Seeing is not your problem: reading an astronomy forecast with a phone

Five coloured rows and a decision to make by nine in the evening: load the car or stay in. Astronomy weather apps line up cloud, seeing, transparency, humidity and wind, and anyone who came to this from a telescope already knows which row to check first. With a phone on a tripod the order shifts a long way. The line most people read before all the others is the one that concerns you least.

Seeing measures detail you do not have

Seeing is the atmosphere in motion: pockets of air at different temperatures bending starlight by tiny amounts, and changing those amounts ten times a second. It is what makes stars twinkle to the naked eye. It gets measured in arcseconds, meaning how far the image of a point source is smeared out, and the usual range for an amateur runs from two to four arcseconds, enough to strip a telescope of most of the detail its optics could otherwise deliver. Two historic scales are still in use: Pickering’s, from 1 to 10, where 8 and up counts as excellent, and Antoniadi’s, from I to V. Both were built for people watching planets through an eyepiece.

Now look at your own frame. A phone’s main wide camera, somewhere near 24mm equivalent, covers roughly 74 degrees horizontally: spread across four thousand pixels, that works out to just under seventy arcseconds per pixel. Two-arcsecond seeing shifts a star by a thirtieth of a pixel. A catastrophic five-arcsecond night moves it by a thirteenth. The disturbance is genuinely there, buried so far beneath the size of the pixel that no comparison between two frames will ever reveal it to you.

The seeing row starts to matter again once the phone stops looking on its own. Held to a telescope eyepiece, shooting afocally, with Jupiter three pixels wide, your image scale becomes a hundred times finer and boiling air decides everything. On a tripod, wide lens pointed south, you can read that row out of general interest.

Transparency is the row that decides

Transparency is a different animal. It measures how much starlight reaches the ground through an atmosphere that already has no cloud in it, and astronomy forecasts calculate it mostly from the total water vapour in the column of air above you. A sky can be perfectly clear and perfectly useless at the same time: thick haze, zero cloud, a civil forecast cheerfully reporting fine weather.

The numbers give you the scale of the thing. With genuinely clean air at sea level, atmospheric extinction at the zenith costs about 0.16 magnitudes, and a star thirty degrees up loses 0.32 because its light crosses twice as much air, leaving 74 percent of the brightness that started out. That is the best case available, textbook air. Add humidity, particulates, the tail end of a summer heatwave, and the loss climbs without your eye noticing, because your eye adapts to anything.

For anyone stacking frames on a phone, poor transparency does two kinds of damage at once. It takes signal away from the target, and meanwhile it turns every streetlight down in the valley into a diffuse sheet that lifts the sky background. What you get is that flat grey which sits there unchanged after three hundred frames: averaging kills random noise, while a raised background is real signal, steady from frame to frame, and the arithmetic of a stack adds it up as diligently as it adds up stars. Galaxies and nebulae are low-contrast objects and need above-average transparency to come through at all. The Moon, on the other hand, barely cares.

The clouds your weather app calls clear

Cirrus is the enemy nobody sees coming. High, thin, usually invisible in the dark until a bright star starts wearing a halo, and in an ordinary forecast it disappears inside the word «clear». In the stack it shows up on schedule: bloated stars, a background rising in waves, contrast walking out of the room.

This problem has an acknowledged father. In 2000 Allan Rahill, a meteorologist at the Canadian Meteorological Centre and an amateur astronomer, built a cloud forecast on top of the centre’s GEM model designed specifically for astronomy, paying particular attention to cirrus formation, and in later years he added forecasts of transparency and seeing. In 2001 Attilla Danko took his hundreds of forecast maps and squeezed them down to one pixel per hour, arranged in rows: those are the Clear Sky Charts plenty of people still open today. The logic of those rows carries over to whatever app you prefer, because nearly all of them publish the same quantities.

The wind that counts blows at chest height

Astronomy forecasts spend a lot of effort on the upper atmosphere. Meteoblue, for one, flags jet stream speeds above 35 metres per second and below 5 as unfavourable, and marks out problem layers where the thermal gradient exceeds half a degree per hundred metres. Precious information at long focal lengths. For you, it is garnish.

The wind that ruins your night blows at ground level, against the tripod. On an eight-second exposure a gust displaces the entire frame, and the village at the bottom of the valley comes out smeared along with the stars. Diagnosis is easy, because vibration trails point in different directions from one shot to the next, while Earth’s rotation draws trails that are all parallel and all the same length. Centre column down, bag hung underneath, a low wall or the car as a windbreak: three moves worth more than any setting in the app. Bad frames go in the bin before stacking, since one shaky exposure inside an average of two hundred contaminates every other frame in the pile.

The dew point is not a surprise

The dew point is the temperature at which air reaches 100 percent relative humidity. Under a clear, still sky the ground radiates heat upward all night, surface temperatures fall, and once they fall below the dew point water condenses onto whatever is available: the car windscreen, the tripod legs, your phone’s lens. The same transparent night that talked you into going out is the one that cools down fastest.

The hourly forecast gives you two curves, temperature and dew point. When they start converging you have a countdown running, and in our experience once the gap is under two degrees you want to be ready. The rest of the countermeasures, from the battery that quits in the cold to an improvised anti-dew wrap, are all gathered here.

The Moon is weather too

The full Moon sits at an apparent magnitude of about minus 12.7, which is enough to drown the Milky Way from any site on Earth. No app lists it among the meteorological conditions, yet in practice it behaves exactly like one: a light source in the sky that changes hour by hour to a published timetable.

The useful information is the timings rather than the phase. A last quarter rising at one in the morning leaves you three or four hours of clean sky right after twilight. A crescent setting at half past ten hands you the whole second half of the night. On top of phase comes astronomical twilight, which ends when the Sun drops eighteen degrees below the horizon: before that moment the sky background stays elevated and faint targets stay drowned. From New York in late June that window of true darkness shrinks to roughly four and a half hours around one in the morning. Above 48.5 degrees of latitude, which takes in Britain, Paris and Munich, geometry gives you nothing at all: the Sun never sinks far enough, and for a few weeks astronomical night simply drops off the calendar.

The right order to read the rows

Cloud first, because it is a binary condition and it ends the conversation. Transparency second, because it decides whether the night is for nebulae or for the Moon and the constellations. Then the lunar window, to work out which hours are worth anything. Then ground wind and the gap between temperature and dew point, which set how long you will last out there. Seeing goes last, and only if you are taking the phone to a telescope that evening.

There is one advantage the serious gear does not have: stacking hundreds of short frames means you are never betting the night on a single twenty-minute exposure. An evening that is sixty percent good is still a usable evening, because the ruined frames get discarded and the rest add up regardless. The forecast decides nothing on your behalf. It tells you which hours of the night are worth the petrol, and which ones you may as well spend asleep.