The Atmosphere

What is actually above your head?

Look up

Five real photographs, from the runway to space. Swipe or tap Next.

1. You live at the bottom of an ocean Every breath, every cloud, every gust of wind is part of one thing: the air wrapped around the Earth. It has weight. The column of air above each square centimetre of you weighs about 1 kg. That weight is what we call air pressure.
2. And yet it is paper thin From the space station, at 400 km, the whole atmosphere is this glowing strip on the edge of the planet. Most of the air's mass sits in the lowest 16 km. Shrink the Earth to a football and the air would be thinner than a sheet of paper.
3. Air is never perfectly clean Besides gases, the air carries dust, salt from sea spray and smoke. This satellite image shows winter haze lying over Delhi and the plains, while the Himalayas stand clear above it. These tiny particles matter: water vapour condenses on them to make fog and cloud. Big cities and industrial areas get more fog and smog because they put more particles into the air.
4. Balloons measure it every day Twice a day, weather stations release balloons that carry small instruments called radiosondes. They radio back temperature, humidity and pressure as they rise, until the balloon bursts around 30-35 km up. Those soundings are how we know the layers you are about to climb through.
5. Heated from below The sun's rays pass through the air and warm the ground first. The ground then warms the air touching it, and that warm air rises. So the air is heated from below, not from above.

Takeaway: The atmosphere is a thin, heavy, invisible ocean of gas. Weather, flight and life all happen in its lowest few kilometres.

What is air made of?

What air is made of

Every dot is a molecule, moving like the real thing. Tap a gas to find it.

• Nitrogen
• Nearly 4 of every 5 molecules. By volume N₂ : O₂ is about 4 : 1; by weight about 3 : 1.
• Oxygen
• About 1 in 5 molecules. The share stays the same as you climb, but the air thins, so each breath holds less. Above 10 000 ft pilots need supplementary oxygen.
• Argon
• An inert gas, just under 1%. It plays no part in weather.
• Carbon dioxide
• Only about 0.035% of the air, yet with water vapour it traps the Earth's heat: a greenhouse gas.
• Water vapour
• Not part of the fixed mix: from almost none over the poles and above 30 000 ft to about 4% in hot, humid tropical air. Warmer air can hold more.

Memory hook: Four nitrogen for every oxygen.

Takeaway: Nitrogen 78%, oxygen 21%, argon about 1%, and a trace of CO2. By volume nitrogen outnumbers oxygen 4 to 1.

What else is floating in the air besides gas?

Dust, salt and smoke

Real pictures. Swipe or tap Next.

1. Air is never just gas Besides gases, the air carries tiny solid particles: salt from evaporating sea spray, dust from dry lands, smoke and particles from industry.
2. They dim the sun and the view The particles block and scatter sunlight, so they change temperatures and reduce visibility. Haze over northern India in winter is mostly this.
3. Seeds for cloud and fog Water vapour needs a surface to condense on. These particles act as condensation nuclei and make condensation start early, even before the air is quite saturated. In large cities and industrial areas this is why fog and smog (fog mixed with smoke) are so frequent. Particles = condensation nuclei → early fog and smog over cities.

Takeaway: Salt, dust, smoke and industrial particles dim the sunlight, cut visibility and give water vapour something to condense on, which is why big cities get frequent fog and smog.

Which gases change from place to place?

Variable gases

A tiny share of the air, but they decide the weather.

1. Most of the air never changes Nitrogen, oxygen and argon are mixed almost evenly everywhere. A few gases are different: their amount changes from place to place.
2. Cities and industry Burning fuel and wood gives carbon dioxide, carbon monoxide, sulphur dioxide and nitrogen dioxide. They are concentrated over towns and industrial areas.
3. Water vapour from water Seas, lakes and rivers keep adding water vapour, the most important variable gas for weather.
4. Plants and oceans take CO₂ back Plants absorb CO₂, keep the carbon as food and release oxygen. A large amount of CO₂ also dissolves in the oceans. Over the poles and at height there is very little of it.
5. How much water can air hold? Warm air can hold more water vapour. In the tropics air can carry up to about 4% by volume; at the poles and above 30 000 ft it is almost nil, because it is so cold. Air holding all the vapour it can is saturated (relative humidity 100%). Anything less is unsaturated, often loosely called dry air. Water in the air comes in three states: gas (water vapour), liquid (drizzle, rain, showers), solid (snow, ice, hail).

