The atmosphere and stability

Why some days are smooth as glass and others boil with thermals — how rising air cools, what stable and unstable air feel like, inversions, cloud base and thin air.

In this lesson you will

Two mornings at the same hill, with the same light breeze. On the first, the air is like glass: your wing barely moves and the flight down feels like a lift in a quiet building. On the second, the wing nods and rocks, the windsock swings, and by lunchtime the clouds are tall and dark.

The difference is stability: how the air reacts when something pushes it up. It decides whether today is a training day, and it explains most of what you will learn about thermals and clouds.

How rising air cools

Air that rises moves into lower pressure, expands and cools, even though no heat is taken away. Sinking air is squeezed and warms the same way.

Rising air that is not saturated cools at the dry adiabatic lapse rate (DALR), about 9.8 °C/km5.4 °F/1000ft. Its dew point falls only about 1.8 °C/km1 °F/1000ft, so the two close in by about 8 °C/km4.4 °F/1000ft as it climbs¹.
Once the air is saturated, inside a cloud, condensation releases heat and it cools more slowly: the saturated adiabatic lapse rate (SALR). It ranges from about 4 °C/km2.2 °F/1000ft in very warm air to nearly the dry rate in very cold air, and is roughly 4–6 °C/km2.2–3.3 °F/1000ft low down².

The air around you has its own lapse rate, the environmental lapse rate (ELR): how fast the temperature actually falls with height today. Comparing it with the DALR and SALR is the whole secret of stability.

You will also meet the International Standard Atmosphere (ISA): 15 °C59 °F and 1,013.25 hPa30 inHg at sea level, cooling 6.5 °C/km3.6 °F/1000ft up to 11 km6.8 mi³. It is a yardstick for instruments and performance figures. Real days rarely match it.

Stable and unstable air

Picture a bubble of air warmed by a sunny field: a thermal. It is warmer than the air around it, so it rises, cooling at the DALR as it goes.

If the air around it cools more slowly with height than that, the bubble soon becomes as cool as its surroundings and stops. The air is stable. Air that cools more slowly than even the saturated rate, including layers where the temperature stays the same or rises, is absolutely stable⁴.
If the air around it cools faster than the dry rate, the bubble stays warmer and keeps going. The air is absolutely unstable⁴.
In between, the air is conditionally unstable: stable for dry air, but once a rising parcel saturates and is lifted high enough, it becomes unstable and clouds can grow tall⁴.

How high a thermal goes depends on how much more slowly the air around it cools. On a good thermal day the lower air cools at nearly the dry rate, so a thermal that starts a degree or two warmer keeps its lead for a long climb, usually up to the inversion⁴⁵.

Stability changes through the day. Sun heating the ground steepens the lapse rate and destabilises the low-level air; cooling at night stabilises it. Sinking air, cold air arriving low down or warm air arriving aloft all make the air more stable⁶.

Figure: A teaching model of a thermal rising through the air around it. Notice that the thermal line keeps rising only while it is warmer than the line for the air it rises into, that an inversion stops it, and that the thermal reaches its dew point at the cloud base. Try the presets, especially “Moist, unstable, no lid”.

A teaching model of a thermal rising through the air around it. Notice that the thermal line keeps rising only while it is warmer than the line for the air it rises into, that an inversion stops it, and that the thermal reaches its dew point at the cloud base. Try the presets, especially “Moist, unstable, no lid”.

What stable and unstable days feel like

You won’t see a lapse rate, but you can see and feel its effects:

	Stable	Unstable
In the air	Smooth; ridge lift can be very smooth⁷	Bumpy, with thermals, sink and launch cycles
Smoke	Drifts off in a flat ribbon that barely spreads⁸	Loops up and down⁸
Sky	Haze trapped under an inversion⁹; fog in valleys after clear, calm nights¹⁰	Heaped cumulus that can grow tall and turn into thunderstorms¹¹
For you	Light, smooth headwinds in stable air are what first flights are made for¹²	Early students are warned against unstable conditions¹²

Unstable air is not bad air: it is what thermal pilots go looking for. But it is not training air. Gin’s school-wing manual says paragliders are not designed to be flown in turbulent or unstable air¹³.

