A paraglider does not fly in air in general, but in the air of a specific day — its pressure, humidity, temperature and turbulence. In this module we cover how the atmosphere works and where thermals, wind and clouds come from. And how to tell from a few forecast numbers and the view out of the window whether today is a flying day. All of it is built on the syllabus of the official theory exam of the national paragliding federation and tied to the real conditions in Gudauri.
⏱ 130 min📖 20 sections✓ 40 questions
The atmosphere and pressure: the air we fly in
In shortWe fly in the lowest layer of the atmosphere, the troposphere. Pressure falls with height, and the vario measures pressure, not height: near the ground one hectopascal is about 8 metres, at 3000 m already about 11. Before the flight, set the launch height and use QNH: then the instrument counts from sea level, just like the 3200 m (10 500 ft) ceiling of the area.
We fly in the lowest layer of the atmosphere: the troposphere. Over the Caucasus its upper boundary is at about 10–12 km. Almost all weather (clouds, wind, rain and snow) lives right here. Above it the stratosphere begins. At heights of about 10 000 m the jet streams flow: narrow “rivers” of very fast wind. Civil and military aircraft fly in them. In normal flying a paraglider, a paramotor or a hot-air balloon does not go that high: these are the heights of airliners and rare records.
Where we fly: the troposphere, jet streams and our working height range
Atmospheric pressure is the force with which the weight of a column of air presses on each unit of area. The higher you climb, the shorter the column above you and the less it weighs. So the pressure falls. When you go down, it is the other way round: the pressure rises. This is how the vario works: the instrument that shows your height and your rate of climb. It cannot feel height: it measures pressure and converts it into metres.
Pressure is measured with a barometer. The international unit is the hectopascal (hPa). In aviation the same value is called the millibar (mbar): 1 hPa = 1 mbar. Standard pressure at sea level is 1013.25 hPa at +15 °C. Near the ground the pressure falls by about 1 hPa for every 8 metres of climb. But that is near sea level: higher up the air is thinner, and at 3000 m the same hectopascal stretches to about 11 metres. Lowland conversion tables are wrong in the mountains.
Pressure is the weight of a column of air: what the vario really measures and why lowland conversion tables are wrong in the mountains
Two ways to set the instrument.QNH is the pressure reduced to sea level: the instrument shows height above the sea, and maps and airspace limits are given this way. QFE is the pressure at the site itself: the instrument shows height above the site and reads zero at launch. Handy for practising the landing approach, and dangerous near airspace limits.
In practice: before the flight, set the known launch height on your vario. In Gudauri this is 2200 m at the main tandem launch “Panorama”, 2470 m on the Bidara shoulder and 3000 m on Kudebi summit, where the ski lift takes you up in winter. The ceiling of the UGA8 paragliding area (this is the name of the paragliding area over Gudauri) is 3200 m above sea level (10 500 ft). If the instrument is 150 m wrong, you break through this ceiling without even noticing.
QNH and QFE: two ways to set the altimeter, Gudauri launch heights and the ceiling of the area
A detail for pros. Pressure changes during the day, and the sensor also reacts when its case heats up in the sun. Over a long day this adds up to tens of metres on the display. There is also a bigger correction: warm air. On a hot day the air column is “stretched”, and above launch the instrument shows less than your real height: if the air is 15 °C warmer than standard, at 700 m above launch the display is about 30–40 m lower than your real height. In summer in Gudauri this error adds to the pressure error in the same direction: towards the ceiling of the area. Hence the rule: keep your height below the ceiling of the area with a margin, not right at it.
Case review. In the morning you set your altimeter at a 2400 m launch with QNH 1008 hPa; by evening the pressure has risen to 1020 hPa. The instrument shows 3100 m, so it seems you still have a hundred metres below the ceiling of the area. In fact 12 hPa is about 96 metres, and your real height is already close to 3200 m. Decision: without a fresh QNH, do not come closer than 150–200 m to the ceiling of the area.
Case review: by evening the instrument shows 3100 m, but you are already at the ceiling of the area
Drag the diagram sideways to see all of it.
Troposphere: all the weather is here
What it is
We fly in the lowest layer of the atmosphere: the troposphere. Over the Caucasus its upper boundary is at about 10–12 km. Almost all weather (clouds, wind, rain and snow) lives right here. Above it the stratosphere begins.