Takeaway: Water vapour, CO, SO₂, NO₂ and methane vary from place to place: more over cities, industry and water. Tiny amounts, big effect on weather and life.

Why do a few trace gases matter so much?

Greenhouse gases

Tap Next to build the picture.

1. Sunlight comes straight through The sun's energy arrives as short waves. The air is almost transparent to them, so most of it passes straight through and warms the ground.
2. The ground sends heat back up A warm surface gives off its own energy, but as long waves (heat radiation). This is terrestrial radiation.
3. A few gases catch it Water vapour, carbon dioxide, ozone and methane are only a tiny part of the air, but they absorb long waves well. They are called greenhouse gases.
4. ...and send part of it back down Having absorbed the heat, these gases radiate it again in every direction, some of it back to the ground.
5. The greenhouse effect So the Earth stays warmer than it would be with no air at all, by roughly 33 °C. Life as we know it depends on it.
6. Too much of a good thing Burning fuel adds CO₂ faster than plants and the oceans can take it up, so more heat is held in. This is global warming, and it needs to be limited. Greenhouse gases: transparent to short-wave (solar), absorb long-wave (terrestrial).

Memory hook: Short waves in, long waves trapped.

Takeaway: Water vapour, CO₂, ozone and methane let sunlight in but trap part of the heat the Earth sends back. That keeps the planet warm; too much of them causes global warming.

What does the ozone layer actually do?

Ozone

Tap Next. Heights are to scale.

1. UV arrives from the sun Part of sunlight is ultraviolet (UV): energetic enough to burn skin and damage eyes.
2. Ozone stops it High up, UV splits oxygen molecules; the loose atoms join other oxygen to make ozone (O₃). Ozone then absorbs UV. There is real ozone between about 10 and 50 km, densest at 20-25 km. (Ozone is only about 0.000007% of the air.)
3. This heats the stratosphere The energy ozone absorbs warms the air. That is why the stratosphere gets warmer with height instead of colder.
4. A shield for life Because ozone takes out most of the UV, little reaches the ground. It protects us from sunburn, skin cancer and cataracts.
5. The ozone hole Chlorofluorocarbons (CFCs, once used in fridges and sprays), halocarbon solvents and similar pollutants drift up to the stratosphere. Sunlight breaks them and frees chlorine and other halogen atoms, which break ozone back into ordinary oxygen. The layer has thinned, and big holes open, mostly over the poles. Max ozone 20-25 km. Appreciable from 10 to 50 km.

Memory hook: Ozone: 20-25 km, UV shield, stratosphere heater.

Takeaway: Ozone forms high up, absorbs the sun's ultraviolet light and warms the stratosphere. Most of it is at 20-25 km. CFCs and similar pollutants have thinned it into holes.

If the air is 480 km deep, why is it so thin up high?

Where the air is

Tap Next. The column in the middle steps is drawn to scale.

1. What air is like Air has weight. That weight presses on everything below it, and the push is called atmospheric pressure. Air can be squeezed and can spread out again. It has no shape of its own and fills any space. It moves, carrying heat and moisture, and it carries heat and electricity poorly (a poor conductor).
2. Gravity packs it low Gravity holds the air to the Earth, and the air turns with the Earth. Each layer of air carries the weight of all the air above it. So the lowest layers are squeezed the most: the same amount of air fits into a much thinner layer near the ground.
3. Half below 6 km Half of all the air lies below about 6 km, roughly 20 000 ft. So a climb to 20 000 ft takes you above half of the atmosphere, even though the whole of it is hundreds of kilometres deep.
4. Three quarters below 10 km Climb to 10 km, where airliners cruise, and three quarters of the air is below you. Only a quarter is left above.
5. 80% below 16 km, 99% below 35 km About 80% of the air lies within 16 km of the ground, and 99% below about 35 km. That leaves just 1% of the air for everything above. Up there the air is so thin that wings get little lift and engines get little air to burn.
6. Thin air, thin oxygen Oxygen is 21% of the air at every height. But higher up there is less air, so each breath holds fewer molecules and less oxygen. Above 10 000 ft, pilots need extra (supplementary) oxygen. Supplementary oxygen above 10 000 ft.
7. No real top The atmosphere is about 480 km deep, but it has no sharp edge. It just gets thinner and thinner until it fades into space.