Common mistake Expecting the smooth morning to last

Symptom: You fly a silky flight at nine, wait for lunch “when it’ll be nicer”, and find strong launch cycles and rough air.
Cause: The sun has been heating the ground all morning, steepening the lapse rate and making the low-level air unstable⁶.
Fix: Expect the air to change through the day. Most training happens early and late for this reason; see Thermals.

Common mistake Reading a blue sky as a gentle sky

Symptom: There isn’t a cloud in the sky, so you expect calm air, and a strong thermal shakes the wing on launch.
Cause: In dry air thermals stop below their cloud base, so no cumulus marks them. They can be just as strong as thermals under clouds¹⁴.
Fix: Judge the air by the forecast, the launch cycles, dust, birds and other wings, not by the clouds alone.

Myth vs fact

Myth: Stable air is always smooth, safe air.
Fact: Stable air is smooth in light wind. In strong wind it can bring wave, rotor and a steep wind gradient.

Stable air resists moving up and down, so disturbances travel further and waves can form¹⁵¹⁶¹⁷.

Inversions

An inversion is a layer in which the temperature rises with height instead of falling. Air rising into it is suddenly colder than its surroundings, so an inversion acts as a lid: it stops thermals and traps haze and pollution below it⁹. Thermals usually rise from the ground to somewhere between 200 m655 ft and 4 km2.5 mi, up to the capping inversion⁵.

Surface inversions form on clear, calm nights and hold smooth, stable air near the ground until the sun heats it away. In strong inversions the temperature can rise by up to about 10 °C18 °F in only a few hundred feet (roughly 100 m330 ft)⁹.

Under a building high-pressure system, sinking air pushes the inversion lower day by day. After the second or third day it is often so low that useful thermals no longer form: warm, sunny, but very stable¹⁸.

Instructor's note Why the wind can change mid-morning

Strong wind shear often sits across an inversion, and stable air near the ground is slowed more than the wind above it (Wind gradient)¹⁷. When the morning inversion breaks up, that faster wind can mix down to launch quite suddenly: a calm 8 a.m. launch is no promise of a calm 11 a.m. one. Low-level wind shear is linked with inversions, fronts, thunderstorms and strong winds aloft¹⁹.

Dew point and cloud base

The dew point is the temperature at which air becomes saturated. A rising thermal cools faster than its dew point falls, so the two close in. Where they meet, cloud forms: the base of a cumulus.

That gives a handy rule. The cloud base of cumulus formed by surface heating is about 125 m410 ft above the ground for each degree Celsius between the temperature and the dew point (the “spread”). The FAA’s version is 305 m1,000 ft for every 4.4 degrees Fahrenheit²⁰.

For example, with 25 °C77 °F and a dew point of 13 °C55 °F, the spread is 12 °C22 °F, so cumulus base is about 1,500 m4,900 ft above you²⁰. Two limits: the answer is height above where you measured, not above sea level, and the rule does not work for layer cloud²⁰.

If the thermals stop below that height, at an inversion, no cloud forms: you get blue thermals with nothing to mark them¹⁴.

Density altitude: thin air

High, hot (and to a lesser extent humid) air is thinner: it has a high density altitude. Your wing flies at the same indicated airspeed and feels the same, but it moves faster through the air and over the ground. True airspeed rises about 2% for every 300 m985 ft, so at a launch 2,000 m6,550 ft up it is roughly 13% higher. Launch runs are faster and longer and landings are faster; at any one site, temperature is what changes most from day to day²¹.

So at a high or hot site, expect to run harder and further before the wing lifts you, and to touch down faster. Your instructor will brief you; USHPA’s P2 ground school asks students to consider altitude, humidity and temperature for launches and landings²¹.

