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

Every hour gets an overall rating from 0 to 100 %: how well it suits observing and imaging. From 85 % it is Excellent, from 65 % Good, from 45 % Average, from 25 % Poor, below that Very poor. Clouds make up most of it – a sky only half covered already caps an hour at 25 %.

The seven-day chart, from top to bottom

• Moon symbol with the illuminated share in per cent, placed above the moon’s highest point. A thin crescent hardly matters; from about half moon it brightens the sky noticeably.
• Sun and moon: the yellow times are sunrise and sunset, the white ones moonrise and moonset. Below them the altitude above the horizon – the sun filled in yellow, the moon dashed in white. A hairline runs from each time to the horizon; where two times fall too close together, only the line remains and the time is shown in the night detail. Light columns are day and twilight, dark ones astronomical night. The red line marks Now.
• Hour bar: 00, 06, 12 and 18 local time, with a weather symbol in between for the six hours around it. The most severe weather wins, so showers beat clouds. At night a clear sky shows the moon instead of the sun – careful: “mainly clear” allows up to almost 50 % cloud.
• Overall, clouds, ECMWF, seeing, transp.: the stronger the blue, the better; light grey is unfavourable, dark grey means no data. Clouds turns bluer the less of the sky is covered. ECMWF shows the cloud cover from a second, independent weather model – when both rows agree, the forecast is more reliable; when they diverge, the night is open. Seeing and transp. (transparency) are estimates; what they mean is explained under the terms below.
• Wind at the ground in km/h; the arrow shows where it blows to. Green up to 10 km/h, light green up to 20, yellow up to 30, orange up to 40, red above. The stronger the wind, the more the telescope shakes.
• Temperature (orange) with each day’s high and low, and dew point (blue) with its low. When the two lines come close, the optics fog up.
• Day bar and night avg.: below the date a colour gradient shows the overall rating hour by hour – red poor, orange and yellow middling, green good; in daylight it stays dark. Above it each night carries its rating, averaged over its astronomically dark hours.

The table

One row per night, noon to noon: when it is astronomically dark, which part of that is dark with the moon down – the moon is below the horizon then, the best time for galaxies and nebulae –, the overall rating of those dark hours, how much of the moon is lit and when it rises (↑) and sets (↓). Where the forecast covers a night only in part, a note below the rating says for how many of the dark hours there are data. Clicking a night opens it in detail. On narrow screens every night appears as a card of its own.

Night in detail

• At the top the weather of every hour as a symbol, below it the twilight band – from bright day through civil, nautical and astronomical twilight to dark night. The astronomically dark period is labelled there; the light-blue strip along its lower edge marks the time with the moon down.
• Sun and moon: rise and set times in a row each, below them both as hills, with the moon phase above the moon’s hill. Then comes the night avg. row as in the seven-day chart.
• Values per hour, shaded: the bluer, the more favourable. Overall % and Seeing % are ratings from 0 to 100. Clouds % is the covered share of the sky, split by height: low up to 2 km, mid 2 to 6 km, high above 6 km – thin high cloud is easy to miss with the naked eye but still spoils images. ECMWF % is the cloud cover from the second model. Then AOD, water vapour, dust, visibility in kilometres (GFS delivers at most 24 km) and rain % as the probability for that hour.
• Below the wind with its direction arrow, wind and gusts in km/h, temperature, dew point, the spread between them and the relative humidity. For the spread, orange means less than 4 °C and red less than 2 °C: dew becomes likely.
• Above the chart it says which weather model the hours come from. The arrows switch nights.

Moving the mouse over a chart or tapping it shows every value for that hour; a tap beside it closes the box again. On a phone both charts can be swiped sideways while the labels on the left stay in place. After loading, the seven-day chart starts at Now and the night detail at sunset.

Terms in brief

• Astronomically dark: the sun is more than 18° below the horizon; the sky is as dark as it gets at that place. In the twilight before, it is still too bright for faint objects.
• Seeing: how steady the air is. Turbulent air makes stars twinkle and blurs fine detail in images of the moon and planets. A cloudless sky with poor seeing is nothing unusual.
• Transparency: how clear the air is – whether haze, dust or smoke swallow the light of faint objects. It matters most for galaxies and nebulae.
• AOD (aerosol optical depth): a measure of haze and dust in the whole air column. Values around 0.1 mean very clear air; from about 0.3 the sky turns noticeably milky.
• Water vapour (mm): how much water the air column holds, counted as the rain it would make if it all fell at once. Below 10 mm the air is very dry, above 30 mm humid.
• Dust (µg/m³): mineral dust near the ground, from deserts for instance. The more of it, the milkier the sky.
• Dew point: the temperature at which moisture condenses as dew. When the air cools down to it, lenses and mirrors fog up – dew heaters help against that.

Where the values come from – and where they are estimates

Clouds, wind, temperature, humidity and the weather symbols 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, and the last two days or so from ICON global at about 11 km, which provides no visibility. The night detail states which model a night comes from.

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 profile: the jet stream at about 10 km altitude, the wind shear between roughly 1.5 and 10 km – how strongly direction and speed change with height – and the wind at the ground. All three make the air turbulent. Where a site is so high that 1.5 km lies inside the terrain, the chart starts from the next pressure level above. The weighting is my own; it 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. It is damped a little at very high humidity near the ground and by a lot of water vapour in the air column, taken from the ECMWF model – only gently, as water vapour absorbs little visible light. Dust is already part of the AOD; the tooltip and the night detail still show it separately, because it explains why a sky looks milky. The CAMS 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. Planets and the best objects of a night are shown by the Observing Planner.

The weather map is an embedded meteoblue service and not part of this page’s calculations: it shows meteoblue’s own satellite and forecast data for the chosen location.

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