The Earth is surrounded by a mixture of gases known as the atmosphere which is held in place by the force known as gravity. The mixture of the atmosphere remains constant and is found to cover the earth up to 30,000 ft at the poles and 60,000 ft at the equator. The boundary of the atmosphere is known as the tropopause.
Within the atmosphere there is normally a decline in temperature of approximately 1.98°C/1000 ft. Pressure also decreases with altitude. Cold temperature increases air density; low pressure decreases air density. Pressure change is the dominant force and as such the air density decreases with altitude. In the atmosphere, small increases in height at low altitude will cause a much greater change in pressure than the same height change at altitude.
Measurement of Atmospheric Pressure
Standard atmospheric pressure, or barometric pressure, is the weight or force exerted by the atmosphere at any given point. This pressure is expressed in different forms by the method of measurement such as pounds per square inch (psi), millimetres of mercury (HG) and inches of mercury. Millimetres of mercury (mm/HG) are used in these notes.
The Standard Atmosphere
Continual fluctuations of temperature and pressure in the atmosphere create problems for engineers and meteorologists who require a fixed standard of reference for aircraft. This standard is known as the International Standard Atmosphere (ISA).
Conditions throughout the atmosphere for all latitudes, seasons, and altitudes are averaged and published by ICAO. The resultant standard atmosphere has specified sea level temperature and pressure and specific rates of change of temperature and pressure with height.
Standard Atmosphere Pressures and Temperatures for Different Altitudes:
- Sea level 760.0 mm/HG +15°C
- 10,000 ft 522.6 mm/HG -05°C
- 18,000 ft 379.4 mm/HG -21°C
- 33,700 ft 190 mm/HG -52°C
- 40,000 ft 140.7 mm/HG -55°C
Physical Divisions of the Atmosphere
The divisions of the atmosphere are primarily physical or meteorological in nature. From meteorology we are familiar with both the troposphere and the stratosphere; both of which are important to the aviator and aviation. To look at the Physiological Effects associated with flight the atmosphere can be split into four zones:
Physiological Zone This area extends from sea level to approximately 12,000 ft. It represents the area of the atmosphere to which the human body is more or less adapted. Only minor physiological problems exist when flying within this zone. Pilots who go higher than their acclimatized levels notice common symptoms such as middle ear blockage and sinus blockage difficulties, shortness of breath, dizziness and headache. Above this zone we are in an environment to which our body is unaccustomed.
Physiological Deficient Zone Existing from 12,000 ft to 50,000 ft this zone, along with the previous zone, is the area in which most flying takes place. Oxygen deficiency becomes an ever increasing problem as we ascend due to the reduced atmospheric pressure.
Partial Space Equivalent Zone This zone extends from 50,000 ft to 120 nm, where pressure changes become very small. The problems for flight over 50,000 ft are the same as those encountered in space. Sealed cabins, pressure suits are necessary as problems now occur with blood and body fluids boiling over 63,000 ft. Gravitational changes on the body make this a space equivalent zone. Only Concorde has operated in this zone.
Total Space Equivalent Zone True space, this zone extends outwards from 120 nm. The physiological problems of this zone are similar to the previous zone.
The air is composed of a mixture of gases of nearly constant proportions:
- Oxygen 20.94%
- Nitrogen 78.08%
- CO2 0.03%
- Other gases 1%
These proportions remain the same at all levels within the troposphere and up to an altitude of 60,000 ft. ICAO has defined the standard atmosphere which assumes:
- Pressure 1013.2mb
- Temperature 15°C
- Density 1225 gm/cubic metre
The temperature lapse rate of 1.98°C/1,000 ft continues up to 36,090 ft. Above this altitude the temperature remains constant at -56.5°C.
Pressure falls 1 hPa per 30 ft gained in the lower levels of the atmosphere (acceptable in the first 5,000 ft)
A temperature change of 3°C or a 10 hPa change in pressure will change the density by 1%.