Chapter 2: The Atmosphere | Human Performance & Limitations

§ 1The Atmosphere

Human beings live their lives in the lower reaches of the atmosphere where temperatures, pressures and oxygen supply are able to support life. The moment we climb, every one of those three variables shifts against us. To fly safely, you must first understand the layer you're flying in and what it is made of.

1.1 The Layers — Troposphere & Stratosphere

Troposphere – Average Top

13 km · 8.1 mi · 43,000 ft

Stratosphere – Reaches Above

100,000 ft

Where Almost All Weather Lives

Troposphere

Where Air is Densest

Lowest Layer (Surface)

The Troposphere height varies. On average it stretches from the Earth's surface to about 13 km (8.1 mi; 43,000 ft). The Stratosphere reaches up to over 100,000 ft. The troposphere contains almost all the weather, the air is densest in this lowest layer, and in fact the troposphere contains three-quarters of the mass of the entire atmosphere.

Figure 2.2 — Vertical structure of the atmosphere: the troposphere holds ¾ of all atmospheric mass and nearly all weather.

1.2 Constituent Gases of the Atmosphere

The earth's atmosphere near the surface is composed primarily of Nitrogen and Oxygen. Together, the two comprise about 99% of the gas in the atmosphere. The remaining 1% is made up of Argon plus traces of other gases.

Composition of the lower atmosphere — exact percentages
Gas	Percentage	Gas	Percentage
Nitrogen (N₂)	78.084 %	Oxygen (O₂)	20.95 %
Argon (Ar)	0.934 %	Carbon Dioxide (CO₂)	0.036 %
Neon (Ne)	0.0018 %	Helium (He)	0.0005 %
Methane (CH₄)	0.00017 %	Hydrogen (H₂)	0.00005 %
Nitrous Oxide (N₂O)	0.00003 %	Ozone (O₃)	0.000004 %

In addition, water vapour is variable but typically makes up about 1 – 4 % of the atmosphere.

Critical principle to remember The relative proportions of these gases remain CONSTANT in the Troposphere and Stratosphere. What changes with altitude is not the percentage of oxygen — it is the total pressure, and therefore the partial pressure of each gas. Oxygen continues to make up 21 % of the air by volume at every altitude; only the partial pressure of oxygen, pO₂, falls.

Mnemonic"Never Ought An Cool Naval Helmsman Make His Nasty Owl" — Nitrogen, Oxygen, Argon, Carbon dioxide, Neon, Helium, Methane, Hydrogen, Nitrous oxide, Ozone — in descending order of abundance. (Optional aid — feel free to invent your own.)

§ 2The Gas Laws

The body responds to barometric pressure changes in temperature, pressure, and volume. These changes are rapid and continuous in the aviation environment. It is therefore essential to know the implication of these changes on our body and take preventive measures to counter them. The gas laws explain to us the science behind what goes on within our body when exposed to changes in pressure and temperature.

Figure 2.3 — The five gas laws of flight and what each one explains in the cockpit.

2.1 Boyle's Law

Definition At a constant temperature, a given volume of gas is inversely proportional to the pressure surrounding the gas. The volume of gas expands as the pressure surrounding the gas is reduced.

As altitude increases → gas expands.
As altitude decreases → gas compresses.
The amount of volume expansion is limited by the pliability of the structure or membrane which encloses the gas.

Examples in aviation & medicine PASG / Air Splints · Respiratory Rate & Depth changes · Flow rates of IV sets · ETT or Tracheal cuff pressures · Trapped gas effects within the body (middle ear, sinuses, GI tract — every pilot has felt these).

Boyle's Law — gas expands as pressure falls — Figure 2.4 — Boyle's Law: as pressure halves, a fixed mass of gas doubles in volume. Trapped gas in sinuses, ears, gut and fillings expands as you climb.

2.2 Henry's Law

Definition The amount of gas in solution is proportional to the partial pressure of that gas over the solution. As the pressure of the gas above a solution increases, the amount of that gas dissolved in the solution increases; the reverse is also true — as the pressure of the gas above a solution decreases, the amount of gas dissolved decreases and forms a "bubble" of gas within the solution.

