The atmosphere & ISA

C. Upper stratosphere

Nacreous (mother-of-pearl) clouds are thin iridescent clouds in the stratosphere, around 20-30 km, seen mostly in high latitudes. Do not confuse them with noctilucent clouds, which are higher still, in the mesosphere.

B. 2 °C per 1000 ft (6.5 °C per km)

The ISA lapse rate is 6.5 °C per km, which is 1.98 - call it 2 - °C per 1000 ft, up to the tropopause at 11 km. 3 °C per 1000 ft is the dry adiabatic rate, a different thing.

A. 16-18 km, and it is the coldest tropopause on earth

The correct answer is: 16-18 km, and it is the coldest tropopause on earth.

B. -5 °C

ISA falls about 2 °C per 1000 ft (1.98). From +15 °C at MSL: 15 - 20 = -5 °C at 10 000 ft.

C. 500 hPa

18 000 ft is roughly 500 hPa - half the sea-level pressure. Keep the set: 850 hPa about 5000 ft, 700 hPa about 10 000 ft, 500 hPa about 18 000 ft, 300 hPa about 30 000 ft.

B. Tropopause

Each boundary takes the name of the layer below it with "-pause" added. The top of the troposphere is therefore the tropopause; the stratopause caps the stratosphere at about 50 km.

B. The layer separating the troposphere from the stratosphere

The tropopause is the lid of the troposphere - the thin transition layer where the normal fall of temperature with height stops and the stable stratosphere begins.

C. Poles

Cold polar air gives a shallow troposphere, so the tropopause sits at about 8-10 km over the poles against about 16-18 km over the equator.

B. 11 km

The ICAO Standard Atmosphere puts the tropopause at 11 km (36 090 ft), with a temperature of -56.5 °C. It is an average, mid-latitude value - the real tropopause is higher over the equator and lower over the poles.

B. -25 °C

15 - (20 x 2) = -25 °C. Learn the pattern: every 5000 ft is 10 °C, so FL100 is -5, FL200 -25, FL300 -45.

A. -8 °C

FL160 to FL090 is 7000 ft lower. At 2 °C per 1000 ft that is 14 °C warmer: -22 + 14 = -8 °C. Going down, it gets warmer.

B. -6 °C

3000 ft lower x 2 °C per 1000 ft = 6 °C warmer: -12 + 6 = -6 °C.

B. 400 hPa

24 000 ft is about 400 hPa (ISA 393 hPa). Between the anchors 500 hPa at 18 000 ft and 300 hPa at 30 000 ft.

C. In the lower troposphere

Warm air holds far more water vapour than cold air, and the air is warmest near the surface, so the vapour is concentrated in the lower troposphere. The stratosphere is almost dry.

Our independent check found that more than one option can be argued for, as worded in the recall. The key gives the answer most references support.

C. Mixing due to turbulence

Below about 80 km the air is constantly stirred by turbulence and convection, so nitrogen, oxygen and argon stay in the same proportions all the way up. Gravity on its own would sort the gases by weight - that only happens higher up, in the heterosphere, where the mixing stops.

A. Weather is mainly confined below this level

Almost all the water vapour, cloud and weather lives in the troposphere. The tropopause also marks where jet streams and clear-air turbulence are found, so its height tells you where the weather stops and where to watch for CAT.

B. Mesosphere

Noctilucent clouds are the highest clouds of all, near the mesopause at about 80 km. They shine after sunset because at that height they are still lit by the sun.

B. 15 °C and 1013.25 hPa

ISA at MSL: temperature +15 °C, pressure 1013.25 hPa, density 1.225 kg/m3. Everything else in the ISA - and every altimeter - is built from these numbers.

A. ISA -10 °C

ISA at FL200 is -25 °C. Actual minus ISA = -35 - (-25) = -10, so the air is 10 °C colder than standard: ISA -10.

B. FL100

700 hPa sits at about 10 000 ft (roughly 3 km), i.e. about FL100.

B. 20-25 km

The ozone layer lies in the stratosphere with its maximum concentration around 20-25 km. Its absorption of the sun's ultraviolet is what warms the stratosphere with height.

B. Equator

Strong surface heating and convection over the equator push the tropopause up to about 16-18 km, against about 8 km over the poles. So the troposphere is deepest over the equator.

C. 0.03%

Carbon dioxide is a tiny fraction of the atmosphere - about 0.03 % by volume - yet it is an important greenhouse gas.