Reading stability before you fly

Forecasters judge stability from a sounding, a plot of temperature, dew point and wind against height. Soaring forecasts turn it into numbers you can use: the trigger temperature the ground must reach before thermals start, how high thermals will go, and an estimated cumulus base²². How to read them is in Forecasts and the go/no-go decision.

Reading the air before a training day

Check the forecast for thermal strength, inversion height and cloud base.
Cue A low inversion and light thermals mean a gentle day; tall clouds forecast by afternoon mean an early finish.
Why Soaring forecasts are built from the same lapse rates as this lesson²².
Look at the sky and the valley when you arrive.
Cue A flat layer of haze, smoke lying in a ribbon, or valley fog mean stable air; heaped clouds and dust swirls mean unstable air.
Why Haze collects under an inversion⁹, and smoke shows the state of the air⁸.
Watch the launch wind for a few minutes.
Cue A steady breeze with no cycles suggests no thermals yet; pulses of stronger wind with lulls between are thermals passing.
Why On thermic days the launch wind comes in cycles²³.
Keep checking as the day warms.
Cue The cycles get stronger and the clouds grow.
Why Heating makes the low-level air less stable through the day⁶.

Standards differ How much of this you are expected to know

Common ground: All the systems include weather theory; APPI and SafePro name stability and inversions explicitly, and reading full soundings comes later²⁴²⁵. SafePro, the international reference, puts thermal turbulence, inversions and katabatic flow in its Stage 2 knowledge.

USHPA (USA): P2 ground school includes considering altitude, humidity and temperature (air density) for launches and landings²¹. P3 pilots give a verbal analysis of conditions, including laminar air and turbulence²⁶.
BHPA (UK): Club Pilot training ends with theory exams, and the CFI must be satisfied that all the theory has been covered and tested²⁷. The detailed syllabus is in a training record that is not public, so follow your school.
APPI (international): Stability and instability and how the day evolves come at Adventure level; pressure systems, air masses and forecast tools at APPI Pilot level; soundings (skew-T) only at Advanced Pilot level²⁴²⁶.

Summarised from each body’s published material, which changes over time. Check the current official documents and follow your school.

Questions students ask

Is unstable air dangerous?

It is rough, and that matters at your stage. For years most Swiss in-flight incidents have happened in thermal or soaring flight²⁸, and training standards keep early students out of unstable air¹². Thermals become your friend later, step by step, with your instructor.

Why does the forecast cloud base differ from my estimate?

The rule of thumb is approximate, the forecast may be for a different place or time, and forecasts may give the base above sea level rather than above the ground. Check which before you compare.

Does humid air really make my wing fly faster?

A little, because humid air is slightly thinner. Height and temperature matter much more²¹.

Do I need to read a skew-T or tephigram?

Not yet. They are charts of a sounding, and APPI, for example, expects them to be mastered only at its Advanced Pilot level²⁶. Knowing what stable, unstable and an inversion mean is what matters now.

Key points

Rising air cools: about 9.8 °C/km5.4 °F/1000ft when dry (DALR), more slowly, roughly 4–6 °C/km2.2–3.3 °F/1000ft low down, inside cloud (SALR).
If the air around a thermal cools more slowly than that, the air is stable and thermals soon stop; if faster, it is unstable and they keep going.
Stable days are smooth and hazy, with smoke in flat ribbons; unstable days are bumpy, with thermals and heaped clouds. First flights belong to stable air.
An inversion is a lid: it stops thermals, traps haze, and can hide stronger wind above it.
Cumulus base is about 125 m410 ft above the ground per degree Celsius of temperature–dew point spread.
Hot, high air is thin: you launch and land faster, so expect longer runs.

Ground to Solo is a study aid. Paragliding is learned in person with a qualified instructor at a recognised school; nothing here certifies you to fly.