In practice
At heights of about 10 000 m the jet streams flow: narrow “rivers” of very fast wind, where aircraft fly. In normal flying a paraglider, a paramotor or a hot-air balloon does not go up there: these are the heights of airliners and rare records. Our layer is the lowest few kilometres; over Gudauri, up to the area ceiling of 3200 m. All our weather is born in the same layer where we fly.
Pressure is the weight of the air column above you
What it is
Atmospheric pressure is the force with which the weight of a column of air presses on each unit of area. The higher you climb, the shorter the column above you and the less it weighs: the pressure falls. When you go down, it is the other way round: the pressure rises. Standard pressure at sea level is 1013.25 hPa at +15 °C.
In practice
This is how the vario works. It cannot feel height: it measures pressure and converts it into metres. In the diagram the pilot climbs to 3000 m, and the instrument sees only one thing: 1013 hPa has changed to 701. Everything the vario shows about height is converted pressure.
One hectopascal: 8 m near the ground, 11 m at 3000 m
What it is
Pressure is measured with a barometer; the unit is the hectopascal, in aviation it is called the millibar: 1 hPa = 1 mbar. Near the ground the pressure falls by about 1 hPa for every 8 m of climb. Higher up the air is thinner, and the same hectopascal stretches: at 3000 m to about 11 m. Like a stack of pillows: the lower ones are squashed by the weight of the upper ones.
In practice
That is why lowland conversion tables are wrong in the mountains. If you count 8 m per hectopascal from the sea up to 3000 m, you get about 2500 m: half a kilometre less than the truth. The vario itself uses the full formula; the danger is quick sums in your head with the lowland rule.
QNH and QFE: where the instrument counts from
What it is
QNH is the pressure reduced to sea level: the instrument shows height above the sea, and maps and airspace limits are given this way. QFE is the pressure at the site itself: the instrument shows height above the site and reads zero at launch.
In practice
QFE is handy for practising the landing approach and dangerous near airspace limits: the area ceiling is given above the sea. In the diagram the pilot has climbed 700 m from a 2400 m launch. On QNH the instrument shows 3100: you see at once that the ceiling is 100 m away. On QFE it shows 700, and you have to remember that the ceiling is 800. For flying near airspace limits, use QNH.
Gudauri launches and the UGA8 area ceiling
What it is
Before the flight, set the known launch height on your vario. In Gudauri this is 2200 m at the main tandem launch “Panorama”, 2470 m on the Bidara shoulder and 3000 m on Kudebi summit, where the ski lift takes you up in winter. The ceiling of the UGA8 paragliding area is 3200 m above sea level, 10 500 ft.
In practice
If the instrument is 150 m wrong, you break through this ceiling without even noticing. In the diagram the vario shows 3050 m and “150 to spare”, but the real height is already 3200. From the 3000 m launch the ceiling is only 200 m above you: there it is especially important to set the instrument by the launch height.
Case review: by evening the instrument shows 3100 m
What it is
In the morning you set your altimeter at a 2400 m launch with QNH 1008 hPa; by evening the pressure has risen to 1020 hPa. The instrument shows 3100 m, so it seems you still have a hundred metres below the area ceiling. In fact 12 hPa is about 96 m, and your real height is already close to 3200 m. On top of that, the case heats up in the sun: over a long day this adds tens of metres.
In practice
Why not 12 × 11 m: a pressure change at sea level arrives higher up weaker; at 3000 m it is only about 8 hPa, and 8 × 11 m is again about 90–100 m, not 130. So for a QNH correction there is one rule at any height: about 8 m per hectopascal. The decision from the lesson: without a fresh QNH, do not come closer than 150–200 m to the area ceiling.
The numbers of the section: troposphere over the Caucasus up to 10–12 km, jet streams at about 10 000 m; standard pressure 1013.25 hPa at +15 °C; near the ground 1 hPa ≈ 8 m, at 3000 m ≈ 11 m; Gudauri launches 2200, 2470 and 3000 m, UGA8 area ceiling 3200 m (10 500 ft); case review: QNH 1008 → 1020 hPa, about 96 m. Pressure at height is calculated with the standard atmosphere.
