About
Clear Sky Chart is a long-standing, free astronomy forecasting resource trusted by amateur astronomers across Canada, the United States, and parts of Mexico. Originally created by the late Attilla Danko and now maintained by CSCCharts, it leverages forecast models developed by Allan Rahill of the Canadian Meteorological Center to deliver highly specialized sky condition predictions. Unlike generic weather apps, Clear Sky Chart focuses on the three factors that matter most to telescope users: cloud cover, atmospheric transparency (how clear the air is for detecting faint objects), and 'seeing' (how steady the air is for resolving fine planetary detail). These predictions are displayed as color-coded hourly blocks, read left to right over a 96-hour window, making it easy to spot the best observing windows at a glance. With over 6,198 individual chart locations spanning astronomy clubs, dark-sky preserves, observatories, star party sites, campgrounds, national and state parks, and more, the tool offers remarkable geographic coverage. Users can search by keyword, browse by region, or enter GPS coordinates to find the nearest chart. The platform is entirely free and has been a foundational planning resource in the amateur astronomy community for decades, helping observers decide whether to chase galaxies, image planets, or stay home and process data on a given night.
Key Features
- 96-Hour Sky Forecast: Displays hourly predictions for the next four days, letting astronomers plan observing windows well in advance.
- Seeing & Transparency Prediction: Goes beyond cloud cover to predict atmospheric transparency for faint-object viewing and air steadiness ('seeing') for high-resolution planetary work.
- 6,198+ Chart Locations: Covers observatories, dark-sky preserves, astronomy clubs, star party sites, campgrounds, and parks across Canada, the USA, and parts of Mexico.
- Flexible Location Search: Find charts by keyword, browse by region or state/province, or enter latitude/longitude coordinates to locate the nearest forecast.
- Canadian Meteorological Center Data: Forecast data is sourced from Allan Rahill's specialized astronomy forecast model at the CMC, providing scientifically grounded sky condition predictions.
Use Cases
- Planning an amateur astronomy observing session by identifying the best clear and dark sky window in the next four days.
- Choosing between galaxy/deep-sky imaging nights versus high-resolution planetary observation based on transparency and seeing forecasts.
- Locating a nearby dark-sky preserve, observatory, or star party site with favorable forecast conditions for a weekend trip.
- Astrophotographers deciding when to set up equipment and whether to process existing data instead of shooting new frames.
- Astronomy club members coordinating group observing events around predicted clear sky windows at a shared site.
Pros
- Completely Free: No account, subscription, or payment required — the full tool and all 6,198+ charts are accessible at no cost.
- Astronomy-Specific Metrics: Uniquely predicts seeing and transparency in addition to cloud cover, giving amateur astronomers data that generic weather services don't provide.
- Massive Location Coverage: Thousands of pre-built charts across North America make it easy to find forecasts for popular observing sites, dark-sky areas, and astronomy parks.
Cons
- North America Only: Coverage is limited to Canada, the United States, and parts of Mexico — astronomers elsewhere cannot use the service.
- Outdated Interface: The website's design and user experience have not been modernized, which can feel dated compared to contemporary forecasting tools.
- No Mobile App: There is no dedicated iOS or Android app, limiting convenience for observers checking conditions in the field.
Frequently Asked Questions
What does the Clear Sky Chart actually predict?
It predicts three sky conditions critical to astronomers: cloud cover (how overcast the sky will be), atmospheric transparency (how well faint objects can be seen through the air), and 'seeing' (how steady the atmosphere is for resolving fine detail on planets through a telescope).
How far ahead does the forecast go?
The chart covers the next 96 hours (four days), displayed as hourly columns that you read from left to right. Blue blocks indicate likely clear and dark skies.
Where does the forecast data come from?
The data is sourced from a specialized astronomy forecast model developed by Allan Rahill at the Canadian Meteorological Center (CMC).
What geographic areas are covered?
Clear Sky Chart covers Canada, the United States, and parts of Mexico (including Baja California, Chihuahua, and Sonora), with over 6,198 individual location charts available.
Is Clear Sky Chart free to use?
Yes, the tool is completely free. No registration or payment is required to access any of the charts or forecasts.