Memory hook: 6 km half, 10 km three quarters, 35 km almost all.

Takeaway: Air is pressed down by its own weight, so most of it sits low. Half of it is below 6 km, three quarters below 10 km and 99% below 35 km. Thin traces go on for hundreds of kilometres.

If the sun is above us, why is the air coldest high up?

Heated from below

Tap Next to add the four ways heat climbs up from the ground.

1. The sun warms the ground, not the air Sunlight passes through the air and hardly warms it on the way. It is the surface, land and sea, that soaks up the sunshine and warms. So the air gets its heat second-hand, from the warm surface below it. The next four steps show how that heat climbs up.
2. 1 · Conduction Conduction is heat passing by touch. Only the thin layer of air right against the warm ground is heated this way. Air is a poor conductor, so this heat cannot spread far up on its own. Something has to carry it higher.
3. 2 · Convection The warmed air is lighter, so it rises in bubbles and currents and carries the heat up with it. Cooler air sinks to take its place. This rising and sinking is convection.
4. 3 · Radiation The warm ground also gives off heat as invisible long waves. This is radiation, and it needs no contact. Part of it is soaked up by the air above, mainly by greenhouse gases such as water vapour and carbon dioxide; the rest escapes to space.
5. 4 · Latent heat When water evaporates from seas and wet ground, it takes heat with it and hides it in the vapour. This hidden heat is called latent heat. When the vapour rises and condenses into cloud, or turns straight into ice (sublimates), that heat is let out into the air.
6. Which matters most? About 23% of the heat goes up as sensible heat, the kind a thermometer feels (conduction, convection and radiation). About 77% goes up as latent heat. Because the heat comes from below, the temperature falls as you go up, at least up to the tropopause. The atmosphere is heated from below. Largest share: latent heat.

Memory hook: Sun → ground → air. Most of it as latent heat.

Takeaway: Sunlight warms the ground, and the ground then warms the air. It does this by conduction, convection, radiation and, most of all, by latent heat set free when water vapour condenses. So the air is heated from below, and it gets colder as you go up.

How does temperature change all the way up to space?

Temperature with height

The key diagram of the whole chapter, built one layer at a time. Heights to scale.

1. Troposphere: colder as you climb Heated from below, the air cools about 6.5 °C per km (the lapse rate). It is generally unstable and well stirred. Here are all the weather, nearly all the water vapour and dust, about 75% of the air, and most flying. Lower troposphere: surface to 2.1 km. Middle: 2.1 to 7.6 km. Upper: 7.6 km to the tropopause.
2. Tropopause: the cooling stops At the top of the troposphere the lapse rate drops to 1-2 °C per km or less: the temperature stops falling. In the ISA this is at 11 km, at −56.5 °C.
3. Stratosphere: warmer as you climb For the first 8-10 km above the tropopause the temperature stays about the same (isothermal). Then it rises, slowly and then sharply, up to about 0 °C at 50 km.
4. Why: the ozone The inversion is caused by ozone absorbing the sun's ultraviolet light, most of it at 20-25 km. Warm air above cold air is very stable: the stratosphere is dry, calm and has no weather.
5. Mesosphere: colder again Above the stratopause at 50 km there is little to absorb sunlight and little mixing, so the temperature falls again, to the coldest level of the atmosphere at the mesopause, about 80 km (about −90 to −100 °C, 173 K).
6. Thermosphere: very hot, very thin Above the mesopause the few molecules left are heated strongly by the sun, so the temperature climbs with height. Up to about 80 km the gases are evenly mixed (homosphere); above, they separate by weight (heterosphere). Mesopause (80 km) is the coldest level: about −100 °C ≈ 173 K.

