How Tonight Score Is Weighted

How Tonight Score Is Weighted

The Tonight score adds up to 100 points: cloud is worth 40, dew 15, transparency 10, dark hours 15 and the Moon 15, plus a 5-point bonus and minus a rain penalty.

Tonight Observing Score · How to Use Tonight Observing Score · Is Tonight Good for Stargazing?

This page lists every weight, band and cutoff the score uses, so you can check any number the tool gives you. The same score appears on the Tonight Observing Score tool, the score dial in Stargazing Weather, and the city pages such as Tonight in Georgetown.

Where the inputs come from

"Tonight" means the night-time forecast hours from this evening to tomorrow morning. Cloud, dew and haze are averaged over those hours.

Tonight's live score for Georgetown or Lethem, with the chips that make it up, the cloud hour by hour and the week's nights.

Inputs and points

| Input | Points | How the points are given | |-------|--------|------| | Cloud cover | up to 40 | On a straight line from average cloud: 0% cloud earns 40, 50% earns 20, 100% earns 0. | | Dew risk | up to 15 | Low 15, moderate 8, high 0. | | Transparency | up to 10 | Excellent 10, good 8, fair 5, poor 2, hazy 0. | | Astronomical dark hours | up to 15 | 3 points per hour of astronomical night, full marks at 5 hours. | | Moon | up to 15 | 15 × (1 - illumination × fraction of the night the Moon is up). | | Clear window soon | +5 bonus | When the best clear viewing window starts within the next 18 hours. | | Rain risk | -8 or -15 | -8 when the chance of rain in tonight's astronomical darkness reaches 30%, -15 when it reaches 50%. |

Cloud 40 + dew 15 + transparency 10 + dark 15 + Moon 15 makes 95 for a perfect night, and the clear-window bonus brings it to 100. The total is rounded and kept between 0 and 100.

The score engine with every input on a slider, starting from the worked examples below.

Each input in detail

Cloud (40 points)

Cloud is the biggest input because no other condition matters under overcast skies. The tool takes the average total cloud cover over tonight's night-time hours and gives 40 × (100 - cloud%) / 100 points, so 20% cloud earns 32 points and 70% earns 12. The chip reads, for example, "20% cloud". It shows as good at 30% or less, as a warning up to 60%, and as bad above that.

Dew risk (15 points)

Dew fogs lenses and eyepieces, and in Guyana it is the most common reason a clear night goes wrong. The risk uses the night's average humidity and the average gap between air temperature and dew point (the "spread"):

The chips read "low dew", "watch dew" or "high dew". The Dew Risk Calculator uses the same rule, and the Dew and Monsoon Stargazing Playbook explains what to do about it.

Transparency (10 points)

Transparency here means haze: how much dust, smoke and sea salt dims the sky. It comes from the night's average aerosol optical depth at 550 nm:

| AOD | Transparency | Points | |---|---|---| | under 0.15 | excellent | 10 | | 0.15 to under 0.25 | good | 8 | | 0.25 to under 0.40 | fair | 5 | | 0.40 to under 0.60 | poor | 2 | | 0.60 or more | hazy | 0 |

This is not telescope "seeing" (how steady the air is, measured in arcseconds), which the score does not estimate.

Astronomical dark hours (15 points)

Astronomical night is the time when the Sun is more than 18° below the horizon, so twilight has fully faded. The score counts the forecast hours that fall inside it and gives 3 points per hour, up to 15 at 5 hours. From Guyana astronomical night lasts about 9 to 10 hours all year, so this input almost always earns the full 15. It matters far from the equator, where summer nights can have little or no astronomical darkness; with none at all the chip reads "no astro-dark window" and earns 0.

Moon (15 points)

The Moon input uses two numbers: how much of the Moon is lit when the page loads (illumination, 0 to 1) and what fraction of the night, from sunset to sunrise, it is above the horizon. Their product is the "washout", and the Moon earns 15 × (1 - washout) points:

A half-lit Moon is up for half the night. About how many of the 15 Moon points does the night keep?

The washout is 0.5 × 0.5 = 0.25, so the Moon earns 15 × (1 - 0.25), about 11 points.

The chip reads "moon 64% lit", or "moon 64% lit · mostly down" when the Moon is more than 30% lit but up for no more than a tenth of the night. A washout of 0.35 or less shows as good and up to 0.65 as a warning. For the exact moonless hours, use Dark Sky Window; the score weighs how much moonlight the night gets, not when it falls. Moon Phases explains when each phase rises and sets.

Clear-window bonus (+5)

The tool looks through the whole 7-day hourly forecast for the best run of night-time hours with cloud at 30% or less (or 60% or less if there is no such run), a rain chance under 50%, humidity under 95%, and a spread above 1.5 °C. Among those runs it prefers long, clear, dark and less humid ones. If the best run starts within the next 18 hours (or started less than an hour ago), the score adds 5 points and the chip reads "clear window soon". If the best run is on a later night, there is no bonus.

Rain penalty (-8 or -15)

The penalty uses the highest chance of rain in any hour of tonight's astronomical darkness (Sun more than 18° down) that is still ahead. Rain on a later night never counts against tonight, even when the best viewing window falls on that night, and a shower in dusk or dawn twilight does not count either. A chance of 30% or more costs 8 points and 50% or more costs 15; the chip reads, for example, "rain risk 40%".

Missing data

When a source does not answer, that input earns neutral points, about half its maximum, so a gap reads as uncertainty rather than as a bad night:

| Input | Neutral points | |---|---| | Cloud | 22 of 40 | | Dew | 8 of 15 | | Transparency | 5 of 10 | | Dark hours | 8 of 15 | | Moon | 8 of 15 |

The chips then read "cloud n/a", "dew n/a", "transparency n/a", "astro dark n/a" or "moon n/a". Until the Moon has been computed, the dark-hours input counts only the moonless dark hours, so the Moon is still taken into account.

Bands: Go, Maybe, Skip

| Score | Label | What it means | |---|---|---| | 70 to 100 | Go | Clear enough to drive for. | | 45 to 69 | Maybe | Watch dew and gaps. | | 0 to 44 | Skip | Cloud, dew, moon, or no dark window. |

The headline next to the label names the one input costing the most points tonight, so a Maybe tells you what to watch.

The bands on the real tool: a dated Georgetown forecast scoring 83, a Go, with the factor chips that add up to it.

Two worked examples

These are made-up nights to show the arithmetic, not forecasts.

A dry-season night on the coast. 20% cloud earns 32. Humidity around 85%, with the air 3 °C above its dew point, makes dew moderate: 8. Good transparency: 8. Nine hours of astronomical night: 15. A Moon 40% lit and up for 60% of the night has a washout of 0.24: 11.4. The best clear window starts this evening: +5. Rain chance 20%: no penalty. Total 79.4, shown as 79, Go.

A wet-season night with a full Moon. 60% cloud earns 16. High dew: 0. Fair transparency: 5. Nine hours of astronomical night: 15. A full Moon up all night: 0. A clear window still opens tonight: +5. Rain chance 40%: -8. Total 33, shown as 33, Skip.

Other places the engine is used

What the score does not include

Why does Tonight Observing Score leave light pollution out?

The score rates the sky's weather and darkness for the night; your site's light pollution is a separate question for the Bortle Map.

How to use the number

Treat it as a go/no-go for driving, then open the hourly forecast for timing. Two pins a short drive apart can score differently when showers or coastal haze sit on one side of town, so move the pin to your actual site. For the whole routine, read Is Tonight Good for Stargazing? and How to Use Tonight Observing Score.

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