Temperature, humidity and the dew point
In shortThe dew point is the temperature at which vapour starts turning into droplets. The difference between the air temperature and the dew point is called the spread, and it tells a pilot more than the humidity percentage. A spread near the ground of 0–2 °C means fog and low cloud, 3–6 °C a low cloud base, 7–12 °C a working day with readable cumulus. Precipitation is measured in millimetres: 1 mm is one litre of water per square metre.
There is always invisible water vapour in the air. How much of it fits depends on the temperature: warm air holds more moisture, cold air holds less. The dew point is the temperature to which you must cool the air for the vapour to start turning into droplets. Not a height and not a place on the map, but a temperature.
This gives the main rule: cloud and fog are born when air full of moisture cools down to its dew point. The vapour settles on tiny nuclei: specks of dust, salt particles, smoke particles. Fog is the same cloud, only lying on the ground.
The dew point: how much moisture fits in the air and how it turns into cloud and fog
The difference between the air temperature and the dew point is called the spread. Relative humidity is a more tricky value: it changes even when no moisture has been added. In the morning in the valley it is +8 °C with a dew point of +7 °C: humidity is about 93 %, and there is fog. By midday the temperature has risen to +22 °C. There is the same amount of vapour, the dew point is still +7 °C, but the humidity has dropped to about 38 %. The air has not become drier, only warmer. That is why the dew point tells a pilot more than percentages do.
The spread over one day: in the morning +8 and +7 with fog, at midday +22 and still +7, with the same amount of vapour in the air
Spread near the ground
What it means
What to expect in flight
0–2 °C
Air close to saturation
Fog, low cloud, launch closes within minutes
3–6 °C
Humid
Low cloud base, clouds grow early and fast
7–12 °C
Working range
Cumulus with a good base, a readable sky
Precipitation (rain, snow, hail) is measured in millimetres (mm), whether it falls as water or as ice. 1 mm is a layer of water one millimetre thick on a flat surface, which is 1 litre per square metre.
One millimetre of precipitation is one litre of water per square metre
A detail for pros. The dew point belongs to the air mass, not to the place: when the air warms up it hardly changes, and when the air rises it drops by 0.2 °C per 100 m. But it changes sharply when a boundary between air masses has passed. A fast rise of the dew point within a couple of hours with the same wind means that a more humid air mass has moved in, and the cloud base will be lower than your morning calculation.
The dew point belongs to the air mass: if it rises fast, the cloud base will be lower than your morning calculation
Drag the diagram sideways to see all of it.
Warm air holds more vapour
What it is
There is always water vapour in the air, and it is invisible. How much of it fits depends on the temperature: warm air holds more moisture, cold air holds less. Like jars of different sizes: at +22 °C, 26 portions of vapour fit, at +8 °C only about 11.
In practice
In the diagram both jars hold the same vapour, 10 portions each: the warm one is 38 % full, the cold one 93 %, almost to the top. This is why a mountain morning often brings dew and fog: overnight the air near the ground has cooled, and the jar has become too small. During the day it will warm up and the fog will go, although there is no less water in the air.
The dew point is a temperature
What it is
The dew point is the temperature to which you must cool the air for the vapour to start turning into droplets. Not a height and not a place on the map, but a temperature. Cloud and fog are born the same way: air full of moisture cools down to its dew point, and the vapour settles on tiny nuclei: specks of dust, salt particles, smoke particles.
In practice
Where the air cools, water appears. If it rises in a thermal during the day, it cools and becomes a cloud higher up. If it cools at night over cold ground, it lies as fog in the valley. Fog is the same cloud, only lying on the ground: at launch you see nothing in it either.
The spread tells you more than percentages
What it is
The difference between the air temperature and the dew point is called the spread. Relative humidity is more tricky: it changes even when no moisture has been added. In the morning in the valley it is +8 °C with a dew point of +7 °C: humidity about 93 %, there is fog. By midday +22 °C, the same vapour, the dew point is still +7 °C, but the humidity has dropped to about 38 %.
In practice
Not drier, only warmer: the jar has grown, but the vapour in it is the same. That is why the dew point and the spread tell a pilot more than percentages. A morning spread of 1 °C is fog that will clear when the air warms up; a spread of 15 °C at midday means dry air and a high cloud base.
Spread near the ground: what to expect in flight
What it is
A spread near the ground of 0–2 °C: air close to saturation; fog, low cloud, launch closes within minutes. 3–6 °C: humid; low cloud base, clouds grow early and fast. 7–12 °C: the working range; cumulus with a good base, a readable sky.
