Weather & Flying Glossary

Every term, acronym, and unit we use in a forecast, in plain language. If a word in your report ever made you squint, it is here.

Ratings & Flyability Thermals & Lift Wind Sky & Clouds Weather Models & Data Flying Terms & Gear Units & Abbreviations Compass Directions

Ratings & Flyability

XC: Flying long distances away from launch instead of staying near the hill. An "XC day" has the lift to actually go somewhere.
XC Level: TowMeUp's headline rating of how good a day is for experienced free-flight pilots. The five levels run from Not Flyable up to Epic (all defined below).
Not Flyable: Blown out, rain, storm risk, or dead air. Nobody who knows better is driving out.
Possible: Flyable if you are local and bored. Sled runs and light lift. Not worth a drive.
Fun: A solid flying day. Thermals, ridge lift, or a good evening glass-off. The regulars will show up.
Exciting: Above average. Strong thermals, real cross-country potential, or beautifully smooth evening air. People rearrange their schedules.
Epic: Take time off work. High, all-day lift with calm winds and easy landings. The kind of day pilots talk about for months.
H2 Suitable: Whether the air is calm enough for Hang 2 (beginner) students. Reported as "yes", "glass-off only" (wait for the smooth evening window), or "no".
Student Air: A flag meaning conditions are calm and forgiving enough for beginner students.
H1 to H5: Hang gliding skill ratings: H1 beginner, H2 novice, H3 intermediate, H4 advanced, H5 master. "H3+" means H3 and above.
P1 to P5: Paragliding skill ratings, from P1 beginner up to P5 master. "P2+" means P2 and above.

Thermals & Lift

CAPE: How much energy the atmosphere has for rising air. More CAPE means stronger thermals, but too much means storms. Measured in J/kg. Rough guide: 50 to 300 is light, 300 to 800 active, over 800 watch for overdevelopment, over 1500 is storm risk.
BL: The churning layer of air near the ground where thermals form and live. Its top is the thermal ceiling.
BL Height: How high the boundary layer reaches, which is roughly how high thermals will carry you. Higher is better. Around 4,000 to 6,000 ft is an exciting day; above 6,000 ft with calm wind is epic.
LI: A stability number. Positive means stable air (weak or no thermals). Negative means unstable and thermic. Below -4 signals severe storm risk.
CIN: A cap of stable air that holds thermals down until the ground heats up enough to break through it.
Buoyancy: The estimated thermal climb rate, in m/s. Higher numbers mean faster climbs. Around 2 to 4 m/s is good, 4 to 6 is strong.
Thermal: A rising column or bubble of warm air. Pilots circle inside it to climb.
Shear / Shear Ratio: How much faster the wind blows up at ridge and thermal height compared to the ground. Low shear means smooth, organized lift. High shear means broken, punchy, tilted thermals that are hard to work.
Thermals (Light / Active / Strong): Forecast flags describing the strength of the day’s thermals, from gentle light lift up to strong, punchy climbs.
Overdevelopment: When thermals and clouds grow too much, shading the ground, killing the lift, and sometimes building into storms.
Freezing Level: The altitude where the air temperature reaches 32°F (0°C).

Wind

Gust: A brief spike in wind speed above the steady average. Gusts over 20 mph are a ground stop. 15 to 20 mph is rough and a no-fly for H2 students.
Wind at 80m: Wind speed about 260 ft above the ground. It is the best stand-in for conditions at ridge and thermal-soaring altitude.
Wind at 10m: Wind speed at the standard surface measuring height, about 33 ft up.
Ridge Soaring / Ridge Soarable: Staying aloft on wind that a ridge deflects upward, no thermals required. "Ridge soarable" means the wind speed and direction are right for it.
Glass-off / Wonder Wind: The smooth, buoyant air in the last 60 to 90 minutes before sunset, as the boundary layer collapses and surface winds go calm. Often the most beautiful flying of the day.
Katabatic Wind: A downslope wind, cool air draining downhill. Common in the evening and early morning.
Anabatic Wind: An upslope wind, warm air rising along a sunlit slope during the day.
Blown Out: Too windy to fly safely.

Sky & Clouds

Cloud Base: The height of the bottom of the puffy cumulus clouds, which marks the top of usable thermal lift. Given AGL. Estimated as (temperature minus dew point, in °F) times 400.
High Cloud Base: A flag meaning cloud base is well above launch, so thermals are tall and there is lots of room to climb and glide.
Cloud Cover: How much of the sky is covered by cloud, given as a percentage.
Cumulus (Cu): Puffy fair-weather clouds. Each one usually caps a thermal, so they act as a road map of the lift.
Precip / Precip Probability: Short for precipitation. The percent chance of rain during the day.
Storm Risk: A flag for a real chance of thunderstorms, driven by high CAPE and a very negative Lifted Index. Do not fly.

Weather Models & Data

Open-Meteo: The weather data service TowMeUp pulls its raw forecasts from.
HRRR: NOAA's high-resolution, short-range US weather model. It is our primary model for the next 48 hours.
GFS: NOAA's global weather model. Good for the broad picture.
NAM: NOAA's regional model covering North America.
ICON: The German weather service (DWD) global model. Currently set aside because it disagreed with the US models too often.
NOAA: The US government agency that runs the HRRR, GFS, and NAM weather models.
Model Agreement / Model Uncertain: We compare several models. When they agree, confidence is high. "Model uncertain" means they disagree on a key number, so trust the forecast a little less.

Flying Terms & Gear

HG: Flying a hang glider, a rigid-winged aircraft launched on foot or by aerotow.
PG: Flying a paraglider, a soft fabric wing that packs into a backpack.
Free Flight: Unpowered soaring flight, meaning hang gliding and paragliding.
Aerotow: Being towed into the air behind a powered aircraft (the tug) on a long line, then releasing to fly free. It is how pilots launch at flatland parks like Lookout.
Tug: The small powered aircraft that tows a glider up during an aerotow.
LZ: The field where pilots land.
Sled Run: A flight with no lift to work: launch, glide straight down, and land. No climbing.
Launch: The takeoff area, whether a ramp, a slope, or a tow line. Also the act of taking off.
USHPA: The national association that governs the sport and issues pilot skill ratings (the H and P ratings).

Units & Abbreviations

AGL: Height measured from the ground directly below you. A 5,000 ft AGL cloud base is 5,000 ft above the field.
ASL / MSL: Height measured from sea level, regardless of the ground below.
ft: Feet.
m: Meters. Multiply by 3.28 to get feet.
mph: Miles per hour.
m/s: Meters per second, used for thermal climb rate. 1 m/s is about 200 ft per minute.
J/kg: Joules per kilogram, the unit for CAPE (thermal and storm energy).
hPa: A pressure unit. Also used to label altitude levels: 850 hPa is roughly 5,000 ft, 700 hPa roughly 10,000 ft.
°F: Degrees Fahrenheit.
%: Percent, used for cloud cover and the chance of rain.

Compass Directions

Wind direction is always where the wind is coming from. A "NW wind" blows from the northwest toward the southeast.

N North

NNE North-northeast

NE Northeast

ENE East-northeast

E East

ESE East-southeast

SE Southeast

SSE South-southeast

S South

SSW South-southwest

SW Southwest

WSW West-southwest

W West

WNW West-northwest

NW Northwest

NNW North-northwest

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