Astro Weather — Instructions & Help

1. Choose your location

Enter a UK postcode or town and select Get forecast, or use Use my location. Location permission is only requested when you press the button. Your choice is saved in your own browser.

2. Read the forecast colours

Good generally favourable; Marginal mixed; Poor unfavourable. Cloud and wind ratings are indicative thresholds, not guarantees. The 48-hour graph shows estimated clear sky (100% minus cloud cover). The hourly cards show cloud, score, wind and humidity.

Bright Moonlight makes faint galaxies and nebulae harder to photograph, but the Moon itself can be a superb target. Dew warnings are guidance: optics may cool below ambient air temperature.

Understanding the hourly forecast cards and observing score

The large percentage on each hourly card is cloud cover, not the observing score. For cloud cover, 0% is ideal (clear sky), while 100% means fully overcast. The separate Score runs in the opposite direction: 100/100 is best and 0/100 is worst.

The current forecast scoring formula starts at 100 and subtracts penalties for cloud cover, wind and relative humidity (RH):

• Cloud: subtract 0.72 points for each 1% cloud cover.
• Wind: subtract 0.7 points for each km/h above 8 km/h.
• Humidity (RH): subtract 0.35 points for each percentage point above 65% RH.

Scores are rounded to whole numbers and cannot fall below zero. For example, 100% cloud, 18 km/h wind and 73% RH gives 100 − 72 − 7 − 2.8 = 18/100, meaning poor observing conditions.

Good means 75–100; Marginal means 50–74; Poor means below 50. The coloured 48-hour clear-sky timeline instead uses clear sky = 100 − cloud cover: green for 70–100% clear, amber for 35–69% clear, and red for under 35% clear.

RH stands for relative humidity. Higher humidity increases dew risk, but the gap between air temperature and dew point is more useful for deciding when to run dew heaters. This score is a simple weather guide, not a prediction of atmospheric seeing, target visibility or equipment safety. Recent weather-station readings may adjust the current assessment, but future hours remain forecast-based.

3. Equipment and targets

Enter your telescope, focal length, camera and preferred observing direction, then save your profile. Seasonal target ideas are suggestions only: the site does not yet calculate precise target altitude, visibility, or whether trees/buildings obstruct your view. Check a planetarium before setting up.

4. Bortle and sky brightness

Automatic Bortle estimates describe modelled sky darkness at your selected location. They do not represent the live weather or tonight's Moon. You can override the class manually. Sky brightness is shown in magnitudes per square arcsecond (mag/arcsec²); larger values generally mean darker skies.

Sky brightness data and required attribution: DarkSkySites.com.

5. Data and limitations

Weather forecasts come from Open-Meteo; UK postcode lookup uses postcodes.io. Forecasts can be wrong, and local cloud can change quickly. Weather Underground station data is optional. Visitors who have their own authorised WU API key may enter it in the Astro Weather settings; the site relays observation requests through PHP. Do not enter a key on a device or website you do not trust.

Using your own Weather Underground station

To connect your personal weather station to Astro Weather:

1. Log in to your Weather Underground account and retrieve your Station ID and PWS API Key.
2. In Astro Weather, select My station / known public station.
3. Paste your Station ID and API Key into the relevant fields.
4. Tick Remember API key on this device if you want the key saved in this browser.
5. Click Save station settings.
6. Finally, click Get forecast to load the forecast and your station observations.

Viewing another Weather Underground station

You may be able to view observations from another public Weather Underground station using your own authorised API key.

For example, when testing the feature in Waltham Abbey:

1. Select Waltham Abbey as your location.
2. Open the link showing nearby Weather Underground stations.
3. Select a station from the map and open its Weather Underground page.
4. Copy its Station ID.
5. Back in Astro Weather, select My station / known public station.
6. Enter the other station's ID alongside your own API key, click Save station settings, then Get forecast.

Important: You still need your own authorised Weather Underground API key. Our testing suggests that your own registered weather station may need to be online and reporting for API access to work reliably. You do not need to own the public station whose observations you want to view.

The Local station observations panel displays the latest available temperature, humidity, wind and rainfall readings, including the age of the observations.

Important: Your station should be online and reporting data to Weather Underground. Testing suggests the API may not return station observations while the station is offline. If no readings appear, check the station's reporting status and confirm your Station ID and API key are correct.

Only use an API key you are authorised to use. Observation requests pass through the WJM PHP relay; Weather Underground controls API availability and quotas. A remembered key may remain in browser storage until you select Clear API key. Only save it on a trusted device.

Station readings help assess current observing conditions and dew risk; they do not change future cloud forecasts. The forecast continues to work when station observations are unavailable.

Weather Underground: station offline or API errors

If your personal weather station loses its internet connection, Weather Underground may stop receiving fresh observations. If the station is shown as offline on the Weather Underground website, Astro Weather may be unable to retrieve live station readings through the WU API. The rest of the astronomy forecast can still work using Open-Meteo.

Check the station's status and last reported time on Weather Underground first. Check that the station and its router have a working internet connection, then allow time for new observations to arrive before testing the WU API again.

An HTTP 503 Service Unavailable error can indicate a temporary WU service problem or an issue with the site's PHP relay; it does not prove that WU has blocked the API key because the station is offline. Do not repeatedly replace your API key unless an authentication error specifically points to the key.

Add Astro Weather to your iPhone Home Screen

1. Open Astro Weather in Safari on your iPhone.
2. Tap the Share button (square with an upward arrow). Depending on your iPhone, this may be in the bottom toolbar or the menu.
3. Scroll down and tap Add to Home Screen. If it is not visible, scroll to the bottom of the actions list and check Edit Actions.
4. Keep the name Astro Weather, then tap Add.

The Astro Weather icon will appear on your Home Screen. It opens the website and requires internet access for live forecasts and station readings. If you already added the shortcut before the icon was introduced, remove the old shortcut and add it again to pick up the new icon.

Sky events, aurora, meteors and comets

The observing location settings collapse automatically once a location has been saved. Select Edit settings to change your postcode, location or Weather Underground station. Your saved settings remain in the browser.

The Sky events section links to the NOAA Space Weather Prediction Center for short-term aurora forecasts, the American Meteor Society meteor-shower calendar for seasonal activity, and NASA/JPL Horizons and the Minor Planet Center for comet information. These links are external references, not guaranteed local visibility predictions. Check local cloud, astronomical darkness, Moon brightness, and the object's altitude before setting up.

Source attribution: forecasts — Open-Meteo; personal weather station observations — Weather Underground; sky brightness — DarkSkySites.com; UK postcode lookup — postcodes.io.

Thank you

Thank you to Open-Meteo, postcodes.io, DarkSkySites.com, the open-source and astronomy communities, and everyone who has contributed feedback, observations and testing. Your help makes this free project better.