Is it going to be clear tonight? Should I set the telescope up in the backyard? If I drive 100 kilometres out of my city’s light pollution, will it be worth it? Find out how you can predict the future with the Clear Sky Chart website.

About Clear Sky Chart

The Clear Sky Chart website can be viewed in a web browser. Astronomy society and club sites often include location-specific charts you can click on to access the full page. You can create applets in Rainmeter for Windows to monitor your favourite charts.

There is an app for iOS mobile devices, but no app for the Android platform.

Features include charts showing: 

• the sky’s transparency or clarity
• the sky’s seeing or steadiness
• predicted cloud cover
• period of light and dark and the effect of moonlight
• air temperature
• humidity
• wind speed
• level of smoke and particulate from forest fires

One of the most popular and well-known tools for Canadian astronomers, the Clear Sky Chart (CSC) website was built by amateur astronomer Attilla Danko using data prepared by Alan Rahill of Environment Canada. There are more than 6,400 free charts available for Canadian locations and beyond.

CSC shows sky clarity predictions up to three nights in advance, updated twice a day. As with any weather report, forecasts can be less accurate the further ahead in time they predict. Nevertheless, this resource presents local data relevant to astronomers in an easy-to-interpret fashion and it is one of the most valuable tools in helping you decide whether to get the gear outside or stay cooped up inside. 

I have little Clear Sky Charts on my Windows desktop, monitoring sky conditions for my favourite locations. When I see a band of dark blue, I get excited — it’s time to do some astronomy!

Reading their charts

Full charts show plenty of detail, like cloud cover, sky transparency, atmospheric seeing and darkness levels. Midnight is marked with a red vertical line.

In the cloud cover, sky transparency and atmospheric seeing rows, you look for those blue “paint chips.” Dark blue is best, whereas white or grey means bad conditions are predicted. In the example, cloud cover is dark blue for the next three nights. Transparency, on the other hand, is cyan-coloured for Friday night and  darker on Saturday. Seeing is also poor on Friday, but better on Saturday. As such, I’d probably plan to go stargazing on Saturday … and clean the eyepieces on Friday night.

As for darkness levels, daylight and twilight times are shown on this row. The bright Moon’s presence will lighten some of those dark blue squares.

When you’re viewing a chart on the website, you can hover your mouse pointer over the little squares to get pop-ups with more detail. Hover over a square under cloud cover and you’ll see the predicted percentage.The transparency and seeing have a rating out of five. Hover over the darkness square to get the Sun or Moon’s altitude.

Below the chart are detailed instructions and indices for the colour codes.

Sponsored charts provide additional features, like  data from the European Centre for Medium-Range Weather Forecasts system, shown in the “ECMWF Cloud” row. Danko initially added this as a backup for when Environment Canada experienced outages and for parts of the Arctic not covered. Sometimes the Environment Canada and ECMWF data disagree, but this can encourage astronomers to look deeper for a better understanding of area cloud patterns. 

On further analysis

Clicking a cloud cover chip (from the top row) will take you to a handy predicted cloud cover map. 

On the chart at the top of this story, I clicked the cloud cover box for Saturday night  (actually Sunday morning, between midnight and 1 a.m.). The map shows the weather looks good and clear for southern Alberta and  Saskatchewan. But note that sneaky little finger of cloud reaching down from the northwest toward Calgary. Will that be a factor?

Charts can be shown in a full size or in a brief two-line format. Full charts also show temperature, humidity, wind and the effect of smoke from forest fires with more colourful squares. The humidity and temperature indicators can range from blue to green, yellow, orange and red. Red in humidity means we will need the dew heaters; red in temperature means short-sleeve conditions.

When forest fires are raging, we need to pay particular attention to the smoke row: once again, blue is good; red indicates particulate will degrade transparency. In fact, a red bar and warning text will appear below a main chart.

Astronomy club websites can show a miniature version of a CSC. The mini-display shows two rows only, cloud cover and transparency. Normally clicking on a mini-chart will pull up the full page.

While people are typically focused on individual charts, the website itself is  rich with general information and tools to help you find charted locations near your home, club observatory and other favourite observing sites.

Canadian support

Brian Gibson, who lives in Mississauga, Ontario, has also produced myCSC, a Clear Sky Chart app for the iPhone (see the Anything Binary website for more details).

As noted, when a chart is sponsored, additional features are unlocked. Users can view forecast history with climate information and past charts for about two weeks. The history charts are fascinating by themselves.

Many RASC centres also contribute financially to CSC, which helps pay for some computer and internet services.

All in all, the CSC resource is widely used across the continent, and all astronomers here should have this amazing Canadian data product in their weather toolkit.

Blake Nancarrow is a double star aficionado, columnist for the RASC Journal, proofreader for the Observer’s Handbook and the interim chair for the RASC Observing Committee. An avid amateur astronomer, he is a member of RASC. Visit his blog at blog.lumpydarkness.com.