Astro Weather

Seven days of forecast at a glance, made for one question: is the night worth it? Clouds at three heights, estimated seeing and transparency, wind, temperature and dew point – plus sun and moon altitude with the moon phase. In Europe the weather data come from the ICON model of Germany’s national weather service, the DWD, elsewhere from the US models GFS and HRRR; a second cloud row shows the ECMWF model for comparison.

How to read the chart

• At the very top is the moon phase at midnight, below it the altitude of the sun (yellow) and moon (white, dashed) with sunrise and sunset times. Light columns are day and twilight, dark columns astronomical night.
• Overall, clouds, ECMWF, seeing, transparency: the stronger the blue, the better. Light grey is poor, dark grey means no data. The ECMWF row shows the cloud cover from a second, independent model: when both rows agree, the forecast is more reliable; when they diverge, the night is open.
• Wind in km/h; the arrow shows where the wind is blowing to. Green is calm, red is stormy.
• Temperature (orange) and dew point (blue). When the two lines run close together, dew on the optics is likely.
• Night in detail shows one night from noon to noon: twilight phases with times, sun and moon altitude and every value hour by hour as a number – clouds by layer and compared with ECMWF, visibility, chance of rain and the gap between temperature and dew point. Red means a gap of less than 2 °C, so dew is likely. Switch nights with the arrows or by clicking a row of the table.
• Moving the mouse over the chart, or tapping it, shows every value for that hour. The table below sums up each night.

Where the values come from – and where they are estimates

Clouds, wind, temperature and humidity come straight from a weather model, up to seven days ahead. Which one depends on the location.

In Europe – more precisely, between 29.5 and 70.5° N and between 23.5° W and 62.5° E – the page uses the DWD’s ICON model. In Central Europe the first two days or so come from ICON-D2 at 2.2 km grid spacing, after that from ICON-EU at 7 km.

Outside Europe the data come from the US weather agency NOAA: in the USA, southern Canada and northern Mexico the first two days or so from HRRR at 3 km grid spacing, after that and in the rest of the world from GFS at about 13 km. The deciding factor was a check for Starfront: over 30 nights, GFS/HRRR and ECMWF came much closer to the hourly cloud reports of the surrounding airports than ICON, which above all forecast clear hours as cloudy too often.

The ECMWF row is a second opinion: total cloud cover from the IFS model of the European Centre for Medium-Range Weather Forecasts at 9 km grid spacing, anywhere in the world. It does not feed into the overall rating.

Night in detail uses the same requests and states above the chart which model the night comes from. The interfaces also offer 15-minute values, but for clouds, wind and temperature they are merely interpolated between the hourly values – so the chart shows hourly values.

Seeing is not provided by any weather model. The chart estimates it from the wind at about 5 and 10 km altitude: fast upper winds mean turbulent air. That gets the trend right, but it is no substitute for a real seeing forecast.

Transparency is estimated from the aerosol optical depth (AOD) of the Copernicus CAMS forecast, damped at very high humidity. These data only reach about four to five days ahead; after that the row stays dark grey.

Overall is a weighting of my own: clouds decide almost everything, seeing and transparency only nudge a clear hour up or down.

Sun and moon are calculated by your browser; nothing is requested for them.

Weather data by Open-Meteo.com (CC BY 4.0) · Models: DWD ICON (Europe), NOAA GFS and HRRR (outside Europe), ECMWF IFS (comparison row) · Aerosol: Copernicus Atmosphere Monitoring Service (CAMS). Seeing, transparency and the overall rating are derived from these data.