The structure of the atmosphere

What is the atmosphere made of?

Separate the nearly constant dry-air mixture from the variable ingredients that create weather.

A mixture, not a single gas

Dry air near the surface contains approximately 78% nitrogen, 21% oxygen and 0.93% argon by volume. The remainder includes carbon dioxide and other trace gases. These proportions are useful rounded values: carbon dioxide changes over time, so it is misleading to memorise an old exact concentration as a permanent atmospheric constant.

Water vapour is excluded from those dry-air percentages. Its proportion varies strongly with temperature and location, from almost none in very cold air to several percent in warm humid air. Water also appears as liquid droplets and ice. Aerosols such as dust, salt and smoke provide surfaces on which droplets and ice particles can form. Their amount and character affect cloud formation and visibility.

The main permanent dry-air gases, especially nitrogen, oxygen and argon, remain well mixed throughout the altitude range of this course. This does not imply a uniform vertical distribution of water vapour or ozone. Climbing to cruise altitude does not substantially change the percentage of oxygen in dry air; it reduces total pressure and therefore the oxygen partial pressure. A percentage and an available pressure are different quantities.

Why is most weather in the troposphere?

Connect the lower atmosphere to the surface that warms and moistens it.

The surface drives the lower atmosphere

The troposphere is the lowest main atmospheric layer. It contains most atmospheric water vapour and most of the atmosphere's mass. Evaporation supplies moisture from the surface. Surface heating, cooling, friction and terrain strongly influence its lower part. Mixing and vertical motion allow heat and moisture to be transported through it, which is why most familiar cloud, precipitation and storms develop here.

Temperature generally decreases with altitude in the troposphere. This is a description of the average environment, not a promise that every individual layer cools with height. A sounding can contain a shallow inversion, an isothermal layer or a different lapse rate while remaining within the troposphere. The boundary layer responds relatively quickly to the ground; above it, the free troposphere is less directly affected by surface friction.

A powerful thunderstorm can reach the tropopause and briefly overshoot into the lower stratosphere. The tropopause is therefore a useful weather boundary, not an impenetrable ceiling. A cruise level above surrounding ordinary clouds can still be exposed to convective overshoots and associated turbulence.

How does temperature divide the atmosphere?

Follow an environmental temperature profile instead of treating every named layer as a fixed flight level.

Use the sign of the temperature change

Starting from the surface, the troposphere has a general decrease of temperature with height. The tropopause is its transition to the stratosphere, where temperature is nearly constant initially or increases with height. In the lower stratosphere, stable stratification strongly resists ordinary convective mixing. It contains very little water vapour compared with the troposphere, so ordinary rain clouds and surface-driven convection are uncommon. It is not a guarantee of no cloud or no wind: exceptional polar stratospheric cloud, strong flow, waves and convective overshoots can occur. Farther up, absorption of solar ultraviolet radiation by ozone contributes to the stratospheric warming with height.

Up to FL650, the relevant sequence is troposphere, tropopause and lower stratosphere. The mesosphere and thermosphere are above this course's altitude range. FL650 is a pressure-altitude designation of 65,000 ft on standard setting; its actual geometric height varies with the atmospheric column.

In the International Standard Atmosphere, the tropospheric lapse rate is 6.5 K per kilometre from sea level to 11 km, where temperature is −56.5 °C. The next ISA layer is isothermal from 11 to 20 km, which includes approximately FL650. A real sounding need not reproduce that reference profile: a tropical tropopause is commonly much higher than the ISA tropopause.

Why is the tropical tropopause higher and colder?

Use latitude, season and the depth of the warm air column to read the upper atmosphere.

Latitude changes the whole column

The tropical troposphere is deep: strong heating and deep convection are associated with a high tropopause, often around 16–18 km, roughly FL520–590, with temperatures commonly near −75 to −85 °C. In polar air the troposphere is shallower, often around 8–10 km, roughly FL260–330, with tropopause temperatures commonly around −45 to −60 °C. These are broad teaching ranges, not limits to use for planning a particular flight.

It can seem surprising that the tropopause above a warm surface is colder. The tropical profile usually continues cooling through a much greater depth before reaching its upper boundary. Compare the temperature at the top of each column, not the two surface temperatures alone. Actual wind and temperature charts or a sounding give the conditions for the flight.

In many extratropical regions, the tropopause is generally higher in the warm season and lower in the cold season. The detailed seasonal cycle, especially in the tropics, also depends on the large-scale circulation; use a regional profile rather than applying one calendar rule everywhere. Warm, deep columns and upper ridges commonly have a higher tropopause than cold columns and upper troughs. Synoptic systems alter this relationship, and a surface pressure value alone cannot determine the tropopause level: the vertical temperature structure matters.

Is the tropopause a smooth shell?

Read the boundary as a set of air-mass structures, especially near strong upper winds.

Different air masses do not fit into one fixed surface

Across the boundary between a warm, deep troposphere and a cold, shallow one, tropopause height can change abruptly. Tropopause breaks, overlapping tropopauses and folds occur near strong frontal zones and jet streams. A cross-section is more informative than a single universal height. Strong horizontal temperature gradients and the associated vertical wind shear help explain why jets are found in these regions.

The tropopause normally limits the depth of ordinary convection because a rising parcel encounters a much smaller environmental lapse rate or a temperature increase. The parcel rapidly becomes colder and denser than its surroundings. This stability helps spread the top of a cumulonimbus into an anvil. Strong storm momentum can nevertheless carry an overshooting top through that boundary.

A clear region close to a tropopause break is not automatically smooth. Strong wind shear around jets can generate clear-air turbulence. Neither the absence of visible cloud nor a blank precipitation-radar display rules that out. Conversely, the presence of a jet does not mean every part of it is turbulent: position, shear, stability and current reports are relevant.

Can stratospheric air be a cabin-air hazard?

Distinguish the protective role of ozone high in the atmosphere from exposure to it inside an aircraft.

The same molecule has different consequences

Stratospheric ozone absorbs part of the Sun's ultraviolet radiation and contributes to the stratospheric temperature structure. It is not distributed uniformly. Stratospheric intrusions and tropopause folds can bring ozone-rich air to jet cruise altitudes, so exposure is not restricted to an aeroplane flying at an exceptionally high geometric altitude.

Ozone is an oxidant and can irritate eyes and the respiratory system. Cabin air drawn from outside can therefore present an exposure issue if ozone is not adequately removed. Aircraft installations may include ozone converters, subject to their certified capability and maintenance requirements. A pilot should use the aircraft's procedures and operational guidance rather than infer a safe exposure from a flight level alone.

The common trap is to reason that 'protective ozone layer' means ozone is harmless to breathe. Its ultraviolet-absorbing role occurs in the external atmosphere; its biological effects depend on inhaled concentration and exposure. This is separate from the low oxygen partial pressure associated with high-altitude unpressurised flight.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.