In practice
The metres in the diagram follow the rule of section 5: base above launch ≈ spread × 125 m. A spread of 2 °C means clouds 250 m above your head, almost on the slope. A spread of 10 °C means a base about 1250 m above launch, room to work. Check the spread for the hour of your flight, not only the morning one.
A millimetre of precipitation is a litre per square metre
What it is
Precipitation (rain, snow, hail) is measured in millimetres, whether it falls as water or as ice. 1 mm is a layer of water one millimetre thick on a flat surface, which is 1 litre per square metre. Snow and hail are measured melted, as the same water.
In practice
Forecasts give precipitation in millimetres for a period, for example “2 mm in 3 hours”: that is 2 litres per square metre. For flying, the fact of precipitation already matters: even drizzle wets the wing, and a wet wing is heavier and tends to go into parachutal stall (more in aerodynamics, section 15). We wait until the precipitation has passed and the wing is dry.
The dew point belongs to the air mass
What it is
The dew point belongs to the air mass, not to the place. When the air warms up during the day it hardly changes: there is the same vapour in the air. In a rising bubble it drops by 0.2 °C for every 100 m: this gives the cloud base rule. And when a boundary between air masses passes, the dew point jumps.
In practice
A practical sign: the dew point has risen fast within a couple of hours with the same wind; a more humid air mass has moved in, and the base will be lower than your morning calculation. In the diagram +7 → +12 °C: the base was about 1875 m above launch, now it is 1250 m. Recalculate your margin to the clouds and to the area ceiling.
The numbers of the section: in the morning +8 °C with a dew point of +7 °C, humidity about 93 %; at midday +22 °C and still +7 °C, about 38 %; spread near the ground 0–2, 3–6 and 7–12 °C; 1 mm of precipitation = 1 litre per m²; in rising air the dew point drops by 0.2 °C per 100 m. Humidity is calculated with the Magnus formula; vapour “portions” are hectopascals of its pressure; the base follows the rule of section 5: spread × 125 m.
Words from this section
Cumulus: a cloud with a flat bottom and a lumpy top, the one that grows over a thermal. For a pilot it is a sign: under it there is most likely lift. Like steam over a pot: the cloud is not there on its own, it is the top of the air flow you are looking for.
In shortThere are two lapse rates: one of the surrounding air, measured by the sounding balloon, and one of the rising bubble itself. A dry bubble cools by 1 °C per 100 m, after condensation starts only by 0.5–0.6 °C. If the air around cools faster than the bubble, the atmosphere is unstable and thermals are strong; if slower, it is stable and the sky is blue. An inversion, where the temperature does not fall or even rises, works like a lid.
The lapse rate shows by how many degrees the temperature changes as you go up. There are two lapse rates, and you must not mix them up.
The lapse rate of the surrounding air is what the sounding balloon measured today. In the standard atmosphere it is 6.5 °C per kilometre, but on a given day it can be anything.
The adiabatic lapse rate is how the rising bubble of warm air itself cools down. Higher up the pressure is lower, so the bubble expands and cools. It does not exchange heat with the air around it: this is what “adiabatic” means. Unsaturated, “dry” air loses 1 °C for every 100 m. As soon as condensation starts, latent heat is released (the energy that the vapour gives back when it turns into a droplet), and the cooling slows down to 0.5–0.6 °C per 100 m. This is the moist adiabat.
Two lapse rates: what the sounding balloon measured and how the rising bubble itself cools
Compare the two lapse rates, and you get the stability of the atmosphere. The surrounding air cools faster than the bubble: the bubble stays warmer and lighter than the air around it all the time and speeds up upwards. This is an unstable atmosphere: strong thermals, high cumulus, a risk of overdevelopment. The surrounding air cools more slowly: the bubble becomes colder than the air around it and slows down. This is a stable atmosphere: a blue sky, weak and low thermals.
There is also a third state, the most important one for thunderstorms. If the lapse rate of the surrounding air lies between the moist and the dry adiabat, the air has two faces. While the bubble is dry, it slows down. Once it reaches condensation and starts heating itself from inside, it shoots upwards. This is conditional instability. This is how you get a day that is calm until noon and stormy by four o’clock.
