RigTonightWhich weather model should astronomers trust? ECMWF, GFS, ICON, GEM and UKMO compared
Every astronomy weather app is built on one or more numerical weather models: physics simulations run by national weather services several times a day. Cloud is one of the hardest things for them to predict, and different models often disagree. Knowing who is who helps you decide whether to set up.
The main models
| Model | Run by | Strengths |
|---|---|---|
| ECMWF (IFS) | European Centre for Medium-Range Weather Forecasts | Widely regarded as the most accurate global model, especially 2–5 days out |
| GFS | US National Weather Service (NOAA) | Free and frequent (every 6 hours); over North America often blended with the high-resolution HRRR |
| ICON | Germany's weather service (DWD) | Very detailed over Europe (down to about 2 km); good with low cloud and fog |
| GEM | Environment and Climate Change Canada | High-resolution version over Canada, worth a close look for Canadian sites |
| UKMO (Unified Model) | UK Met Office | About 2 km over the British Isles; a strong all-rounder in Europe |
Why compare them?
A single forecast can't tell you how certain it is. When five independent models all say "clear", you can trust it. When they range from 0% to 90% cloud, the honest answer is "uncertain", which is still worth knowing before a long drive to a dark site.
RigTonight shows each model on its own row and combines them into one clear-sky score, with a confidence rating based on how far apart they are. Hover over an hour on the Tonight tab for a plain-English summary, or over a model's name for what it is.
Reading cloud forecasts like an astronomer
- Thin high cloud often shows as partial cloud cover, but it ruins contrast for faint targets. Be cautious with "50% cloud".
- High humidity near saturation means dew, haze or fog even under a clear forecast. Run dew heaters, and expect poor transparency.
- Gaps between clouds can be worth shooting with a fast setup. A "patchy" verdict means short subs and a watchful eye.
- Forecasts for tonight beat forecasts for next week. Beyond about three days, trust the trend more than the hour-by-hour numbers.
Your location matters
Models handle coasts, mountains and valleys differently. Over a few months you'll notice which one tends to be right where you image. Keep notes, or check the Tonight tab at sunset against what you see.
Cloud is only one part of a good night. Is tonight good for astrophotography? covers darkness, the moon, transparency, seeing and dew too.