Memory hook: Down, up, down, up: tropo, strato, meso, thermo.

Takeaway: Temperature falls through the troposphere, stays at −56.5 °C then rises through the stratosphere (ozone), falls through the mesosphere to the coldest level (about −90 to −100 °C at 80 km), then rises in the thermosphere.

What do the layers look like wrapped round the Earth?

The layers in 3D

A real NASA picture of the Earth with a slice cut out, so you can see the layers like the layers of a cake. They are drawn much thicker than real; the labels give the real heights. Drag to turn it.

• The troposphere: a thin skin All our weather, and almost all flying, happens in this lowest layer. It is 16-18 km deep over the equator and only 8-10 km over the poles.
• It bulges at the equator Look at the orange band in the cut: it is thicker over the equator than over the poles, because warm tropical air rises higher before it stops cooling.
• The stratosphere above it Above the troposphere lies the stratosphere, up to about 50 km. Ozone in it soaks up sunlight, so it gets warmer as you climb. Warm air on top of cold air is very stable, so this layer is calm and dry, with almost no cloud and no weather. Its winds are strong and steady, which is why long-range aircraft like to cruise near its base.
• The mesosphere From 50 to 80 km the air gets colder with height, down to the coldest air of all. Above it, the thin, hot thermosphere fades into space.

Takeaway: The layers are shells round the whole Earth. The lowest, the troposphere, is a thin skin that is thicker over the equator. The stratosphere, mesosphere and thermosphere sit on top of it.

What changes as you climb from the runway to space?

The climb

Drag the altitude, tap a stop, or press Climb. Watch the sky, the temperature and the pressure change.

• Troposphere
• From the ground to the tropopause: 16-18 km at the equator, 8-10 km at the poles (11 km in the ISA). Three quarters of the air and nearly all the water vapour. All weather happens here.
• Stratosphere
• Up to about 50 km. First steady in temperature, then warming as ozone absorbs ultraviolet light. Dry, stable, no weather.
• Mesosphere
• From 50 to 80 km. Temperature falls again; its top, the mesopause, is the coldest place in the atmosphere.
• Thermosphere
• Above 80 km. A few molecules heated to hundreds of degrees by the sun. Contains the ionosphere, which bends radio waves, and the aurora.
• Runway
• Sea level in the standard atmosphere: 15 °C and 1013.25 hPa. The air is densest here, about 1.225 kg in every cubic metre.
• Fair-weather clouds
• Warm air rising off the ground cools until its water vapour condenses. Puffy cumulus mark the tops of those thermals.
• 10 000 ft
• Above about 10 000 ft the air is thin enough that pilots need supplementary oxygen.
• Half the air is below you
• Air is squeezed by its own weight, so it is packed low down. By about 6 km half the atmosphere is beneath you, and pressure has halved.
• Everest
• 8.8 km. Clouds pile against the Indian side of the Himalayas while Tibet stays dry. Over the poles the troposphere would already be ending here.
• Airliners cruise
• FL360 is about 11 km: the tropopause of the standard atmosphere. Outside it is −56.5 °C. Jet streams and clear-air turbulence sit just below the tropopause.
• Tropopause over India
• Over India the tropopause is about 16 km. A thunderstorm cannot rise through it, so its top flattens into an anvil, as in this photo from the space station.
• Into the stratosphere
• NASA's ER-2 flies up here, about 20 km. The sky overhead is almost navy blue. The air is very dry and calm: there is no weather to fly through.
• Ozone layer
• Ozone is spread from 10 to 50 km, densest at 20-25 km. It absorbs the sun's ultraviolet, which shields us and warms this layer. The map shows the seasonal hole over Antarctica.
• Weather balloons burst
• Radiosonde balloons swell as the pressure drops and usually burst around 30-35 km. Ninety-nine per cent of the air is now below you.
• Stratopause
• The top of the stratosphere, about 50 km. The warming from ozone peaks here, close to 0 °C.
• Meteors burn up
• The mesopause, about 80 km: around −100 °C (173 K), the coldest place in the atmosphere. Grains from space burn up here as shooting stars.
• Noctilucent clouds
• The highest clouds of all, made of ice at 80-85 km. Seen in polar summer just after sunset, lit by a sun that has already set below.
• Aurora
• Particles from the sun make the thin gas glow green and red, from about 100 to 300 km. Near the poles: northern and southern lights.
• Space station
• About 400 km. Everything you climbed through is that thin blue line on the horizon. The air thins out into the exosphere and, in the end, into space.