Three states of the atmosphere: we compare the lapse rate of the surrounding air with the path of the bubble
Let us do the numbers. The forecast gives +20 °C at 2000 m and +13 °C at 3000 m. The lapse rate is 0.7 °C per 100 m, between the dry adiabat (1.0) and the moist one (0.55). A dry thermal will run out of power, but a cloud, if it forms, will go up by itself. On a day like this you start early and finish early.
The calculation explained: 0.7 °C per 100 m lies between the moist and the dry adiabat
Lapse rate of the surrounding air
State
What happens in the sky
0.6–1.0 °C per 100 m
Conditionally unstable
A working day, but keep a margin for overdevelopment
less than 0.55 °C per 100 m
Absolutely stable
Weak low thermals, ridge soaring instead of thermals
0 or temperature rising
Inversion
A lid. It will not let you higher
The lapse rate table: what each one gives in the sky
A detail for pros. On a hot midday the first tens of metres above dry ground can be superadiabatic: the temperature falls faster than a degree per hundred metres. Such a layer does not live long, it keeps breaking away. This is where dust devils are born, and this is why a thermal near the ground at these hours is a ragged jet, not a bubble.
Case review. Two days in a row, the same +24 °C at the ground. On the first day the cloud base was at 1200 m and the thermals gave 2 m/s; on the second day the cumulus had grown into towers already by one o’clock. At the ground there was no difference at all: the difference was in the air mass higher up. Decision: do not look at the temperature at the ground, look at the temperature difference between the ground and the 700 hPa level, which is about 3000 m. This is what tells you whether the day will be a working day or an explosive one.
Two days with the same +24 °C at the ground: the difference was higher up
Drag the diagram sideways to see all of it.
There are two lapse rates, and you must not mix them up
What it is
The first is the lapse rate of the surrounding air: what the sounding balloon measured today. In the standard atmosphere it is 6.5 °C per kilometre, but on a given day it can be anything. The second is the adiabatic one: how the rising bubble itself cools. Higher up the pressure is lower, the bubble expands and cools, and it does not exchange heat with the air around it: this is what “adiabatic” means. A dry bubble loses exactly 1 °C for every 100 m. As soon as condensation starts, latent heat is released (the energy that the vapour gives back when it turns into a droplet), and the cooling slows down to 0.5–0.6 °C per 100 m.
In practice
The orange and the turquoise lines in the diagram are a ruler. It is the same every day, you do not need to learn it again. Only the blue line changes: the air that is already above you. The whole forecast for a paraglider comes down to one question: where is the blue line today compared with these two. Switch the states and watch: the ruler stays, the day changes.
The air around cools faster than the bubble
What it is
The surrounding air loses temperature faster than the bubble itself. So the bubble is always warmer and lighter than the air around it, and the higher it goes, the bigger the difference. The lines move apart, they never cross, and the climb speeds itself up. This is an unstable atmosphere: strong thermals, high cumulus, a risk of overdevelopment.
In practice
The table of the section has no such row, and that is not a mistake. A lapse rate steeper than 1.0 through the whole working layer is rare; more often it is a thin layer near the ground, shown in the “Superadiabatic” state. But if such air stands over kilometres, the day starts early, speeds up fast and ends with a thunderstorm. Do not plan to “fly as long as possible”: plan to land before the sky closes.
Less than 0.55: absolutely stable
What it is
The surrounding air cools more slowly than the bubble. After only a couple of hundred metres the bubble becomes colder than the air around it and slows down. The point where its line meets the line of the air around is the top of the thermal: the bubble will not go higher, however long you circle.
In practice
A blue sky, weak and low thermals, ridge soaring instead of thermals. This is not “bad weather”: it is the best day for first solo flights and for an evening flight along the slope. Make your plan along the slope, not along a route: you will not be able to climb anyway, but the air is calm and predictable.
From 0.6 to 1.0: conditional instability
What it is
The lapse rate of the surrounding air lies between the moist adiabat (0.55) and the dry one (1.0), and the air has two faces. While the bubble is dry, it slows down and runs out of power. But if it has reached condensation, latent heat is released inside, the cooling slows down to 0.55, and now it is the bubble that cools more slowly than the air around it. It shoots upwards.
