Forecasts, observations and the night ahead
By late afternoon, the forecast that helped you keep the evening clear has a different job to do. You need to decide when to put the rig outside and whether the first gap will last. A weather model looks ahead; an observation tells you what has been reported recently. Read them together, with their times in mind, before treating a clear patch as the start of the night.
A model run is a fresh calculation
A weather model starts from an estimate of the atmosphere and calculates how it may develop. A new run starts again with newer information. Cloud timing can change between runs because the starting conditions have changed, and small differences can alter when a front reaches your site. A forecast moving the clear spell later is a reason to revisit the plan, even if the afternoon still looks much the same outside.
The Quiet Sky looks for a newer model run every few minutes while it is open. That does not mean a new run exists each time it checks. Nor does an hourly chart mean the underlying model is recalculated every hour. Use the later forecast to reassess the session; the clear sky forecast guide explains which parts of the night to compare.
A nowcast starts much closer to the present
A nowcast looks a short way ahead, drawing heavily on recent observations. Watching cloud move in satellite images can help estimate where an existing cloud edge may go next. It is less straightforward when cloud is forming or breaking up as it moves. An image of the sky now and a prediction for later answer different questions, even when they sit beside each other.
Reading only a model forecast leaves out the check against what the sky has just done. The Quiet Sky puts recent observations beside the forecast so you can compare the two. It does not provide a satellite nowcast. Use the observations to question a promising forecast without assuming the patch overhead will stay clear. The cloud cover guide explains why that patch may miss cloud in front of a low target.
Read the time and place on an observation
Near a saved site, The Quiet Sky shows the latest nearby airport weather report, including its age and distance. Saved sites need an account. The cloud figure shown covers cloud below 8 km, so it does not settle the question of high thin cloud. A nearby report is useful context, but the airport and your telescope are looking through different pieces of sky.
Account holders can also connect their all-sky camera and see it beside the forecast. Check that view against the part of the sky you intend to use. Bright stars on a camera image do not by themselves tell you how much faint detail will survive in a long exposure. Keep seeing and transparency in mind as well.
Use the evening check to revisit the plan
In the evening, The Quiet Sky compares the models' last three hours with nearby observations and says when the observations do not match the forecast. Signed-in users can get the next couple of hours rescored from the model that has matched better. The satellite part of this check covers Europe and Africa. This uses observations to inform which model supplies the next scores; it does not replace the forecast with an observation.
If the check points away from the forecast you used earlier, look again at the start and end of the usable spell. Decide whether to delay setup or aim for a shorter run, using the session planning guide to count what can fit. Even a model that has followed the last few hours can get the next change wrong. Keep checking the sky as dusk approaches.
Leave room for a different ending
When the models disagree about tonight, a later finish can be tempting. The Quiet Sky compares the model chosen for your area with a second model from Environment Canada over the same hours. Large differences bring a score range, with the dial filled to the pessimistic end. Extra clear hours from the second model appear on the chart, but the verdict stays with the main forecast.
Plan around the time you can use without depending on those extra hours, then reassess if the sky holds. Check the forecast again near dusk and look outside before starting the sequence. The guides can help you interpret the numbers; the first frames will tell you what your own telescope is collecting.