Free tool · Ontario Telescope
Is tonight worth setting up for?
An ordinary weather forecast tells you whether to bring an umbrella. It does not tell you whether the seeing will hold at high power, when the moon gets out of the way, or whether the dew is going to beat you to it. This one does. Tell us where you observe from and we will score every hour of the next seven nights.
The Ontario Telescope Astronomy Weather Forecast is a free, location-based observing forecast for amateur astronomers and astrophotographers. It scores cloud cover by layer, atmospheric transparency, astronomical seeing, moonlight, wind and dew risk for every hour of the next seven nights, anywhere in the world, and reports twilight times, moon phase, visible planets, well-placed deep-sky objects and any meteor shower near peak.
01Where are you observing from?
Observing conditions change over surprisingly short distances — a site an hour up the highway can be under clear sky while the city sits under lake-effect cloud. Everything below is computed in your browser; your coordinates go to the weather service to fetch the forecast and nowhere else.
Observing site
What are you doing tonight?
The three modes weight the same forecast differently. Visual cares most about cloud and moonlight. Astrophotography punishes thin cirrus and wind far harder, because both ruin a sub-exposure that visual observing shrugs off. Planetary is dominated by seeing and barely notices the moon at all.
02Tonight
One number for the night, taken from the best three consecutive hours rather than a single lucky gap in the cloud.
03Hour by hour
Only night hours are shown — from just before dusk to just after dawn. Each block is one hour. Green is good, red is not.
04The week ahead
If tonight is a write-off, this is where you find the night worth booking off for. Accuracy is good for two or three days and gets progressively softer after that.
05Darkness and the moon
The two things a weather app never tells you. Astronomical darkness — sun more than 18° below the horizon — is when faint deep-sky objects actually become workable. In southern Ontario it disappears entirely for a few weeks either side of the summer solstice.
06What’s up tonight
Computed for the middle of tonight’s best window at your location, so the list changes as the week goes on and as you move the map pin.
Planets
Worth pointing at
Showpiece objects currently more than 25° above your horizon, ranked by how easy they are to see from where you are standing.
Meteor showers
07How the score is worked out
No black box. Here is exactly what goes into the number.
The four things that decide a night
Astronomers care about a different set of variables than the weather app on your phone, and they do not always move together. A crisp, cold, crystal-clear night after a cold front can have terrible seeing. A hazy, humid night can have superb seeing. Both are useful for different things.
- Cloud cover — split into low, middle and high layers rather than one lumped percentage. Low and mid cloud is opaque and ends the session. High cirrus is a veil: you can still observe the moon and planets through it, but it quietly destroys deep-sky contrast and ruins long exposures. That is why the three modes weight it differently.
- Transparency — how much water vapour and haze sits in the air column above you. Driven mainly by humidity at 700 and 500 hPa, roughly 3 and 5.5 km up. Poor transparency dims faint fuzzies and makes the sky background glow; it barely affects the moon.
- Seeing — atmospheric turbulence, the thing that makes stars twinkle and planets boil at high power. The dominant driver is the jet stream, so we read wind speed at 250 hPa (about 10 km up) and add surface wind and low-level instability. Good seeing is what lets you push past 200×.
- Moonlight — a full moon raises the sky background by several magnitudes and takes most galaxies off the table. We combine phase with the moon’s actual altitude hour by hour, because a 90% moon that has not risen yet is not a problem.
The one people forget
Dew. When the air temperature falls to within a degree or two of the dew point and the wind drops, every optical surface pointed at the sky starts fogging — corrector plates and finder scopes first. It ends more sessions in Ontario than cloud does. The dew row in the chart is the temperature-dew point spread, adjusted for wind.
Reading the chart
Each column is one hour of night. The top row is the combined score; the rows underneath show you which ingredient is responsible when the score is bad. A column that is green everywhere except seeing is a perfectly good deep-sky night that will disappoint you on Jupiter. A column that is green except moonlight is a fine planetary and double-star night.
Scores above about 60 are worth setting up for. Above 78 is a genuinely good night, which in southern Ontario you get perhaps two or three times a month. Below 22, do something else.
Where the data comes from
Weather comes from Open-Meteo, which blends national forecast models — for Canada that is mainly Environment Canada’s HRDPS and GDPS, plus ECMWF and the American GFS. Sun, moon and planet positions are computed in your browser from standard astronomical algorithms; nothing about the sky needs a server.
Model resolution is a few kilometres, so a forecast cannot see a single valley or the microclimate a large lake creates along its shore. Treat the first two nights as reliable, the third and fourth as a good steer, and anything beyond that as a planning aid rather than a promise.
Gear that changes what a marginal night is worth
Some of the conditions above can be fought and some cannot. Cloud wins every time. Dew, moonlight and mediocre transparency can all be worked around.