In practice
This is both the most common working day and the parent of thunderstorms: calm until noon, towers by four. The diagram shows both branches: the orange, dry one dies at 670 m; the turquoise one, from the cloud base, goes up without stopping. The practical lesson: watch not the strength of the thermal but the height and the growth speed of the cumulus. If it grows up faster than you climb, go and land.
Zero or rising temperature: an inversion
What it is
In an inversion layer the temperature does not fall with height, and sometimes it even rises. A bubble that was warmer than the air around it lower down hits this layer and almost at once becomes colder than the air around it. It goes no further: in the diagram you see the orange line stop right at the bottom edge of the band.
In practice
This is a lid over the valley. A classic Gudauri picture: in the morning the valley is hazy, everyone’s thermals stop at the same height, and above it the air is smooth. An inversion usually breaks from the bottom up by midday, so the day “switches on” suddenly, not smoothly. If everyone lands from the same height, it is not that you circle badly: it is the lid.
We calculate the lapse rate from two forecast points
What it is
The forecast gives +20 °C at 2000 m and +13 °C at 3000 m. The difference is 7 °C per 1000 metres, which is 0.7 °C per 100 m. This lies between the moist adiabat (0.55) and the dry one (1.0), so the day is conditionally unstable. Note: the diagram shows only the line between the two levels from the forecast; you must not extend this line down to the ground, nobody measured it there.
In practice
A dry thermal will run out of power, but a cloud, if it forms, will go up by itself. On a day like this you start early and finish early. The calculation itself is two numbers from the forecast and a division in your head: ten seconds. This is the fastest check of “will there be anything to catch today”, and you do it before you get into the car.
A detail for pros: the superadiabatic layer
What it is
On a hot midday, above dry hot ground, the first tens of metres can be superadiabatic: the temperature falls faster than a degree per hundred metres. On the main diagram this layer is two pixels, so here it is shown separately and large. It does not live long: it keeps breaking away.
In practice
This is where dust devils are born. A thermal near the ground at these hours is a ragged jet, not an even bubble; at launch you feel it as jerky, “biting” air right at the ground. If you see a dust devil near launch, it is not a danger in itself, it is a message: the air below breaks away raggedly, so fly the wing actively in the first metres.
Two days in a row, the same +24 °C at the ground
What it is
On the first day the cloud base was at 1200 m and the thermals gave 2 m/s. On the second day the cumulus had grown into towers already by one o’clock. At the ground there was no difference at all: the difference was in the air mass higher up.
In practice
That is why you look not at the temperature at the ground, but at the temperature difference between the ground and the 700 hPa level, which is about 3000 m. In the diagram: on the calm day it is +6 °C at 3000 m, on the explosive day −3 °C, with the same ground. These nine degrees up there decide whether the day will be a working day or an explosive one. The thermometer at launch will tell you nothing about it.
The numbers of the section: standard atmosphere 6.5 °C/km; dry adiabat 1.0 °C per 100 m, moist 0.5–0.6. The table of the section: 0.6–1.0 is conditionally unstable; less than 0.55 is absolutely stable; 0 or rising is an inversion. The case “+20 at 2000 and +13 at 3000” gives 0.7 °C per 100 m.
Words from this section
Overdevelopment: when clouds have grown too much and the day has turned into a thunderstorm day. Like dough that has risen and is climbing out of the pot: it started nicely and ended with cleaning up.
Dust devils: small spinning columns of dust on hot ground. A sign that the air near the ground breaks away raggedly, not in even bubbles. Like a tiny tornado on a dusty road: not a danger in itself, but a message about the character of the day.
Ridge soaring: flying along a slope in the air that rises up it. Russian pilots call it “dynamic” in slang. Like a surfer on a wave near the shore: the wave goes up the slope, and you stay in it.
The upper-air diagram: how a pilot reads an emagram
In shortAn emagram is a vertical cut through the atmosphere: height up the side, temperature along the bottom. You read it in five steps: find the launch level, draw a dry adiabat upwards from the day’s forecast maximum, where it crosses the air curve is the top of the thermal, where the two dashed lines of the bubble meet is the cloud base, then compare these two heights. Base below the top of the thermal: there will be clouds; above it: a blue day.
An emagram is a graph of a vertical cut through the atmosphere. Height or pressure goes up the vertical axis, temperature along the horizontal one. Two curves are drawn on it: the air temperature and the dew point at each height. The data come from a radiosonde or from a model calculation. Pilots also call this graph an upper-air diagram or a sounding.
