Cloud cover and the aurora

Aurora forms 80 to 500 km up, far above any weather. Cloud sits between you and it. A strong storm under full overcast shows nothing from the ground, while a modest display under a clear sky can be memorable. SWPC puts it first among its conditions: the skies must be clear and free of clouds.

Cloud can also differ between nearby spots. One grid square can be overcast while the next one is forecast clear. That is why Glow reads cloud per grid square and shows it by interval instead of as one number for the night.

The NWS forecast grid carries a skyCover value, the percent of the sky covered by cloud, for time intervals through the coming days. Forecast text uses words instead, and the NWS glossary ties each word to octants, eighths of the sky covered by opaque cloud.

Glow prints its own short label beside each percent. Those labels are not NWS terms, and Glow says so on the page. The percent is the NWS figure; the label is a reading aid. At 71 percent and above Glow writes cloudy, which in octants is roughly 6 of 8 or more.

A forecast percent is not a photograph. It covers a whole grid square for a time interval, so a 30 percent interval can still put a bank of cloud on your northern horizon. Glow rounds coordinates to 2 decimal places, about 1 km, before looking up the square, so nearby points share one forecast.

For a US point Glow calls the api.weather.gov points endpoint, which names the forecast office and the grid square, then reads that square's skyCover series. The cloud cover tool keeps every interval that overlaps tonight's civil night and draws them under the darkness timeline, so you can see a clear gap between two cloudy spells.

The live logbook on each US place page, such as Duluth or Buffalo, shows the interval that covers the current time. If none does, it shows the next interval within a short lookahead and labels it so.

Outside rough boxes around the US states and Puerto Rico, Glow does not call NWS. The pages for Tromsø, Reykjavik and Yellowknife say there is no live forecast. Their month tables show NASA POWER's mean cloud amount for 2001 to 2020 instead, a cited average, not a forecast for any night.

• Check the cloud timeline in the afternoon, then again after dark. The NWS grid can change between the two.
• Look for gaps. A 2 hour clear window inside the 10 PM to 2 AM core is worth more than a clear evening.
• Move if you can. The chance tool reads cloud for any US coordinates, so you can compare a few spots.

Cloud never enters a combined score on Glow. The tools page explains why: cloud, darkness and the OVATION cell come from different sources and fail in different ways, and merging them hides which one is missing.