How to Use Stargazing Weather
Read the Tonight score, cloud layers, dew risk, transparency (haze) and the 7-night outlook to decide if a night is worth the drive.
What this tool answers
Will the sky over your site be clear, dry and dark enough tonight, or later this week, and which hours are best? It uses the Open-Meteo forecast, CAMS haze data and the Sun and Moon positions for your pin. It does not forecast telescope seeing. For the ideas behind it, read Is Tonight Good for Stargazing?.
What you see on the screen
- Location bar: place and coordinates, with My location and Change.
- Headline: a condition such as Clear night or Fog, a Go, Maybe or Skip headline, and the Tonight score dial (0 to 100).
- Chips: score factors (such as "25% cloud" or "moon 40% lit"), then Transparency with an AOD value and Dew risk with the dew spread (Δ).
- Best viewing, then Current conditions: Cloud Cover, Visibility, Humidity, Dew spread, Temp, Wind.
- Cloud layers chart for 24 hours (12 on phones, with a Show 24h button), then the 7-night outlook cards.
- Side column (below on phones and tablets): Observer with Use my current location, and Plan the next step links: Check your Bortle, What's in the sky, Confirm moonless dark window.
Step by step
- Open Stargazing Weather.
- Set the site you will stand at: My location for GPS, or Change for Guyana Presets or Manual Coordinates.
- Read the dial and the headline.
- Read the chips to see what is costing points.
- Read Best viewing for the best run of hours.
- Check the Cloud layers chart around the hours you plan to be out.
- If tonight looks poor, read Best night this week above the outlook cards.
- Tap Confirm moonless dark window to check Moon times in Sun/Moon Times & Dark Window.
- Reload the page close to departure; forecasts change.
The real tool with a dated forecast for Georgetown (fetched 6 Oct 2026, 15:50 GYT): overcast in the afternoon, a Go score of 83 and a clear window from 20:00 to 02:00.
How to read the results
Tonight score: 70 to 100 is Go, 45 to 69 is Maybe, below 45 is Skip. Points: cloud 40, dew 15, transparency 10, astronomical dark 15, Moon 15, plus 5 for "clear window soon". A rain chance of 30% or more during tonight's astronomical darkness costs 8 points, and 50% or more costs 15. A missing input earns about half points and its chip reads "n/a". How Tonight Score Is Weighted has the full method; Tonight Observing Score shows the same score on its own page.
Tonight's live forecast for Georgetown or Lethem as the tool reads it: the dial and its chips, cloud layers hour by hour, Best viewing and the 7-night outlook.
Cloud: total cloud of 10% or less rates excellent, 30% or less good, 60% or less fair, 85% or less poor, and above that overcast. The chart stacks Low, Mid and High cloud for each hour. A rain icon marks hours with a 60% or higher chance of rain, and a droplet marks humidity of 90% or more. Tonight avg is the mean cloud over the next 14 night hours.
Dew risk: dew spread is air temperature minus dew point. Risk is high when humidity is 92% or more or the spread is 2 °C or less, moderate at 80% or 4 °C, and low otherwise. When it is high, a line warns "Fog / heavy dew likely". The Dew and Monsoon Stargazing Playbook covers what to do about it.
Transparency: aerosol optical depth (AOD) measures haze and smoke. Below 0.15 is excellent, below 0.25 good, below 0.4 fair, below 0.6 poor, and 0.6 or more hazy.
The tool's dew and haze rules: move humidity, dew spread and haze, and watch the risk and the points.
Best viewing: the best run of night hours in the whole forecast, labelled now, tonight or a weekday. Each hour needs 30% cloud or less (60% if no hour qualifies), rain chance under 50%, humidity under 95% and dew spread above 1.5 °C. Moonless dark hours earn a bonus and humid runs a penalty. It shows the times, a quality word, mean cloud and mean humidity.
Shaded hours in the chart (legend Moonless dark) have the Sun more than 18° below the horizon and the Moon below the horizon.
7-night outlook: one card per evening, with hours after midnight counted in the evening before. Each card shows a score with Go, Maybe or Skip, mean cloud, mean humidity with a dew level, and "Nh moonless" for the moonless dark hours. "No moonless dark" means the Moon is up for all or nearly all of the astronomical night, not that there is no night. Each card scores its night by the dial's rules, with that night's own Moon and rain chance; only the clear-window bonus is left out, and the Tonight card shows the dial's own score.
A clear Saturday falls on the night of the full Moon. Does its card in the 7-night outlook lose points for the Moon?
- No: every card gets the same Moon points
- Yes: each card weighs that night's own Moon and rain, as the dial does (the right answer)
- Only the Tonight card counts the Moon
- The outlook leaves the Moon out entirely
Each outlook card scores its night by the dial's rules, with that night's Moon and rain chance; only the clear-window bonus, which means "clear soon", is left out.
Times are the site's local clock; for a Guyana pin that is Guyana time (America/Guyana, UTC-4).
Using it from Guyana
- Astronomical night lasts about 9 to 10 hours all year in Georgetown, so cloud, dew and the Moon decide most scores.
- Pin the site you will drive to. A Georgetown forecast may not match a site far inland.
- Coastal Guyana has two wet seasons (mid-April to the end of July, mid-November to the end of January), and the Rupununi one (about April to August). In wet months, look for short clear gaps in the chart.
- On humid nights, check the dew chip before you pack, and bring a dew heater or lens hood.
- On a Skip night, the Time-of-Day Observing Guide has astronomy tasks that do not need a clear, dark sky.
What it does not do
- It does not forecast seeing (steadiness, in arcseconds). Transparency here is haze only.
- It does not rate light pollution. Use the Bortle Map.
- It does not list Moon or twilight times. Use Sun/Moon Times & Dark Window.
- It is a model forecast. Tropical showers can build fast, so treat tonight and tomorrow as the most reliable.
Common mistakes
- Reading Current conditions as tonight. Those are values for right now.
- Treating one cloudy hour as the whole night, or one clear hour as a clear night.
- Ignoring the Moon when the sky is clear. Check the moon chip and the dark window.
At 15:00 Current conditions shows 20% cloud. What does that tell you about tonight?
- Tonight will be 20% cloudy
- Tonight scores Go
- Only the sky right now; read the cloud chart and the outlook for the night (the right answer)
- Nothing: the figure is a monthly average
Current conditions are values for right now. The Cloud layers chart and the night's outlook card cover the hours you will be out.