A score you can argue with
The number is a shortcut, not a verdict. Every penalty is listed on the city page so you can decide whether it matters for the equipment you are carrying.
The score starts at 100 and loses points for conditions that interfere with looking up. The Moon and darkness are calculated from the city's coordinates. Cloud and wind come from NWS for US cities and MET Norway internationally, across the dark window.
Cloud cover carries the most weight
Average sky cover across the dark hours drives the base score. A little cloud is normal and not fatal; a solid overcast is. We also look at the maximum cloud cover, because one clear hour surrounded by thick cloud is not the same as a consistently clear night.
Darkness and Moon
- Twilight-only nights are capped low. In far-northern summer, the Sun never gets far enough below the horizon for true astronomical darkness; we say so instead of inventing a night.
- Moonlight costs points in proportion to how bright the Moon is and how long it is above the horizon during the dark window.
- Short dark windows are capped, because a twenty-minute gap between twilight and dawn is not a stargazing session.
Weather that ends a session
- Precipitation chance above 30% starts to matter; a high chance is a large penalty.
- Gusts strong enough to shake a tripod cost points, even when the sky is clear.
- Dangerous cold, high humidity, and dew are called out. They are comfort and equipment problems, not cloud problems.
What the labels mean
| Score | Label | Reasonable expectation |
|---|---|---|
| 85–100 | Excellent | Clear, dark, and worth planning around. The Milky Way may be obvious from a dark site. |
| 70–84 | Good | A solid night with some compromise: a bright Moon, a little cloud, or a short window. |
| 50–69 | Fair | Workable for bright objects and gaps. Do not drive two hours on the strength of the number alone. |
| 25–49 | Poor | Mostly cloud, strong moonlight, or very little darkness. A night for binoculars from the porch, if anything. |
| 0–24 | Skip | The forecast or the sky is working against you. Stay in. |
| — | Unknown | Cloud data does not reach this night yet. Darkness and Moon details are still shown. |
Alerts
Scores for a selected session
The session planner uses a separate score for your chosen window and goal. The Sun must be below -6° for Moon and planet viewing, -12° for casual stargazing, or -18° for faint objects. Both ends of each 30-minute sample must meet that limit. A session needs a current hourly cloud forecast covering at least 75% of its eligible time; coarse or missing intervals do not count.
The score starts at 100 and subtracts 0.95 points for each average cloud percentage point above 5. Moonlight can subtract up to 30 points for faint objects or 12 for casual stargazing, scaled by illumination and time above the horizon. Moon and planet sessions have no Moon penalty, but need at least one target reaching 10° altitude. Precipitation probability and gusts reduce the score where available. Sessions shorter than an hour are capped at 45. No score predicts atmospheric seeing, light pollution, smoke or terrain.
Email alerts use a separate threshold: a score of 70 or better. The alert is intentionally quiet. A night that merely scores “fair” does not generate mail.
Known limits
A city is not a field. Light pollution, a ridge, coastal fog, smoke and the floodlight next door can change the view. Forecast grids cover an area rather than one observing spot. Missing precipitation or gust fields mean unknown conditions, not zero risk. We withhold scores and detailed observing recommendations when forecasts are old, incomplete or too coarse. Check the source and sample times on the city page, then check conditions again before travelling.