In normal physiologic function, this law can be seen in the transfer of gas between the alveoli and the blood. This is significant physiologically for the occurrence of evolved gas disorders, e.g. decompression sickness. It explains the hypoxia experienced with increasing altitude — as the pressure of gases is reduced with ascent, the amount of gases dissolved in solution decreases, and this leads to hypoxia and may lead to nitrogen bubble formation.

Everyday example Bottle of soda. With the cap on, the gas within the solution is at equilibrium. With the cap removed, the gas pressure decreases and bubbles are released into the solution. Your blood does exactly the same thing on a sudden cabin depressurisation.

Clinical link — DCS (decompression sickness) A pilot who has been SCUBA diving cannot climb shortly after surfacing because Henry's Law is still working against them — dissolved nitrogen in their blood will come out of solution as bubbles when ambient pressure falls. (Strict wait-times before flying after diving are covered in a later section.)

2.3 Charles' Law

Definition The pressure of a gas is directly proportional to its temperature with the volume remaining constant. Temperature increases make gas molecules move faster, greater force is exerted, and volume expands.

The law explains:

The temperature changes associated with rapid decompression.
The pressure changes that induce temperature changes with an oxygen cylinder (a freshly-charged O₂ bottle feels warm; a discharging one feels cold).

Classic illustration Shaving cream can be placed into a fire — heat raises the internal pressure of the can until the structure fails. Same physics governs the rapid heating of an oxygen cylinder during fast filling.

2.4 Graham's Law — Law of Gaseous Diffusion

Definition Gases diffuse or migrate from a region of higher concentration (or pressure) to a region of lower concentration (or pressure) until equilibrium is reached. The physiological significance is in the explanation of gas exchange.

SOP — How your lungs use this law Oxygen moves from the alveoli into the blood, and from the blood into the tissues due to this phenomenon. The whole architecture of the respiratory system relies on Graham's Law for survival.

2.5 Dalton's Law — Law of Partial Pressures

Definition Partial pressures describe the distribution of certain gases in a mixture and follows Dalton's law which states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the gases which compose the mixture. As altitude increases — gases exert less pressure. This explains the hypoxia that occurs with flight to higher altitudes.

Worked example — Oxygen at different altitudes (Dalton's Law in action)

At Sea Level: O₂ = 21 % & pO₂ = 21 % × 760 mm = ≈ 160 mmHg

At 8,000 ft: O₂ = 21 % & pO₂ = 21 % × 565 mm = ≈ 119 mmHg

The barometric pressure at 36,000 ft is one-fourth of that at sea level. Hence the quantity of oxygen available is proportionately low. Whatever the air pressure, oxygen continues to make up 21 % of the air by volume. In other words, the proportion of oxygen in the air always stays the same whatever the altitude. The partial pressure of Oxygen decreases with altitude as does the total pressure of air.

Exam-critical takeaway The percentage of O₂ in air does NOT change with altitude — it is always 21 %. What kills the pilot is the falling partial pressure of that oxygen, because diffusion across the alveolar membrane depends on pressure gradient, not percentage.

Quick-Reference — The Five Gas Laws of Aviation Physiology
Law	One-line Statement	What it Explains in Flight
Boyle's	P × V = constant (at fixed T) — pressure ↑ → volume ↓	Trapped-gas pain in ears, sinuses, teeth, GI tract; tracheal-cuff & IV behaviour
Henry's	Gas dissolved ∝ partial pressure over the solution	Decompression sickness, hypoxia mechanism, alveolar gas transfer
Charles'	P ∝ T (at fixed V) — warming raises pressure	Rapid-decompression temperature drop; O₂ cylinder heating
Graham's	Gases diffuse from high → low concentration	O₂ from alveoli → blood → tissues; CO₂ in opposite direction
Dalton's	Total P = Σ partial pressures	Why high-altitude hypoxia happens despite air being "21% oxygen"

Human Performance & Limitations · Module A — The Ocean of Air The Atmosphere