How to read the UULKA score

The big number on UULKA's home page answers one question: how good is the sky for stargazing during tonight's dark hours? This guide explains what the number is made of, how to read the rest of the page around it, and where its limits are.

The short version

Score	Rating	What to expect
80 to 100	Excellent	Clear, dark and dry. Go out.
65 to 79	Good	A rewarding night, perhaps with some moonlight or a little cloud.
35 to 64	Fair	Worth a look if you are flexible; check the best window.
0 to 34	Poor	Cloud, rain or a very bright Moon will spoil most of the night.

The score is not a measure of how pretty the sky is in general. It is a forecast of how much of the night sky you will be able to see, hour by hour, from your location.

What is scored: only the dark hours

UULKA scores only astronomically dark hours: the part of the night when the Sun is more than 18 degrees below the horizon and twilight has completely faded. Sunset, dusk and dawn are shown on the timeline but are never scored. Near the poles in summer there may be no true darkness at all. In that case UULKA falls back to nautical darkness (12 degrees below the horizon) with a reduced score and a clear note, or tells you there is no score because twilight lasts all night.

The tonight score is the average of the dark hours that are still ahead of you. If you check at 1 am, the hours before 1 am are not counted.

The four ingredients

Each dark hour starts with 100 points. Four things can take points away.

Cloud: up to 50 points

Cloud is by far the most important factor, because it simply hides the sky. UULKA uses the forecast's low, middle and high cloud separately and counts them differently: low and middle cloud are close to opaque, while thin high cloud (cirrus) lets much of the starlight through, so it costs fewer points than the same coverage of low cloud. A hundred per cent of high cloud is much better than a hundred per cent of low cloud, and the score reflects that.

Moonlight: up to 30 points

The Moon is the brightest thing in the night sky and it floods the atmosphere with scattered light. The penalty combines two things: how much of the Moon is lit (its illumination) and how high it is above the horizon. A full Moon high overhead takes the full 30 points. A full Moon that has not risen yet, or has already set, takes none. That is why the best window on a moonlit night is often the stretch before moonrise or after moonset, and why the Moon appears in the night planner with its own curve.

Humidity: up to 10 points

Damp air is hazier, scatters more light and drops dew on lenses, mirrors and windscreens. The humidity score is full marks in dry air and falls to zero as the air approaches saturation.

Haze and fog estimate: up to 10 points

UULKA estimates how transparent the air will be from three forecast values: relative humidity, the gap between the air temperature and the dew point, and forecast fog. When the air temperature is only a degree or two above the dew point, haze, mist or fog are likely. This is labelled an estimate everywhere it appears. It is not a measurement, and UULKA does not forecast "seeing" (the twinkling and blurring caused by turbulence), because no free forecast source provides it reliably.

Safety nets: caps

Two rules stop an otherwise decent-looking hour from fooling you.

• Cloud cap. If cloud is almost total, the hour is capped at 20 out of 100, however dry and moonless it is.
• Rain cap. Any measurable rain caps the hour at 5 out of 100.

When a cap changes your score, the "Why this score?" card says so in a separate line.

Light pollution: you tell us

UULKA does not look up light pollution automatically. If you know the Bortle class of your sky (1 is a pristine dark site, 9 is the centre of a large city), choose it in the "Why this score?" card. UULKA then reduces every hour's score slightly. A Bortle 8 city sky takes roughly a quarter off a perfect night. The choice is stored only in your browser. If you leave it unset, there is no adjustment, which is the right setting for a dark-sky trip.

Reading the rest of the page

Best window. The best run of consecutive dark hours (three hours when the night is long enough). On a partly cloudy or moonlit night it is often much better than the overall score, so check it before you decide to stay in.

Key times. Full darkness start and end, the next moonrise or moonset, when the Milky Way core is above 10 degrees, the next meteor shower peak, and the best window, each with a countdown.

Why this score? Plain-English bars for cloud, moonlight, humidity and haze, plus any caps or light-pollution adjustment. The bars add up to the headline score, so you can see exactly where points went.

Night planner. One row per target worth looking for: the Moon, the bright planets, the Milky Way core and a handful of showpiece objects such as the Pleiades, the Orion Nebula and the Andromeda Galaxy. Each curve shows its altitude from sunset to sunrise, the shaded band is full darkness, and the marker is when it is highest. Anything that never climbs above 10 degrees during the dark hours is left out and listed underneath.

Hour by hour and the next 7 nights. Bars coloured by rating for tonight, and a card for each of the coming nights.

Which nights to trust

The first two to three nights come from an hourly forecast; beyond that the forecast steps are six hours long and fog and one-hour rainfall are not available. UULKA marks those later nights "less certain". A forecast five nights out is a hint, not a promise; check again the evening before.

What the score cannot tell you

• Local effects. Valley fog, sea breezes, a hill to your south, or a streetlight across the road are invisible to a weather model.
• Short gaps in cloud. Hourly values smooth over breaks that may last only minutes.
• Seeing. Planetary detail and double-star splitting depend on turbulence, which UULKA does not forecast.
• Light pollution unless you enter it yourself.

A quick routine

1. Open UULKA after lunch and look at tonight's score and best window.
2. If it is fair or better, read "Why this score?" to see whether cloud or Moon is the limit.
3. Check the night planner for what is up and when.
4. Look at the forecast again an hour before you go out, and then trust the sky over the screen.

Clear skies.