What is drawn on it:
the temperature curve: how the surrounding air cools with height;
the dew point curve: how much moisture there is at each height;
the distance between the curves: the spread; where they meet, there will be cloud;
the sloping grid: the adiabats along which a bubble cools.
How to read it in five steps.
Find the launch level on the temperature curve.
From the day’s forecast maximum, draw a dry adiabat upwards.
Where it crosses the curve of the surrounding air is the top of the thermal: above it the bubble is colder than the air around it.
The point where the bubble’s temperature meets its dew point is the condensation level, that is, the cloud base.
Compare the top of the thermal and the base. Base below the top: there will be clouds. Base above it: a “blue” day, you go by the birds and the vario.
A common question. The solid green line is the moisture of the surrounding air, and it does not have to meet the temperature at the cloud base. The base is set by the two dashed lines: the temperature of the bubble and its own dew point. The bubble cools by 1 °C every 100 m, but its dew point drops by only 0.2 °C, so the spread between them closes by 0.8 °C every 100 m. Where it has closed, condensation starts: that is the cloud base.
The same five steps as a picture. The base is set by the two dashed lines: the temperature of the bubble and its dew point. The solid lines are the air around, and they do not have to meet at the cloud base. A teaching diagram: the numbers are round so you can do the sums in your head.
Let us go through it step by step. The three pictures below answer the most common question: why the base is taken from the two dashed lines and not from the solid green line.
The dew point is an everyday thing. A glass, a mirror and your breath in the cold: each time the air cooled below its dew point, and the moisture became visible.The two green lines are about different things. The solid one describes the air around, the dashed one the bubble itself. The base is calculated from the bubble’s lines.The same calculation in a column. The spread melts by 0.8 °C every 100 m: 12 °C at the ground reaches zero at 1500 m.
What to look for with your eyes. A bend of the temperature curve to the right: a stable layer or an inversion; these bends are the ceilings. The curves moving apart high up: the upper atmosphere is dry, there will be no high cloud. The curves coming closer at middle heights: a humid layer is moving in, and by midday it will cover the sky.
Number guidelines from the practice of soaring meteorology. A convective layer 1500 m thick or more: a day with strong and high thermals. A spread of 2–4 °C at the top of the layer: nice, readable cumulus. A spread of more than 10–15 °C in the upper troposphere: high cloud will not cover the sun. These are guidelines, not laws.
A detail for pros. A model emagram and a real sounding are not the same thing. The model smooths the terrain: in the mountains its “ground level” can be hundreds of metres below the real launch, and the whole lower part of the graph is made up. Take the lapse rate and the humidity higher up from the model, but set your starting point by the real launch height.
Case review. The 14:00 emagram shows a part from 2200 to 3000 m that runs parallel to the dry adiabat, and a sharp bend to the right at 3000 m. What the pilot sees: a blue sky, bits of cumulus all at one level, not growing. What it is: convection has reached a stable layer and stopped. Decision: the ceiling of the day is 3000 m, you will not get up to the 3200 m ceiling of the area. We plan the route in the 2200–3000 m layer and calculate glides from this ceiling, not from the permitted height.
Drag the diagram sideways to see all of it.
An emagram is a vertical cut through the atmosphere
What it is
Height goes up the side, temperature along the bottom. Two curves are drawn on the graph: the blue one is the air temperature at each height, the green one is its dew point, that is, how much moisture it holds. The data come from a radiosonde (its path is shown as a balloon in the diagram) or from a model calculation. The light sloping grid is the adiabats: the paths along which a rising bubble cools.
In practice
The first thing you do with your eyes is look at the distance between the blue and the green lines. This is the spread. Where the lines are far apart, the air is dry; where they meet, there is cloud. Pilots also call this graph an upper-air diagram or a sounding, and weather forecast sites draw it for your point for any hour.
Step 1. Find the launch level
What it is
An emagram does not start at your launch, but lower: at the valley or even at sea level. So first you find on the blue curve the height you will fly from. Everything below it has nothing to do with your flight.
In practice
In Gudauri this is 2200, 2470, 2750 or 3000 m, depending on the launch. In the example it is 2200 m, where the morning sounding showed +16 °C. Mark this point straight away: everything after this is built from it, and a mistake here will move both the top of the thermal and the base.