Memory hook: Cold, warm, cold, hot: the temperature zig-zags as you climb.

Takeaway: Troposphere (weather, cooling) → tropopause → stratosphere (ozone, warming) → mesosphere (coldest) → thermosphere (aurora, hot but empty).

What is up there above the weather?

Above the weather

Real photographs from orbit and from the ground. Swipe or tap Next.

1. Stable, dry and calm Above the weather, the stratosphere has strong, steady horizontal winds and almost no cloud, which is why long-range aircraft like to cruise near its base. In high latitudes in winter, rare iridescent mother-of-pearl (nacreous) clouds form in its upper part.
2. Night-shining clouds Noctilucent clouds are the highest clouds of all, near the top of the mesosphere. They are seen only rarely, over polar regions in summer, lit by the sun after it has set below the horizon.
3. Where meteors burn up Grains of rock from space hit the thin air of the mesosphere so fast that they burn up as shooting stars. The mesopause, at about 80 km, is the coldest level of the atmosphere: about −100 °C (173 K).
4. Aurora and radio waves Above 80 km the temperature climbs again. From about 60 km up, the air is electrically charged: the ionosphere, which reflects radio waves round the Earth. Bursts from the sun disturb it and can black out HF radio. The thin golden line along the Earth's edge here is airglow, from this layer.
5. The edge of space The outermost layer reaches out about 10 000 km. Its few molecules move very fast, so their temperature is very high, but they are so far apart that there is hardly any heat. Some escape into space.

Takeaway: Nacreous clouds in the stratosphere, noctilucent clouds and meteors in the mesosphere, aurora and the radio-reflecting ionosphere in the thermosphere, and the exosphere fading into space.

Can you put each fact in its layer?

Which layer?

Tap a fact, then the layer it belongs to.

• Almost all clouds and rain
• Cools about 6.5 °C per km
• Holds about 75% of the air's mass
• Ozone warms it
• Mother-of-pearl (nacreous) clouds
• Very dry and stable, no weather
• Coldest place, about −100 °C
• Meteors burn up
• Noctilucent clouds
• Aurora
• Ionosphere bends radio waves

Takeaway: Weather and cooling: troposphere. Ozone and calm: stratosphere. Coldest and meteors: mesosphere. Aurora and radio: thermosphere.

Can you actually see the layers?

Layers from space

A real sunset photographed from orbit. Tap each band of colour.

• Troposphere
• The orange band: the lowest, densest layer. Its dust and water vapour scatter the setting sun's light into reds and oranges.
• Stratosphere
• The bright white band above: thin, dry air with very fine particles that scatter sunlight.
• Mesosphere and above
• Air so thin that the blue fades into the black of space.
• The sun
• The sun, setting behind cloud tops in the troposphere. Its light crosses hundreds of kilometres of air to reach the camera.

Takeaway: At sunset, light passes sideways through the air. The orange band is the dense troposphere; above it the stratosphere glows white and blue.

Why is the tropopause twice as high over the equator?

Higher at the equator

A slice of the air from the pole (left) to the equator (right).

1. The troposphere is not the same depth everywhere It reaches up 16-18 km over the equator but only 8-10 km over the poles.
2. Why: rising heat Over the hot equator, strong convection carries warm air very high before it stops cooling. Over the cold poles the air does not rise so far. Rule: the warmer the surface, the higher the tropopause.
3. Higher means colder Because the air keeps cooling all the way up, the high tropical tropopause is very cold (−70 to −75 °C); the low polar one is warmer (−40 to −45 °C).
4. What sets its height Five things: surface temperature, latitude, season, the spread of land and sea, and the weather situation of the day (synoptic situation). It is lower in winter than in summer. Tropopause height: equator 16-18 km, poles 8-10 km, India 16-16.5 km.