Dew control
Heater strips and a controller turn a night with a two-degree dew point spread from a forty-minute session into a full one.
Browse dew controlFilters
UHC and O‑III filters claw back nebula contrast under moonlight and light pollution. They do not help galaxies.
Browse filtersMore free tools
ISS passes, focus stepping, flat frames, guide scope matching and double-star resolving calculators.
Free utilities08Glossary
The terms this page uses, defined plainly. If you have arrived here from a search engine or an AI assistant looking for one of these, this is the short version.
| Term | What it means |
|---|---|
| Astronomical seeing | The blurring and twinkling caused by turbulence in the atmosphere, quoted as the apparent size of a star image in arcseconds. Under 1″ is excellent; over 4″ means high magnification is wasted. The dominant driver is wind shear high up, especially the jet stream near 250 hPa. |
| Transparency | How much of an object’s light reaches the ground, set by water vapour, dust and haze in the air column. Poor transparency dims faint galaxies and nebulae and brightens the sky background. Largely independent of seeing. |
| Astronomical twilight | Sun between 12° and 18° below the horizon. True darkness begins below 18°. Above roughly 49° latitude it does not happen at all near the summer solstice — which is why northern Ontario gets no astronomical dark in June. |
| Dew point spread | Air temperature minus dew point. Below about 2 °C with light wind, dew forms on optics, finders and camera lenses. A dew shield helps; heater strips solve it. |
| Cloud layers | Models report low, middle and high cloud separately. Low and middle cloud is opaque and ends the night. High cloud is cirrus — thin ice crystal cloud that passes enough light for the moon and planets but wrecks deep-sky contrast and long exposures. |
| Magnitude | The astronomical brightness scale. Lower is brighter, and one magnitude is a factor of about 2.5. The faintest stars visible to the unaided eye from a dark site are around magnitude 6. |
| Jet stream | A fast, narrow ribbon of wind about 10 km up, near the 250 hPa level. Strong jet stream winds overhead are the single best predictor of poor seeing, no matter how clear the sky looks. |
| Light pollution | Artificial skyglow. A property of your site rather than the weather, so it does not vary night to night — but low cloud over a town reflects it back down, which is why a partly cloudy night is far worse from the city than from a dark site. |
09Questions
What is an astronomy weather forecast?
It predicts the variables that decide whether a night is usable for observing, rather than whether it will rain. Cloud cover split into low, middle and high layers; atmospheric transparency; astronomical seeing; moonlight; wind; dew risk; and twilight times. A consumer forecast reports none of those except cloud, and will happily describe a night under heavy high cirrus as clear.
Why does this disagree with my weather app?
Because it is answering a different question. A consumer forecast optimises for whether you will get rained on, and it will happily call a night with 40% high cirrus “clear”. For visual observing that is a nuisance; for a two-hour imaging run it is a lost night. We also fold in seeing, transparency, moonlight and dew, none of which appear in a normal forecast at all.
What is the difference between seeing and transparency?
Seeing is turbulence and limits how much magnification is useful, so it governs planets, the moon and double stars. Transparency is haze and water vapour dimming starlight, so it governs how faint a galaxy or nebula you can detect. They are independent and often opposed: crisp cold air behind a front usually gives excellent transparency and poor seeing, while still, hazy, humid air often gives poor transparency and superb seeing. This is why the tool scores them separately instead of collapsing everything into one “clear” number.
What is a good score, really?
60 and up is worth setting up for. 78 and up is a night you should clear the calendar for. If you observe from southern Ontario you will notice the numbers are lower than you might like — that is not pessimism in the model, it is the Great Lakes.
The seeing forecast says excellent but the stars are twinkling madly.
Seeing forecasts model the free atmosphere: the jet stream and the upper-level turbulence that no site can escape. They cannot model your own back garden. Heat pouring off a roof, a driveway or a brick wall that has been in the sun all day will wreck the seeing in the first few metres above your telescope, and no forecast can see that. Set up on grass, away from buildings, and give the optics an hour to reach ambient temperature.
Does this account for light pollution?
No. Light pollution is a property of your site rather than the weather, so it does not vary night to night and would only add a constant offset to every score. It does interact with the forecast though — low cloud over a city reflects light back down and makes a partly cloudy night from town far worse than the same forecast from a dark site.
How far ahead is it actually worth trusting?
Two nights, confidently. Three or four as a strong hint. Past that, use it to spot which night of the coming week has the moon out of the way and a plausible weather pattern, then check again 48 hours out before you drive anywhere.
Can I use this outside Canada?
Yes. The weather models and the astronomy both work worldwide, and the search box will find towns anywhere. The equipment advice is written for Ontario conditions, but physics does not care where you live.