Is tonight good for stargazing?
UULKA scores the night sky from 0 to 100 anywhere in the world, shows the best hours to look, and explains exactly why: cloud, moonlight, humidity and haze.
Weather: MET Norway
How the score works
UULKA scores every astronomically dark hour of tonight from 0 to 100 and averages them. Daylight and twilight hours are never scored. Each hour starts from 100 points that can be lost to four things:
- Cloud: up to 50 points. Low, middle and high cloud are weighed separately. Thin, high cloud blocks less starlight, so it costs less than the same amount of low cloud.
- Moonlight: up to 30 points. The fraction of the Moon that is lit, multiplied by how high it is. A Moon below the horizon costs nothing.
- Humidity: up to 10 points. Damp air is hazier and dews up lenses.
- Haze and fog estimate: up to 10 points. Worked out from humidity, how close the air temperature is to the dew point, and forecast fog. It is an estimate of transparency, not a measurement, and UULKA does not forecast "seeing" (twinkling).
Two safety nets keep the score honest: if cloud is almost total the hour is capped at 20, and any measurable rain caps the hour at 5. If you tell UULKA your Bortle sky class it applies a small reduction for light pollution.
Weather comes from MET Norway's forecast model, the Moon and planets are calculated locally, and nothing is invented: when the forecast does not supply a value (fog beyond a few days, for example) that part is simply left out. Read the full guide to the score or see our editorial policy.
Frequently asked questions
What does the UULKA score mean?
The score is a number from 0 to 100 for how good the sky looks for stargazing during the dark hours of tonight. 80 and above is excellent, 65 to 79 is good, 35 to 64 is fair and below 35 is poor. It is built from cloud cover (the biggest factor), moonlight, humidity and an estimate of haze, and heavy cloud or rain caps it at a low value whatever else is going on.
How accurate is the forecast?
Cloud is the hardest thing to predict. The first two to three nights come from an hourly forecast and are the most reliable; later nights use a coarser six-hourly forecast and are marked as less certain. Forecasts are never perfect, especially for scattered cloud and fog, so treat the score as a good guide, check the sky yourself, and expect surprises near the edges of weather fronts.
Why does the Moon change the score so much?
A bright Moon scatters light through the atmosphere and washes out faint stars, galaxies and the Milky Way. UULKA multiplies how much of the Moon is lit by how high it is in the sky, so a full Moon overhead costs the full moonlight allowance, while a Moon below the horizon costs nothing, however full it is.
What counts as dark?
Only astronomical darkness is scored: the time when the Sun is more than 18 degrees below the horizon and the sky has stopped glowing from twilight. In places near the poles in summer that never happens; UULKA then falls back to nautical darkness (12 degrees) with a reduced score, or says that there is no true darkness at all.
Does UULKA show seeing or transparency?
UULKA does not forecast astronomical seeing (how much the stars twinkle), because no free weather source provides it reliably. It does show a haze and transparency estimate, worked out from humidity, the gap between air temperature and dew point, and forecast fog. It is clearly an estimate, not a measurement.
Does the score know how much light pollution I have?
Not automatically. If you know the Bortle class of your sky (1 is a pristine dark site, 9 is an inner city), choose it in the Why this score card and UULKA will reduce the score slightly for brighter skies. The choice is saved only in your browser.