Memory hook: Hot below, high above, cold on top.

Takeaway: The warmer the surface, the higher the tropopause: 16-18 km over the equator, 8-10 km over the poles, lower in winter. The high tropical tropopause is also the coldest.

Can the North Pole be warmer than the equator?

Reversal above 8 km

Two temperature soundings, one over each.

1. Over the equator Warm at the ground, and it keeps cooling all the way to a tropopause near 17 km, where it is about −75 °C.
2. Over the pole Cold at the ground, but the tropopause comes early, near 9 km. Above it the temperature stops falling and stays near −45 °C.
3. The lines cross Above about 8 km the polar column is warmer than the equatorial one at the same height: the temperature (and density) pattern reverses above 8 km. Above 8 km the lowest temperatures are over the equator.

Memory hook: Low lid at the pole = warmer up high.

Takeaway: Above about 8 km the polar air has already stopped cooling (its tropopause is low), while equatorial air keeps cooling up to 16-18 km. So high up the poles are warmer, and denser air is reversed too.

Is the tropopause one smooth surface?

Three tropopauses

A slice from the North Pole (left) to the equator (right).

1. The polar tropopause From the pole to about 45-60° latitude it lies near the 300 hPa level, about 8-9 km. In winter, when polar air pushes south, it is sometimes noticed over Srinagar.
2. The middle tropopause Between the two breaks it lies near 200 hPa. Over India it is about 11.5 km with about −45 °C; in winter it can reach as far south as 23° N.
3. The tropical tropopause From the equator to about 35-40° it lies near 100 hPa. Over India it is about 16-16.5 km.
4. The breaks Where one step meets the next the tropopause changes height suddenly. These breaks (or folds) are near 60° and 40° latitude; the one near 40° is the more marked.
5. Jet streams live at the breaks The polar front jet stream sits at the southern end of the polar tropopause; the subtropical jet stream at the southern end of the middle tropopause.
6. Over India India (8-37° N) is under the tropical tropopause, at 16-16.5 km, with the subtropical jet overhead in the north in winter. Tropopause breaks: about 40° and 60°. The 40° one is more prominent.

Memory hook: 300, 200, 100 hPa from pole to equator. Jets at the breaks.

Takeaway: The tropopause comes in three steps: polar (~300 hPa), middle (~200 hPa) and tropical (~100 hPa), with breaks near 60° and 40° where the jet streams blow. Over India the tropical one is at 16-16.5 km.

Where does the weather stop?

The lid on the weather

A thunderstorm over Africa, photographed from the space station. Tap the pins.

• Flat top
• The anvil. The storm's rising air reached the tropopause and could rise no further, so it spread sideways. Clouds rarely go higher than the tropopause.
• Overshooting top
• The strongest updraught punches a little way into the stratosphere before falling back: a sign of a very violent storm.
• Spreading downwind
• Strong winds just below the tropopause stretch the anvil out downwind.
• Small cumulus
• Ordinary fair-weather cumulus, only 1-2 km tall, deep in the troposphere.

Memory hook: Anvil top = tropopause.

Takeaway: The tropopause acts as a lid: rising storm air hits the stable stratosphere and spreads out sideways into a flat anvil.

Is it warmer or colder than standard today?

ISA calculator

Set the altitude and the outside air temperature. The ISA deviation works itself out.

• Hot day at sea level, 35 °C
• 2 km, +5 °C
• FL300, −40 °C
• FL390, −56.5 °C
• 19 km, −60 °C

Memory hook: Actual minus ISA.

Takeaway: ISA deviation = actual − ISA. ISA falls 1.98 °C per 1000 ft (6.5 °C/km) from 15 °C at sea level, and stays −56.5 °C from 11 to 20 km.

What exactly is in the ICAO standard atmosphere?

