Composition and Structure of the Atmosphere Questions and Answers

Answer: A

Nitrogen makes up about 78 per cent of dry air by volume, oxygen about 21 per cent and argon just under 1 per cent, leaving all other gases to share the remaining fraction. Nitrogen is chemically inert under most atmospheric conditions, which is why it has accumulated over geological time while more reactive gases have been cycled through the surface reservoirs. Committing the three leading percentages to memory is worthwhile because they anchor every calculation of the mean molecular mass and the specific gas constant of air.

Answer: B

Oxygen constitutes 20.95 per cent of dry air by volume, close enough to 21 per cent for practical purposes. Its concentration is maintained by photosynthesis against the losses to respiration, combustion and oxidation of crustal minerals. The other values quoted belong to nitrogen, argon and carbon dioxide respectively, and mixing them up is a common slip that a moment's attention to the ordering prevents. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

Argon makes up 0.93 per cent of dry air, an order of magnitude more than carbon dioxide, and it is entirely inert. Nearly all of it is argon-40 produced by the radioactive decay of potassium-40 in the crust, which is why the terrestrial atmosphere is so much richer in argon than the cosmic abundance would suggest. Water vapour is excluded by the specification of dry air, and neon is present in far smaller amounts. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Nitrogen, oxygen and argon are described as permanent because turbulent mixing keeps their proportions uniform throughout the homosphere. Water vapour, carbon dioxide and ozone are the principal variable gases. The terms permanent and trace answer different questions, one about variability and the other about abundance, and a gas can be both permanent and present only in trace amounts. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Water vapour ranges from almost nothing over a cold desert to about 4 per cent by volume in the humid tropics, a variation no other constituent approaches. Its variability is what makes atmospheric thermodynamics interesting, since the phase changes carry enormous quantities of latent heat. Nitrogen, oxygen and argon keep constant proportions, and although all constituents thin with height their relative abundances do not change within the homosphere.

Answer: B

The troposphere contains about 80 per cent of the atmospheric mass and almost all of the water vapour, and its temperature falls with height at an average of about 6.5 kelvin per kilometre. Its depth varies from about 8 kilometres at the poles to about 17 in the tropics. Virtually all weather occurs within it, which follows from its containing the moisture and from its lapse rate permitting convection. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

The stratosphere extends from the tropopause to about 50 kilometres, and its temperature increases with height because ozone absorbs solar ultraviolet radiation. That temperature inversion makes it very stably stratified and suppresses vertical mixing, which is why volcanic aerosol injected into it persists for years. The layers run troposphere, stratosphere, mesosphere and thermosphere in order of increasing altitude. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Absorption of ultraviolet radiation by ozone heats the upper stratosphere, so the temperature rises from about 217 kelvin at the tropopause to near 270 kelvin at the stratopause. The resulting strong stability suppresses vertical motion. Note that the temperature maximum lies well above the ozone concentration maximum, because the ultraviolet is absorbed higher up where the radiation is still intense. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

The stratopause near 50 kilometres is the temperature maximum separating the stratosphere from the mesosphere above. Temperatures there approach 270 kelvin, comparable with the surface, though the pressure is only about one hectopascal. The four pauses, at roughly 11, 50, 85 and beyond, each mark a reversal in the sign of the temperature gradient.

Answer: B

The mesopause near 85 kilometres reaches about 130 kelvin in the summer polar region, the coldest temperatures found anywhere in the Earth's atmosphere. That extreme cold is produced by the wave driven circulation forcing ascent there, not by radiative processes. It is where noctilucent clouds form, and their existence is direct evidence of the temperature.