Step 2. A dry adiabat from the day maximum
What it is
From the forecast maximum temperature at launch level, draw a line upwards along the sloping grid: along the dry adiabat, 1 °C for every 100 m. This is the path along which a warm bubble cools, until condensation starts in it.
In practice
You take the day maximum, not the morning temperature: by midday the ground will warm up, and the bubbles will leave it warm. In the example the forecast gives +22 °C at launch. The line is dashed: it is not the air, it is our calculation of the bubble’s path.
Step 3. The top of the thermal is the crossing
What it is
Where the dashed bubble line crosses the blue curve, the bubble has the same temperature as the air around it. Higher up it is colder and heavier than the air around: the climb stops. This is the top of the thermal.
In practice
In the example the top is 4200 m: 2000 m above launch. A layer of 1500 m or more is a day with strong and high thermals. Watch how the bubble overshoots the top and sinks back: in the air it is the same, near the top the thermal gets weaker and falls apart, you cannot climb higher.
Step 4. The base is where the two dashed lines meet
What it is
The base is set by the two dashed lines: the temperature of the bubble and its own dew point. The bubble cools by 1 °C every 100 m, but its dew point drops by only 0.2 °C, so the spread between them melts by 0.8 °C every 100 m. Where it has melted to zero, condensation starts: that is the cloud base.
In practice
A common question: why not by the solid green line? The solid line is the moisture of the air around, and it does not have to meet the temperature at the base. Count in your head: the spread at launch is 12 °C, divide by 0.8: that is 15 times 100 m, so 1500 m. The base: 2200 + 1500 = 3700 m.
Step 5. Compare the base and the top
What it is
If the base is below the top of the thermal, the bubble reaches condensation in time: there will be clouds. If the base is above the top, the bubble runs out of power before it becomes a cloud: the day is “blue”.
In practice
In the example the base is 3700 m and the top is 4200 m: there will be cumulus, and it will show where the thermals are. On a blue day there are thermals, but you cannot see them: you go by the birds, by other pilots and by the vario. A guideline from practice: a spread of 2–4 °C at the top of the layer gives nice, readable cumulus.
What to look for in ten seconds
What it is
Three signs are visible without any calculation. A bend of the blue curve to the right: a stable layer or an inversion; these bends are the ceilings. The curves far apart high up: the upper atmosphere is dry, there will be no high cloud. The curves close together at middle heights: a humid layer is moving in, and by midday it will cover the sky.
In practice
A guideline from the practice of soaring flight: a spread of more than 10–15 °C in the upper troposphere means high cloud will not cover the sun. These are guidelines, not laws. An emagram is a forecast; in the air, check it against the sky.
Case review: the 14:00 emagram
What it is
The curve goes at an even slope from 2200 to 3000 m (convection works there) and at 3000 m bends sharply to the right. What the pilot sees: a blue sky, bits of cumulus all at one level, not growing. What it is: convection has reached a stable layer and stopped.
In practice
Decision: the ceiling of the day is 3000 m, and you will not get up to the 3200 m area ceiling, however long you circle. We plan the route in the 2200–3000 m layer and calculate glides from this ceiling, not from the permitted height. Bits of cloud all at one height are a hint you can see right from the ground.
A detail for pros: model and real sounding
What it is
A model emagram and a real sounding are not the same thing. The model smooths the terrain: in the mountains its “ground level” can be hundreds of metres below the real launch. Then the whole lower part of the graph describes air that is not there at your launch: there is a mountain there.
In practice
Take the lapse rate and the humidity higher up from the model, but set your starting point by the real launch height. In the diagram the model puts the ground at 1600 m, but the real launch is at 2200 m: the bubble starts from here, from the real ground, and the forecast maximum is taken for it.
The numbers of the section: dry adiabat 1 °C per 100 m, the bubble’s dew point −0.2 °C per 100 m, the spread melts by 0.8 °C per 100 m (12 °C closes in 1500 m). Guidelines from the practice of soaring flight: a convective layer from 1500 m means a strong day; a spread of 2–4 °C at the top of the layer means readable cumulus; a spread higher up of more than 10–15 °C means high cloud will not cover the sun. A teaching example: the numbers are round so you can do the sums in your head.
What follows is for our students
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Language: the sections above are in English. Behind the code the full module text and the question bank are currently in Russian — the English versions are being prepared.
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