrease in altitude.
The exact definition of altitude provided by the altimeter can be of vital importance to
mountain climbers out in bad weather with poor visibility.
Using the altimeter
To use the altimeter properly it is essential to strictly observe several rules. The altitude
indicated could otherwise be just an approximate value and not good enough to provide
accurate bearings under critical visibility conditions. If you have an altimeter, you must be
able to rely on it. It is therefore a good idea for every user to check that the instrument is in
correct working order every now and then. For this purpose it is necessary to measure the
altitude of two points in the terrain, which are situated close to each other and which are
specified exactly in the map. If the difference between the altitudes given in the map and
the reading of the altimeter is more than ten meters, it is advisable to have the instrument
tested by an expert.
Fig. 4 The altimeter reacts to changes in the air pressure. It is therefore able to provide experienced mountain climbers with a reliable
weather forecast, when also paying additional attention to the cloud development. The altimeter should therefore always be part of the
mountain climber's essential equipment.
To obtain accurate information with a tolerance of max. approx. +/-10 meters, the following
rules must always be observed:
1. To determine the altitude always use the altitude given in the map as much as
possible, instead of the altimeter. Without a map for adjustment, the altimeter can only
serve as a barometer and its use is strictly limited.
2. Always check the altitudes given by the altimeter using points given in the map. Repeat
this time and time again. These points should, if possible, not vary by more than 500
meters in height, and not be more than 2 to 4 kilometers apart.
3. Also make sure not to carry the instrument under a jacket (where it is subject to high
temperatures and humidity), but to keep it in a bag or rucksack. In addition to this,
always take two readings after tapping gently on the dial (needle play) and then take
the average value. This is not necessary with electronic altimeters.
Correcting the altimeter reading
If, when walking, it is not possible to readjust the altimeter using points which differ by less
than 500 meters in altitude, it is essential to carry out a correction in order to provide
accurate readings. Incorrect altitude readings can also occur when the temperature has an
influence on the density of the air and thus on the vertical pressure gradient. When
climbing in hot air the decrease in pressure is slower than in the standard atmosphere on
which the calibration of the altimeter is based. In cold air the decrease in pressure is
faster.
Standard atmosphere:
Altimeters are calibrated on the basis of the standard atmosphere. This specifies the
pressure and the temperature available at a certain altitude. In the standard atmosphere,
at sea level, the air pressure is 1013.25 hectopascal (or millibar) and the temperature is
15°C. It has furthermore been specified that the decrease in temperature is 0.65 degrees
every 100 meters of the ascent.
An altimeter can therefore only indicate the exact difference in altitude in the atmosphere
which has been taken as a basis for the calibration. Should the actual atmosphere not
coincide with the standard atmosphere, the altitude reading must be corrected. This
correction depends on the difference between the average temperature in the air column
climbed and that which would be present in the standard atmosphere. The actual
temperature can be measured using a thermometer which the climber has with him and
the temperature of the standard atmosphere at a certain height can be easily calculated. If
the altitude reading is to be corrected it is therefore important to know whether, after
passing through an air column, the air pressure decreased faster of less fast than in the
corresponding layer in the standard atmosphere.
Case
1:
The average value of the temperature in the air layer
passed through is greater than the average value in the
standard atmosphere:
In this case, the difference in altitude indicated by the
altimeter must be increased by 4 per mil every degree
centigrade of the difference between the two average
values. This increase is necessary because the actual air
pressure decreases more slowly than in the standard
atmosphere.
Case
2:
The average value of the temperature in the air layer
passed through is less that the average value in the
standard atmosphere: In this case the air pressure in the air
column passed through decreases more quickly than in the
standard atmosphere. For this reason it is necessary to
reduce the reading of the difference in altitude by 4 per mil
every degree centigrade of the difference between the two
average values.
Example for the correction of an altimeter reading (in case 1)
Altitude of starting point
3000 m (according to map)
Altitude of point of arrival
4000 m (according to altimeter)
Difference in altitude
1000 m
Measured temperature at 3000 m
8° C
Measured temperature at 4000 m
0° C
Standard temperature at 3000 m
15-(30 x 0.65)
= approx. -5°C
Standard temperature at 4000 m
15 -(40 x 0.65)
= ca. -11°C
Average temp. in the air column passed through
(0 + 8) :2
= 4°C
Average temperature in the standard atmosphere
(-5 + [ -11]) :2
= -8°C
Difference between the two average temperatures
12 °C
Correction
12 x 0.004 x 1000
= 48 m
Corrected difference in altitude
1000 + 48
= 1048 m
Corrected altitude at point of arrival
3000 + 1048
= 4048 m
Correction of the altimeter reading can be omitted when the difference between the
average values of the air column passed through and the standard atmosphere is less
than 5 °C or if the difference in altitude is less than 500 m.
The calculation given above is quite troublesome and is extremely difficult to carry out
when up on a mountain in strong wind and under low temperature conditions. It is
therefore better to avoid this step by using the map to frequently readjust the altimeter.
You should, however, always be aware of the fact that a correction has to be carried out if
the altitude reading needs to be as accurate as possible.
Additional remark
The significance played by the altimeter in obtaining one's bearings has recently become
less. More and more mountain climbers are also using pocket receivers for the satellites of
the American Global Positioning Systems (GPS). These instruments weigh less than half a
pound and, when handled correctly, provide an extremely reliable determination of one’s
position. With the GPS receivers available on the market today, it is possible to determine
one’s actual position right down to an accuracy of +/-10 meters. These devices are,
however, less suitable for the determination of the actual altitude itself. For this reason, a
good altimeter will continue to be an important piece of equipment for mountaineers.