Pressure — the earliest warning
Sea-level pressure does not tell you tomorrow's sunshine; it tells you the AIR MASS IS CHANGING. A steady fall of a few hPa over a day means a trough or front is arriving: wind will back or veer, high cloud thickens first, and afternoon convection runs stronger than usual. A rise means drier, cooler air moving in — clear skies, but usually a stronger gradient wind.
In the tropics pressure falls 2–3 hPa by itself between about 10:00 and 16:00 and recovers overnight. That is the atmospheric tide, not a warning. This forecast therefore flags a drop only when it is FASTER than that daily rhythm, compared against the same hours the day before.
Surface wind — direction matters as much as speed
A launch works only when the wind blows INTO the slope. At the same 5 m/s, straight onto the face inflates the wing evenly and the launch is easy; from behind the wing collapses, and across the face you launch crooked and drift into trees. That is why the arrows here are coloured by each site's own direction rules, not by the number alone.
Mountain passes add a trap: wind funnels through a gap and accelerates right at the launch edge. The same direction is smooth when light and vicious when strong, so each site has a separate 'venturi' rule that only triggers once the speed is up.
Upper winds and shear
Wind at 925, 850 and 700 hPa (roughly 750 m, 1,500 m and 3,000 m) decides more of the flight than most people expect. Calm on the ground with 8–10 m/s at 500 m above launch means thermals are torn apart as soon as they leave the slope: the wing gets rough and you drift backwards. Soaring low on the face is fine; climbing is not.
A sudden jump between layers is WIND SHEAR. The danger is that the anemometer on launch reads gentle while the air just above is not. The Airgram tab stacks four levels for exactly this: wind increasing gradually with height is normal, a sudden jump or a sharp change of direction is what you avoid.
Inversion — the invisible lid
Air normally cools with height. An inversion is a layer that is WARMER above than below, usually formed overnight under clear skies. It acts as a lid: a rising thermal bubble stops there, so the climb ceiling is low and flights are short.
An inversion also traps fog and low cloud in the valley until late morning — the reason a pass can be white over while the rain forecast reads zero. Once the sun burns through, the lid breaks and the fog clears within an hour; this page gives the expected clearing time so you can plan when to head up.
Thermals — the fuel of the flight
The ground absorbs radiation, heats the air against it, and bubbles of warm air break away and rise: that is a thermal. Two numbers describe it — CAPE (how much convective fuel there is) and the mixing-layer top (how high a bubble can get). This page calls that top the thermal ceiling.
The scale here rates towards SMOOTH: moderate thermals mean a long, comfortable flight, while rough ones make the wing buck and the landing field gusty. Cross-country pilots read it the other way round — they want it strong, so the same number means opposite things to the two of you.
Cloud, rain and thunderstorms
Cloud base comes from the spread between temperature and dew point (about 125 m per 1 °C), plus the height difference between the model's grid cell and the real launch. A base below launch height means the launch is inside cloud: no slope, no landing field, no flying.
Rain is graded in millimetres per hour, not per cent — the model's per cent means 'rain somewhere in a 25 km cell', which reads 90% every day of the wet season. The real danger is not the raindrops but the THUNDERSTORM: before the cloud arrives its downdraught sweeps the launch, reversing the wind and multiplying its speed within minutes. A 40% storm risk is already reason enough not to take off.
