Clouds and weather hazards

Most of the weather that ends a flying day arrives in plain sight. The clouds start telling the story hours ahead: small and crisp in the morning, taller by lunchtime, dark and heavy by mid-afternoon.

Gin’s school-wing manual says most serious paragliding accidents come from pilots misjudging the weather¹, and often the signs were there, as in the rain-cell accident later in this lesson². This lesson is about reading the sky early enough to be on the ground, packed away, before it matters.

The life of a cumulus

A cumulus sits on top of a thermal. A growing one has a sharp, well-defined base with lift under it; a dissipating one looks fuzzy and ragged, with little lift or even sink beneath. Some cumulus “cycle” quickly, forming and dying within a short time. A dark patch under the base means a deeper cloud and a better chance of a thermal there³.

You can estimate the base before you fly: about 125 m410 ft above the ground for each degree Celsius between the temperature and the dew point⁴ (see The atmosphere and stability). Paragliders fly by visual flight rules, which means staying out of cloud⁵; how far you must keep from it depends on where you fly (Airspace and the law).

Figure: Six skies to recognise. Notice the sharp base of a growing cumulus against the ragged edge of a dying one, a tower much taller than it is wide, and rain that never reaches the ground.

Overdevelopment

Overdevelopment is cumulus growing into towering clouds and, often, thunderstorms. Towering cumulus show unstable, turbulent air and often grow into cumulonimbus; on unstable days any cumulus can quickly grow into a towering cumulus⁶. It needs three ingredients: moisture, unstable air and something to lift it, such as a front, converging winds, upslope flow, outflow from earlier storms, or sea and valley breezes. When the air is moist as well as unstable, thermal soaring weather can turn into thunderstorm weather quickly⁷.

Signs to watch for⁶:

• clouds growing taller than they are wide, with crisp, cauliflower tops⁸;
• one big cloud growing while the small cumulus around it disappear;
• virga or showers falling from the base;
• near a dying storm, new dark, firm bases: a new cell is forming.

Thunderstorms

A thunderstorm cell grows through towering cumulus, mature and dissipating stages. Its updraughts can exceed 15 m/s2,960 ft/min, its hazards peak late in the mature stage, and strong winds aloft spread its top into an anvil. Sources differ on how long a cell lasts, from about half an hour to about an hour, and multicell storms keep regenerating for hours¹¹.

Gust fronts

As a storm matures, its rain-cooled downdraught hits the ground and spreads out well ahead of it as cool, gusty air. The leading edge is the gust front: a sudden and sometimes violent change in the wind that can arrive before any rain. Warning signs are a line of dust or debris, spray or dark streaks on lakes, blowing smoke, and a shelf or roll cloud on the storm’s leading edge, which marks the top of a very turbulent zone¹².

How far ahead? The sources measure different things¹³:

The safe reading: outflow can travel tens of kilometres and outrun the storm, so no distance is safe.

Figure: The outflow of a thunderstorm (not to scale). Notice that the wind at the landing field may be light, even blowing towards the storm, then swings round into violent gusts when the gust front arrives, before any rain.

Downbursts and rain showers

A microburst is a small, intense downdraught that hits the ground and spreads out in every direction; the only visible clue may be virga or a ring of blowing dust. The FAA describes them as under 4 km2.5 mi across, lasting 5–15 min, with downdraughts up to 30 m/s5,900 ft/min; a university text says 1–4 km0.6–2.5 mi and only 2–5 min¹⁴.

You don’t need thunder for this. A local rain cell can produce a violent outflow of air cooled by evaporating rain, and downbursts can happen without heavy rain. In 2025 a pilot who underestimated a rain cell met increasingly gusty, turbulent air on the way down, suffered several large collapses and a spin, and crashed².

Lightning

Lightning can strike up to about 16 km9.9 mi from a thunderstorm, so if you can hear thunder you are probably within striking distance, and there is no safe place outdoors when storms are around. A training sailplane in Britain was struck with the storm still a few kilometres away⁹.

Cloud suck

Cloud suck is strong lift near the base of a cloud that draws you up into it. Lift often strengthens near cloud base, because condensation inside the cloud releases heat that makes the rising air more buoyant. Under an otherwise flat base it can increase dramatically in a hollow, and pilots trying to leave have found themselves climbing fast with cloud all around. Under a cumulonimbus the inflow can exceed 5 m/s980 ft/min and suck a sailplane into the storm⁸.

One school’s warning signs are clouds more than about 900 m2,950 ft deep with crisp cauliflower tops, where it expects stronger lift within about 300 m985 ft of the base, and a stepped base; it advises working any lift under such clouds on the side with open blue sky, so an escape route is close⁸.

Avoiding cloud suck matters far more than escaping it. Pilots have been sucked into cloud even on big ears and full speed bar, and some high-performance wings cannot do effective big ears. Gin’s school-wing manual says rapid descent techniques must be practised in calm air with plenty of height, and that careful weather assessment and watching the sky are the best ways never to need them¹⁵.

If you are being drawn up, descend and move away early, for example with big ears and speed bar, or leave the lift with big ears and look for sinking air. Don’t throw your reserve to escape a cumulonimbus: you would most likely be carried up into the cloud, hanging helplessly beneath it¹⁶. Descent techniques are covered in Blown back and cloud suck and practised with your instructor.

Wave and lenticulars

When strong wind blows across a ridge into stable air, the air can oscillate in waves downwind: mountain wave. MeteoSwiss gives the conditions as ridge-level wind of at least 25–40 km/h16–25 mph, as perpendicular as possible and ideally increasing with height, with stable air at ridge height. Smooth, lens-shaped lenticular clouds can mark the wave crests, but wave can exist in dry air with no clouds. Under the crests, near or below ridge level downwind, rotor zones of severe to extreme turbulence form, worst on the leading edge of the first rotor, and a rotor cloud can look like a harmless cumulus. Below lenticulars the wind is often variable and gusty¹⁷.

USHPA lists lenticular clouds among its weather danger signs for P2 students¹⁸. A calm launch under lenticulars is no comfort: the strong wind is just above you. Foehn, which often brings them, is covered in Local winds.

Virga, rain and fronts

Virga is rain falling from a cloud base and evaporating before it reaches the ground. It can mean a cloud has reached towering-cumulus or thunderstorm stage⁶, and the evaporating rain cools the air and drives outflow and downbursts beneath it². Rain itself is outside the operating limits of the school-wing manuals checked, because a wet wing is much more likely to go into a parachutal stall¹⁹ (see Stall and parachutal stall).

Fronts change the weather for hours. Cold fronts often bring a narrow band of showers or thunderstorms, gusty, variable wind as they pass, then rapid clearing with gusty, turbulent wind behind. Warm fronts bring widespread layer cloud, rain, low cloud and poor visibility ahead of them²⁰.