Forecasts and the go/no-go decision

The evening before a training day, your weather app shows light wind and sunshine. Your instructor says it will probably be a no: the app showed one number for one height, while your instructor looked at the wind higher up, the gusts and whether the afternoon will build into storms.

As a student you don’t make the final call; your instructor does. But the habit of briefing yourself starts now, and it protects you once nobody makes the call for you.

Forecasts are planning tools

A forecast tells you what kind of day to expect, not what will happen on your launch. Nobody can forecast an individual thermal, and internet weather is only as good as its source and how recently it was updated. The Swiss federation’s advice after its 2025 accidents: rain, foehn and thermal apps cannot clear you to fly, and if the pilots launching before you can only keep their wings open with a lot of skill, that is the real condition¹.

Pressure charts: the big picture

Start with a surface pressure chart, also called a synoptic chart. Its lines are isobars, joining places of equal pressure. Closely spaced isobars mean strong wind; widely spaced isobars mean light wind³.

In the northern hemisphere the wind circulates clockwise out of an anticyclone (high) and anticlockwise into a depression (low); in the southern hemisphere, the reverse. Above the friction layer the wind blows roughly along the isobars. Near the ground, friction turns it across them towards the low, by about 10° over water and up to 45° over rugged land, and in mountains the surface wind is hard to read from the isobars at all³.

Look for fronts too. A cold front often brings showers or thunderstorms and gusty, variable wind as it passes, then gusty, turbulent wind behind it; a warm front brings layer cloud, rain, low cloud and poor visibility ahead of it⁴. With its gusty wind, a front crossing your site makes it a no-go day for a student⁴⁵.

High pressure often brings settled days with light winds⁶; after a few days its sinking air can push the inversion so low that thermals stay weak⁷.

The wind higher up

Forecasts of winds aloft are usually drawn for pressure levels: the 850-hPa chart shows the wind at about 1,500 m4,900 ft above sea level and the 700-hPa chart at about 3,000 m9,850 ft, varying with the temperature⁸. For a mountain launch near 1,500 m4,900 ft, the 850-hPa wind is roughly what you meet once airborne.

The wind aloft is usually stronger than at the surface and blows from a different direction. About 600 m1,950 ft above the ground it is typically 20–40° to the right of the surface wind in the northern hemisphere (to the left in the southern), because friction slows only the lowest few hundred metres; a calm launch can sit under much stronger wind².

How strong is too strong? For you, anything above your limits at the heights you will launch and fly. Even powered-aircraft pilots call about 37 km/h23 mph at ridge level strong, with rough air and rotor downwind of rugged terrain⁹, and winds aloft above about 46 km/h29 mph are a classic cause of low-level wind shear¹⁰.

Will it overdevelop?

Next, stability: will the day stay smooth or build into showers and thunderstorms (the atmosphere and stability)?

Forecasters answer it with a sounding: a plot of temperature, dew point and wind against height above one place, drawn on an emagram. Soaring forecasts built from soundings give a trigger temperature (the surface temperature needed before thermals start), the maximum height of the thermals and an estimated cloud base¹¹. You don’t have to read soundings yet, but you should know what your instructor is looking at.

Two numbers summarise instability, and many forecast sites show them¹²:

• CAPE (convective available potential energy): any value above 0 J/kg means unstable air and a chance of thunderstorms, and thunderstorm days often exceed 1,000 J/kg.
• lifted index (LI): a negative value means unstable air.

Treat them as warning lights, not precise measurements. Thunderstorms need moisture, unstable air and something to lift it, such as converging winds or a sea breeze, and thermal weather can turn into thunderstorm weather quickly when the air is moist¹³. High CAPE with a clearly negative lifted index usually makes it a no-go day for a student⁵, and you never launch or land with a thunderstorm approaching¹⁴.

Gusts

The average wind is only half the story. In US airport reports (METARs) the wind is a two-minute average, and a gust figure appears only when the wind jumps by about 19 km/h12 mph or more between peaks and lulls¹⁵. Conventions differ between countries, but the lesson is the same everywhere: a report with no gust figure can still hide strong gusts. That is a lot for a student: USHPA’s recommended limit for Novice (P2) pilots is peak gusts of 24 km/h15 mph, building no faster than 8 km/h5 mph in 5 s¹⁶.

Gusts matter because the force of the wind on your wing grows with the square of its speed: twice the wind gives about four times the force¹⁷. Compare the forecast gusts with the average; the difference is the gust factor¹⁵, and a large one is a red flag even when the average looks fine.

Live stations and webcams

In the morning, switch from forecasts to observations. Online weather stations show the live wind, gusts and direction, often with a graph of recent hours. The graph tells you more than the latest number: is the wind building, steady or dying? APPI asks its students to judge exactly this short-term trend¹⁸.

Know where each station is and how high its sensor sits. On flat, open ground a 10 m35 ft mast reads more than you feel at head height¹⁹, but a hilltop launch is often windier than a station in the valley, because the wind speeds up over the crest²⁰. Airport reports are also a few minutes old when you read them¹⁵.

Webcams let you read the on-site signs before you leave home: how trees and flags move, whitecaps on a lake, and towering cumulus building¹⁹²¹²². On site, many instructors teach students to measure the wind with a hand-held wind meter rather than judge it by feel²³.

Brief yourself

Run the same routine every time; your instructor’s is fuller.

Student limits

Your instructor sets your limits for the day, usually tighter than any published figure²⁵. The published student limits of each training body (USHPA, BHPA, APPI and SafePro) are compared side by side in Reading the wind; read them before you decide.

Making the call

One manufacturer’s manual puts it bluntly: most serious accidents come from misjudging the weather, not from equipment failure, and paragliders are not designed for turbulent or unstable air or rain²⁶. So the rule is deliberately cautious: if any item is outside your limits, or anything leaves you in doubt, the answer is no. The DHV’s version is the old pilots’ saying “a doubt is a no”, and it says most accidents could be prevented by keeping to it²⁴.

Watch for small problems adding up: the BHPA’s review of serious incidents found misjudged conditions and small adverse factors piling up, and in the FAA’s personal-minimums method two or more marginal areas mean you don’t go²⁷.

A no-go is a good decision, not a timid one, and there will be many better flying days²⁴. Personal minimums and checking yourself are covered in Go/no-go and personal minimums.