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.
We fly in the lowest layer of the atmosphere — the troposphere. Over the Caucasus its upper boundary lies at roughly 10–12 km. Almost all the weather — cloud, wind, precipitation — lives right here. Above it the stratosphere begins. At heights around 10,000 m the jet streams flow: narrow “rivers” of very fast wind. Civil and military aircraft travel in them. A paraglider, a powered paraglider and a hot-air balloon don’t get that high in normal flying: these are airliner heights and rare records.
Atmospheric pressure is the force with which the weight of a column of air presses on a unit of area. The higher you climb, the shorter the column above you and the less it weighs. So the pressure falls. Going down it is the other way round: the pressure rises. That is what a vario works on — the instrument that shows your height and your rate of climb. It does not feel height at all: 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 quantity 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 at sea level: higher up the air is thinner, and at 3,000 m the same hectopascal already stretches over about 11 metres. Lowland conversion tables lie in the mountains.
Two ways to set your instrument. QNH — pressure reduced to sea level: the instrument shows height above the sea, and that is how charts and airspace boundaries are defined. QFE — the pressure at the site itself: the instrument shows height above it and reads zero at launch. Handy for practising approaches, and dangerous where airspace is concerned.
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 less. The dew point is the temperature to which the air has to be cooled for the vapour to start turning into droplets. Not a height and not a place on the map, but a temperature.
From which comes the main rule: cloud and fog are born when moisture-saturated air cools to its dew point. The vapour condenses onto the tiniest nuclei — specks of dust, salt particles, combustion products. Fog is the same cloud, only lying on the ground.
The difference between the air temperature and the dew point is called the spread. Relative humidity is a trickier quantity: 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 around 93 %, and fog. By midday the temperature has risen to +22 °C. There is just as much vapour, the dew point is still +7 °C, but the humidity has fallen to about 38 %. It has not got drier, it has got warmer. That is why the dew point tells a pilot more than the percentages do.
| Spread near the ground | What it means | What to expect in flight |
|---|---|---|
| 0–2 °C | Air on the edge of saturation | Fog, low cloud, the launch closes in minutes |
| 3–6 °C | Damp | Low cloudbase, cloud develops early and fast |
| 7–12 °C | The working range | Cumulus with a good base, a readable sky |
Precipitation — rain, snow, hail — is measured in millimetres (mm), and its physical state has nothing to do with it. 1 mm is a layer of water one millimetre thick on a horizontal surface, that is, 1 litre per square metre.
The temperature gradient shows how many degrees the temperature changes with height. There are two gradients, and they must not be confused.
The environmental lapse rate — what the sounding actually measured today. In the standard atmosphere it is 6.5 °C per kilometre, but on any particular day it can be anything.
The adiabatic lapse rate — how the rising bubble of warm air itself cools. There is less pressure higher up, so the bubble expands and cools. It exchanges no heat with its neighbours while doing so — that is what “adiabatic” means. Unsaturated, “dry” air loses 1 °C for every 100 m. As soon as condensation begins, latent heat is released — the energy the vapour gives back as it becomes a droplet — and the cooling slows to 0.5–0.6 °C per 100 m. That is the saturated adiabat.
Compare the two gradients and you have the stability of the atmosphere. The surrounding air cools faster than the bubble: the bubble stays warmer and lighter than its neighbours all the way up and accelerates. That is an unstable atmosphere — strong thermals, high cumulus, a risk of overdevelopment. The surrounding air cools more slowly: the bubble becomes colder than its neighbours and slows down. That is a stable atmosphere — a blue sky, weak and low lift.
There is a third state as well, and it is the most important one for thunderstorms. If the environmental lapse rate lies between the saturated and the dry adiabat, the air behaves two-facedly. While the bubble is dry, it slows down. Once it reaches condensation and starts heating itself from within, it breaks away upwards. That is conditional instability. That is how you get a day that is calm until noon and stormy by four o’clock.
Working it out in numbers. The forecast gives +20 °C at 2,000 m and +13 °C at 3,000 m. The gradient is 0.7 °C per 100 m, between the dry adiabat (1.0) and the saturated one (0.55). A dry thermal will run out of steam, while a cloud, if one forms, will go up on its own. A day like that is started early and finished early.
| Environmental lapse rate | State | What the sky does |
|---|---|---|
| 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 lift, ridge lift instead of thermals |
| Zero, or temperature rising | Inversion | A lid. It will not let you through |
An emagram is a graph of a vertical slice of the atmosphere. Height or pressure runs up the vertical axis, temperature along the horizontal one. Two curves are plotted on it: the air temperature and the dew point against height. The data come from a radiosonde or from a model calculation. Pilots also call such a graph an upper-air diagram or a sounding.
What is drawn on it:
How to read it in five steps.
A common question. The solid green line is the moisture of the surrounding air, and it does not have to meet the temperature curve at the base. The base is set by the two dashed lines: the temperature of the bubble and its own dew point. The bubble cools 1 °C per 100 m while its dew point falls only 0.2 °C, so the spread between them closes by 0.8 °C every 100 m. Where it closes, condensation begins: that is the base.
Let us take it step by step. The three pictures below answer the most common question: why the base is read from the two dashed lines and not from the solid green one.
What to look for with your eyes. A bend of the temperature curve to the right — a stable layer or an inversion; those bends are the ceilings. The curves diverging high up — the upper atmosphere is dry and there will be no high cloud. The curves converging at middle heights — a moist layer is moving in and will cover the sky by midday.
Numerical benchmarks from soaring meteorology in practice. A convective layer 1,500 m thick or more — a day with strong, high lift. A spread of 2–4 °C at the top of the layer — handsome, readable cumulus. A spread of more than 10–15 °C in the upper troposphere — high cloud will not shut the sun out. These are benchmarks, not laws.
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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.