ISA and JSA

Every number of the ISA, then the jet standard atmosphere. Tap Next.

1. Why a standard atmosphere? The real atmosphere changes every day. To design and test aircraft, compare how they perform and calibrate altimeters, everyone needs the same imaginary atmosphere. ICAO's version is the International Standard Atmosphere (ISA).
2. At mean sea level The ISA starts with dry air (no water vapour) at mean sea level. Temperature 15 °C (288.15 K), pressure 1013.25 hPa, density 1225 g/m³, and gravity 980.665 cm/s².
3. Up to 11 km Going up, the temperature falls 6.5 °C per km (1.98 °C per 1000 ft). This goes on up to 11 km, which is 36 090 ft. That height is the ISA tropopause, the top of the weather layer.
4. From 11 to 20 km Above the tropopause the temperature stops falling. It stays at −56.5 °C from 11 km up to 20 km (65 617 ft).
5. From 20 to 32 km Higher still, it warms again by 1 °C per km (0.3 °C per 1000 ft), reaching −44.5 °C at 32 km (104 987 ft).
6. Jet standard atmosphere (JSA) The ISA suits aircraft below about 30 000 ft. For very high flying, space flight and engine design the JSA is used instead. It starts at 15 °C at sea level and cools a steady 2 °C per 1000 ft, with no tropopause: the dashed line just keeps going. ISA deviation = actual − ISA. Warmer than ISA is +, colder is −.

Memory hook: 15 · 1013.25 · 6.5 · 11 · −56.5 · 20.

Takeaway: The ISA is an agreed, made-up atmosphere. At sea level it is dry air at 15 °C, 1013.25 hPa and 1225 g/m³. It cools 6.5 °C per km up to 11 km, stays at −56.5 °C up to 20 km, then warms 1 °C per km up to 32 km. The JSA simply cools 2 °C per 1000 ft, with no tropopause.

Can you work out ISA temperatures without a calculator?

ISA sums

The lecture's shortcut and its exam-style questions. Tap Next.

1. One yardstick for everyone If every airfield calibrated altimeters to its own air, aircraft from different places would read different altitudes at the same height. ICAO's ISA is one imaginary atmosphere everyone uses: dry air, 15 °C, 1013.25 hPa and 1225 g/m³ at sea level, falling 1.98 °C per 1000 ft (6.5 °C/km) to −56.5 °C at 11 km (36,090 ft), constant to 20 km, then warming slowly.
2. 15 minus twice the thousands No calculators in the exam, so round 1.98 to 2: ISA temperature = 15 − 2 × (altitude in thousands of feet). Check: at 2000 ft, 15 − 4 = 11 °C.
3. Never below −56.5 At 45,000 ft the shortcut gives 15 − 90 = −75 °C, but in the ISA the temperature stops falling at the tropopause.
4. The FL330 question It is −48 °C at FL300 and the tropopause is at FL330. What is the temperature at FL350? Count down 2 °C per 1000 ft to the tropopause, then stop.
5. Count the gap It is −9 °C at FL130. What will it be at FL80, with the ISA lapse rate? The gap is 5000 ft, so 10 °C warmer lower down.
6. Actual minus ISA 20 °C at sea level: +5 °C deviation. 1002.25 hPa at sea level: −11 hPa. −45 °C at 6 km, where ISA is 15 − 6.5 × 6 = −24 °C: deviation −21 °C.
7. The lecture's last question It is 3 °C at FL70. At what altitude will it be 11 °C, with the ISA lapse rate?

Memory hook: ISA °C = 15 − 2 × (feet ÷ 1000), but never colder than −56.5.

Takeaway: Why ISA exists: one agreed atmosphere to calibrate instruments and compare the real day against. In the exam, round 1.98 to 2 °C per 1000 ft: ISA = 15 − 2 × (height in thousands of feet). Stop at −56.5 °C above the tropopause. Deviation = actual − ISA.

Numbers to know

The figures the exam keeps asking for.

Pairs that catch students out

Don't mix up

Each pair looks alike. The rule underneath tells them apart.

