ToolLiveComputed
Will it be cloudy during tonight's dark hours?
Answer
NWS forecasts 0% to 4% sky cover during tonight's dark hours at Duluth; 10 h 36 min of civil night falls in periods at 30% or less.
| Local time | Sky cover | 30% or less |
|---|---|---|
| Thu 20:00 to 21:00 | 3% | Yes |
| Thu 21:00 to 22:00 | 2% | Yes |
| Thu 22:00 to 23:00 | 3% | Yes |
| Thu 23:00 to 01:00 | 4% | Yes |
| Fri 01:00 to 02:00 | 2% | Yes |
| Fri 02:00 to 03:00 | 1% | Yes |
| Fri 03:00 to 04:00 | 2% | Yes |
| Fri 04:00 to 05:00 | 1% | Yes |
| Fri 05:00 to 06:00 | 0% | Yes |
| Fri 06:00 to 07:00 | 1% | Yes |
Worked example
NWS forecasts 50% to 81% sky cover during tonight's dark hours at Fairbanks; no period of civil night is at 30% or less.
Fairbanks is a US point, so NWS answers with the forecast office and grid square that hold it. On this request that is office AFG, grid 492,119, with 4 skyCover periods overlapping tonight's civil night.
The strip under the timeline draws each period as a bar whose height is the sky cover percent. Green bars are periods at 30% or less; gray bars are cloudier. The bars line up with the twilight bands above, so you can see which dark hours are the clear ones.
What NWS skyCover is
The National Weather Service publishes forecast grids for the United States through api.weather.gov. Each grid square carries time series of forecast fields. One of them is skyCover: the percent of the sky covered by cloud, given for periods of one or more hours.
Glow first asks the points endpoint which office and grid square hold your coordinates. It then reads the gridpoint forecast and keeps every skyCover period that overlaps tonight's civil night, from the first dusk to the last dawn. Points are cached for an hour and grids for 15 minutes, and each request waits at most 3 seconds.
How to read the strip
The top band is tonight from local noon to noon. Periwinkle shades are civil and nautical twilight, the darkest navy is astronomical night, and the pale hatch shows hours when the moon is above the horizon while it is dark. The light band marks 10 PM to 2 AM local time.
Under it, each NWS period is one bar. A period can span several hours, and the bar spans the same hours. The verdict line adds up the minutes of civil night that fall inside periods at 30% or less. That threshold is Glow's reading aid, not an NWS category.
Outside the United States
NWS forecasts stop at the US border, and api.weather.gov returns no grid for Canada, Iceland or Norway. Glow does not swap in forecast there. The page says a live reading is not available and points you to your national weather service; town pages show NASA POWER's long-term average, which is not a forecast.
Near borders and coasts a point inside the rough US boxes can still return no grid. In that case the tool reports that NWS returned nothing rather than guessing.
Questions
- Can you see the northern lights through clouds?
- Cloud hides whatever is behind it. Thin or broken cloud can leave gaps, which is why the tool shows each forecast period instead of one number for the night.
- Why does the tool only work in the US?
- Glow reads live cloud forecasts only from the National Weather Service, which covers US coordinates. It does not substitute another forecast where NWS has no data; town pages show NASA POWER's long-term average, labelled as such.
- What does 30% or less mean on the strip?
- It is Glow's cut-off for a mostly open sky in the forecast period. NWS reports the percent; the 30% line is a reading aid.
- How often does the cloud forecast change?
- Glow re-reads the NWS grid at most every 15 minutes. The retrieval time is printed next to the office and grid ids.
- Why is there no strip tonight?
- Either the place has no civil night tonight, the point is outside NWS coverage, or NWS did not answer within 3 seconds. The verdict line says which.