B. 11 km

In middle latitudes the tropopause sits at about 11 km, between the equatorial 16-18 km and the polar 8 km - which is also the height the ISA uses.

A. Is almost constant

In the lower stratosphere the temperature stays almost constant with height (isothermal), before it starts to rise with height higher up because ozone absorbs solar ultraviolet.

A. 78 %

Dry air is about 78 % nitrogen, 21 % oxygen and 0.93 % argon, with a tiny amount of carbon dioxide and trace gases.

B. The fall of temperature with height stops

Through the troposphere the temperature falls with height. At the tropopause that fall stops and the temperature becomes nearly constant (and higher up in the stratosphere it rises). That change in lapse rate is how the tropopause is identified.

B. Latitude only

The tropopause is highest over the warm equator (about 16-18 km) and lowest over the cold poles (about 8-10 km). A warmer surface gives a deeper troposphere, so the height changes with latitude (and season), not with longitude.

B. Terrestrial radiation heating the air from below; ozone absorbing solar ultraviolet

The troposphere is heated from the bottom by the earth, so it is warmest at the surface and cools with height. The stratosphere is heated from within - ozone absorbs the sun's ultraviolet - so it warms with height.

A. 5.5-6 km (18 000-20 000 ft)

Pressure halves by about 18 000 ft (500 hPa is roughly 5.5 km). Since pressure is the weight of the air above, half the atmosphere's mass is below that level - and about three-quarters is inside the troposphere.

A. 10 °C warmer than ISA

ISA at 10 km = 15 - (6.5 x 10) = -50 °C. The actual -40 °C is 10 °C warmer than standard.

C. 16–16.5 km

Over India the tropical tropopause is high and cold, at about 16-16.5 km. It extends from the equator to about 35-40 degrees latitude, where it breaks and gives way to the lower polar tropopause.

A. Troposphere

The troposphere is the lowest layer, from the surface to the tropopause. Almost all the weather and water vapour are found in it.

A. Greenhouse gases

Water vapour, carbon dioxide and methane absorb the earth's long-wave radiation and re-radiate part of it downwards. This greenhouse effect heats the troposphere from below and gives it its lapse rate.

C. 10 000 ft

Above about 10,000 ft the oxygen partial pressure is low enough that judgement and night vision start to suffer, so supplemental oxygen is needed above that level.

A. 2°C/1000 ft

The Jet Standard Atmosphere assumes a lapse rate of 2 °C per 1000 ft with no tropopause, so it keeps cooling at the same rate all the way up. It is used for jet performance data.

B. Heat from the earth's surface

Air is almost transparent to incoming sunlight. The sun heats the ground, and the ground then heats the air in contact with it by conduction, convection and its own long-wave radiation - so the atmosphere is heated from below.

A. Warm

Carbon dioxide and water vapour let sunlight through but absorb the long-wave radiation the earth sends back and re-radiate part of it downwards - the greenhouse effect that keeps the lower atmosphere warm.

A. 75 %

Air is compressed by the weight above it, so it is densest near the surface: about three-quarters of the atmosphere by mass, and nearly all its water vapour, lies in the troposphere.

C. Stratosphere

In the stratosphere the temperature is constant or rises with height (an inversion), so a rising parcel is always colder than its surroundings - the layer is very stable and there is little vertical motion or weather.

A. Troposphere

Water vapour is evaporated from the surface and carried up by convection, and the air can hold less of it the colder it gets. So it is concentrated in the lowest layer - above about 30 000 ft there is very little left. That is why effectively all cloud and weather is a tropospheric business.

B. Decreases with increasing latitude

A warm surface gives a deep troposphere. Over the hot equator the tropopause is pushed up to about 16-18 km; over the cold poles it sits at only about 8-10 km. So as you go polewards - increasing latitude - the tropopause gets lower.

A. -56.5 °C

In the ICAO Standard Atmosphere the temperature falls to -56.5 °C at the 11 km tropopause and is then held CONSTANT at that value up to 20 km. 17 km lies inside that isothermal layer, so the temperature is still -56.5 °C.

C. -75 to -80 °C

The equatorial troposphere is very deep, so air goes on cooling all the way to 16-18 km and arrives far colder than anywhere else, about -75 to -80 °C. The -56.5 °C figure is the ISA, mid-latitude tropopause. This is why the coldest air at cruising levels is found over the tropics, not over the poles.