• Nacreous clouds: Mother-of-pearl colours, high-latitude winter, stratosphere. Noctilucent clouds: Silvery-blue night clouds, polar summer, upper mesosphere (80-85 km). Nacreous: stratosphere. Noctilucent: night-shining, higher, mesosphere.
• Tropical tropopause: 16-18 km, about 100 hPa, colder (−70 to −75 °C). Polar tropopause: 8-10 km, about 300 hPa, warmer (−40 to −45 °C). Higher tropopause, colder tropopause. Above 8 km the poles are warmer than the equator.
• Homosphere: Up to 80 km: gases well mixed by gravity and stirring. Heterosphere: Above 80 km: gases separate out by weight. Homo = same mix all the way up to 80 km.
• ISA: 6.5 °C/km to 11 km, then −56.5 °C. For aircraft below about 30 000 ft and altimeters. JSA: 2 °C per 1000 ft all the way, no tropopause. For very high flight and engines. J for Jet: one straight lapse rate.
• Sensible heat (23%): Felt as temperature: conduction, convection, radiation. Latent heat (77%): Hidden in water vapour, released when it condenses. Most of the air's heat arrives hidden inside water vapour.

Quick test

10 questions to check this chapter. Each answer explains why.

1. An airliner climbs out of Chennai into a cloudless sky and levels at FL370. In which layer is it cruising?

   • Troposphere
   • Mesosphere
   • Stratosphere

   Answer: Troposphere. Over the tropics the tropopause is near 16-17 km (FL520-560), so FL370 is still well inside the troposphere.

2. Why is the tropopause highest over the equator?

   • Coriolis force is strongest there
   • Strong heating and convection lift the whole column
   • The air there is coldest

   Answer: Strong heating and convection lift the whole column. Vigorous convection over the warm tropics carries the troposphere's top up to 16-18 km; over the poles it sits near 8 km.

3. Climbing through the stratosphere, the temperature

   • stays the same, then rises
   • keeps falling at 6.5 °C/km
   • rises at once, then falls

   Answer: stays the same, then rises. In the ISA it stays at −56.5 °C to 20 km, then ozone heating makes it rise towards the stratopause.

4. Which gas absorbs most of the sun's ultraviolet and warms the stratosphere?

   • Nitrogen
   • Carbon dioxide
   • Ozone

   Answer: Ozone. Ozone, densest at about 20-25 km, soaks up UV and heats the layer around it.

5. Almost all of the atmosphere's water vapour is found

   • in the troposphere
   • in the stratosphere
   • spread evenly to 50 km

   Answer: in the troposphere. Vapour comes from the surface and condenses out as it rises, so nearly all of it, and all the weather, stays low.

6. An OAT gauge reads +5 °C at 6000 ft. The ISA deviation is

   • +2 °C
   • −8 °C
   • +8 °C

   Answer: +2 °C. ISA at 6000 ft is 15 − 2 × 6 = +3 °C; actual minus ISA is 5 − 3 = +2 °C.

7. A sea-level airfield reports exactly standard conditions. The METAR shows

   • 20 °C and Q1013
   • 15 °C and Q1013
   • 0 °C and Q1000

   Answer: 15 °C and Q1013. The ISA starts at 15 °C and 1013.25 hPa at mean sea level.

8. Dust, smoke and salt in the air matter to pilots mainly because they

   • cut visibility and help fog form
   • warm the stratosphere
   • make the air heavier

   Answer: cut visibility and help fog form. Particles scatter light and act as nuclei for condensation, so cities and dusty plains get haze, smog and early fog.

9. Flying at FL450, the outside air temperature in the ISA would be

   • −75 °C
   • −45 °C
   • −56.5 °C

   Answer: −56.5 °C. The ISA lapse rate stops at the tropopause (11 km, about 36,000 ft); above it the temperature stays at −56.5 °C.

10. In which layer do meteors burn up and noctilucent clouds glow?

    • Mesosphere
    • Troposphere
    • Thermosphere

    Answer: Mesosphere. The mesosphere (about 50-85 km) is the coldest layer; meteors burn there and rare noctilucent clouds form near its top.