How the app picks a weather model for your area
Two forecasts can put the end of the cloud an hour apart. If that hour is the only dark stretch before your target drops behind the roof, the difference matters. The model behind the forecast is worth understanding before you plan the run. The Quiet Sky checks how cloud forecasts have held up in your region and uses that evidence when choosing what to show.
Why cloud forecasts differ
Weather models divide the atmosphere into a grid and calculate how conditions may develop. A finer grid can represent smaller features in the landscape, which can matter when cloud hangs over nearby hills. Models also differ in how they calculate cloud forming and clearing at different heights. Two models can put different amounts of low cloud over the same place even when both expect the same weather system to pass.
They start new runs at different intervals and look different distances ahead. A regional model may give a detailed view of the next few days while a global model reaches further. More detail does not settle whether tonight's cloud will clear on time. A model that has followed the weather well in one area may be less useful elsewhere, or on a different night. The cloud cover guide explains what those cloud amounts mean at the telescope.
Check what was forecast before the night
Every night, The Quiet Sky ranks weather models against cloud reported by airport weather stations in regions with saved sites. Through the day it keeps copies of forecasts made hours and days ahead. Later, it compares those copies with what the airports reported for the forecast times. That tests the forecast you could have used to plan an evening, rather than a revised calculation made after the cloud arrived.
An airport report describes the sky at the airport, so it is evidence for the surrounding region rather than a measurement above your telescope. The checks concern cloud forecasts, not the sharpness of stars in your subs. Keep seeing and transparency in view when deciding what to shoot. A model's recent cloud record cannot settle every part of an imaging night.
Keep a choice until the evidence changes
Some forecasts leave you to pick a model yourself; The Quiet Sky uses recent regional checks to make that choice. It considers models and blends of models. A choice needs enough recent evidence, without cloud amounts leaning too far towards clear or covered skies, and must show useful forecasting skill. It must also be updated regularly. The model already in use stays unless another shows a clear improvement, so one good night does not prompt a switch.
The UK and Ireland form one region, with geographic tiles used elsewhere. Only regions containing a saved site are ranked. Saving a site needs an account, but the chosen model or blend supplies the cloud, the score and the verdict for everyone in that region. Elsewhere, the forecast uses the model Open-Meteo selects for the location. This is a regional choice, not a separate judgement for each garden.
Follow the forecast out to seven nights
The chosen model may stop before the end of the week, or lack some hours. A second model fills missing hours, and another carries the forecast out to the full 7 nights. The week ahead therefore need not come from one model throughout.
Use the later nights to set aside a possible evening, then return closer to dusk before carrying out the rig. A continuous chart does not mean the weather several nights away is as settled as it looks. The guide to choosing a clear sky forecast explains how to fit those hours around your target.
Read the checks beside the main forecast
The Quiet Sky still compares the same hours with a second model from Environment Canada. If that model keeps the sky clear longer, its extra hours appear on the chart while the verdict stays with the main forecast. Keep them as possible extra time when planning an imaging session.
The evening check compares the models' last three hours with nearby observations and says when they do not match. Signed-in users can get the next couple of hours rescored from the model that has matched better. Observations remain beside the forecast. Read the forecast and nowcast guide before revisiting tonight's forecast, or return to the guides for help with an individual weather number.
For /how-it-works
Read the astronomy weather guides for help interpreting cloud cover, seeing and transparency, and deciding what to check before setting up.