CASANOVA COLLECTION?

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.  Since 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.

Fig. 3 Decrease of pressure in the earth's atmosphere
The air pressure does not drop uniformly with the increase in altitude

H?e = altitude
H?e nimmt zu – increase in altitude
Druck nimmt ab – drop in pressure
Druck – pressure
H?e in km – altitude in km

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.