“Everybody talks about the weather, but nobody does anything about it” — including astronomers. We can, however, at least get the information we need to decide if the skies will be good enough to set up a telescope at a dark site—or even in the backyard.

Astronomy Stargazing Forecast Sites & Apps

The first place many astronomers turn for observing forecasts is Attila Danko’s Clear Sky Charts.  It assembles data from the Canadian Meteorological Centre into a stargazing forecast spanning 180 hours.  A chart for a particular location uses rows of colored squares to indicate cloud cover, transparency, and seeing. There are charts for thousands of locations in the U.S. and Canada, including the sites of major star parties.

Clear Sky Charts
In Clear Sky Charts, darker blue squares indicate better conditions regarding cloud cover, seeing, and transparency.

A somewhat newer app, Egg Moon Studio’s Scope Nights, has become a favorite of amateur astronomers. Scope Nights works in similar fashion to Clear Sky Charts but compiles data from the U.S. National Oceanic and Atmospheric Administration (NOAA), the European numerical forecast model, and other sources to provide worldwide stargazing forecasts. Scope Nights is beautiful, and its simple graphics make stargazing conditions clear at a glance. Both daily and hourly forecasts are available. The app also displays light pollution conditions on a map.

ScopeNights screen
Scope Nights’ main screen displays the astronomy forecast for six nights.

Astrospheric, which bills itself as “advanced weather forecasting for North American astronomers,” is similar to Clear Sky Charts, but adds considerably more information. In addition to the basic forecast, there are satellite images, an extended cloud forecast, and lunar information, including phase and rise/set times.  Like Clear Sky Charts, Astrospheric uses colored squares to indicate conditions, but they are also summarized when you mouseover: “cloud cover 82%,” “transparency, cloudy,” “seeing average.”

Astrospheric screenshot
A stargazing forecast on the Astrospheric app shows similar data to Clear Sky Charts, but with additional information, including a satellite image of cloud cover.

All three of these websites are also available as apps for both iOS and Android devices.

General Weather Forecast

Although not aimed specifically at astronomers, major weather services offer apps that display conditions and forecasts. While most of these do have some astronomy information, and some even feature astronomy pages, astronomy data are usually limited to basics like sunrise, moonrise, and maybe an occasional feature on special events like meteor showers.

The problem with these apps is their inconvenience on a dark observing field. All require plenty of scrolling, and getting details often requires reading small text. Most are also advertising-heavy.

Even so, I’ve used both the webpages and apps of The Weather Channel, Wunderground, and Accuweather with great success during dark sky expeditions. Among their most valuable features are their radar and satellite images that make it easy to see when bad weather is coming.

DIY Stargazing Forecast

Being able to put together an accurate weather forecast requires not just high-tech weather instruments, but years of training and experience. Nevertheless, by checking a few indicators it’s more than possible to get an idea of the night’s stargazing weather.

Clouds

Clouds, besides blocking the sky, are the primary means of predicting weather without a forecast. More specifically, cloud layers moving in different directions indicate bad weather on the way. There’s an old saying about “mares’ tails” (cirrus clouds) and “mackerel scales” (altocumulus clouds): “Mackerel scales and mare’s tails make lofty ships carry low sails.”  It’s true these clouds often appear in advance of rain. If both are visible, bad weather will arrive within 36 hours.

Mares' tails cirrus clouds
"Mares' tails" cirrus clouds
James St. John / CC BY 2.0

Keep a sharp eye out for “towers” of cumulus clouds. Patches of cumulus clouds are common on warm afternoons, but towering formations (called cumulonimbus) indicate the possibility of severe weather.

Rings

Rings around the Moon or (less frequently) the Sun are produced by ice crystals in the upper atmosphere, typically in high-altitude cirrus clouds, and can mean that rain is on the way.

Wind

Wind is a less accurate indicator of storms but is still useful. Easterly or southeasterly winds often presage rain. Not sure of wind direction? The old routine of sticking an index finger in your mouth and holding it up to the wind actually works. The side of your finger toward wind flow will feel cooler.

Whether your forecast source is an astronomy weather app, a website, or just an index finger, paying attention to weather will make observing a less frustrating experience. Who wants to ignore the weather, haul a ton of equipment to a dark site, and spend the evening under clouds?



Alphabetical List of Forecasts:

Comments


Image of John M. Wiley

John M. Wiley

January 2, 2010 at 6:49 am

I like http://www.wunderground.com for local forecasts. In my experience, they're a bit more accurate three or more days out than NWS.

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Image of John Barrett.

John Barrett.

September 12, 2010 at 12:54 pm

This is invalueablely FANTASTIC(sorry,can't underline as yet), you guys at Sky and Telescope are working way too hard. Just keep up the good work. Now with more accurate forecast as well as simply looking up, this is one tool that is absolutely necassary. I Thank You!!!!!

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Image of J. Kelly Beatty

J. Kelly Beatty

July 8, 2021 at 7:25 pm

Rod did discuss Astrospheric, including a screen grab.

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Image of GaryN

GaryN

September 3, 2021 at 2:12 pm

Astrospheric should really be top of list for anyone in the US and Canada. It's the only one of the bunch that readily makes the data available on the map, and it's the only one that includes wildfire smoke in the transparency forecast - something that has become an absolute requirement for pretty much anyone hoping to find clear skies. There are free apps for Android and iOS and a website at https://www.astrospheric.com

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