What colour is the sky right now

Pick a city and the tool paints the clear sky above it right now — or at any moment you choose — as the three colours of its gradient, from overhead to the horizon, with their hex codes and the CSS line to paste. It names the light you are in, blue hour or golden hour or full day, says where the sun stands, and tells you when the light changes next. Free, no account, nothing stored.

What it does, and why it is here

The question is idle and everybody asks it: what colour is the sky right now, what colour will it be tonight. The answer is computable, because a clear sky runs through the same sequence everywhere as the sun goes down — blue, then gold at the horizon, then the pinks and violets of the blue hour, then indigo, then black — and the one thing that drives the sequence is the sun’s height, which an ephemeris knows to the minute for any place on Earth.

So the tool reads the sun’s height and bearing for your city at the moment you ask, places it on that sequence, and blends the two nearest colours of a clear evening into the sky you would see. It is a model, and it says so: clouds, haze and dust are not in it. What it gives you is the sky that the light alone would make.

What you fill in

• City — picked from the list, which fills in the coordinates and the time zone. That is all the tool needs.
• When (optional) — a date and time on the city’s clock. Empty means right now. Tonight’s blue hour, last Tuesday’s sunset, the sky over the town you were born in at the minute you were born: all the same question.

What you get back

The light, first: golden hour, blue hour, nautical twilight, full day, night — the names photographers and sailors use, on the thresholds they use them at. Under it, how high the sun is and which way, and the day’s sunrise and sunset.

Then the sky itself, painted top to bottom, and its three colours as hex codes: overhead, half-way down, at the horizon line. Below them the CSS gradient, ready to paste as a background. Designers keep it; the rest of us send it.

Then the card — the same swatch with the sun where it stands, above the horizon line by day, sinking under it after sunset — and the moment the light changes next: golden hour at 19:43, nautical twilight at 20:51.

The live version

Under the answer there is a snippet to copy. It embeds the same picture as an image that redraws itself every ten minutes: paste it into a page and it becomes the sky over our town right now, turning gold in the evening and dark at night without anyone touching it. It carries only the city, so there is nothing personal in the address.

Where it stops

A clear sky. An overcast one is grey, a dusty one is red at noon, and the tool knows neither. Below eighteen degrees under the horizon the night is simply night, and the tool does not announce the exact minute it begins. What it announces exactly are the boundaries the sun defines: dawn and dusk, civil and nautical, the golden hour’s two ends, and the horizon.

Nothing is stored

The city and the moment are read from your request, used for the computation and dropped. The address of the answer carries them, so it can be sent to someone and recomputes when they open it. No account, no cookie.

Questions

Why is the sky blue, and why does it turn gold at the end of the day?

Air molecules scatter short wavelengths far more strongly than long ones, so the light arriving from every direction of a clear sky is mostly blue. Near sunset that same light crosses a far longer stretch of atmosphere before it reaches you, the blue has been scattered out of it along the way, and what is left is red and gold. The tool does not simulate the scattering: it reads how high the sun is and takes the colours a clear sky has at that height.

What are the blue hour and the golden hour, exactly?

Both are defined by the height of the sun, not by the clock — which is why they last twenty minutes in the tropics and half an evening in Iceland. These are the boundaries this tool uses.

The horizon the tool uses sits 0.833° under the geometric one, and not at zero, because the sun is a disc rather than a point and because the atmosphere bends its light: when the sun looks as though it is touching the horizon, it has in fact already set.

Can I use these colours in a design?

That is what the hex codes and the CSS line are there for. The three swatches are the sky overhead, half-way down and at the horizon line, and the gradient under them is ready to paste as a background.

Does it work for a moment other than now?

Yes. Leave the when box empty for the present minute, or give it any date and time on the city’s own clock — tonight’s blue hour, a sunset from last year, the sky over the town you were born in at the minute you were born.

Why does the answer not match what I can see out of my window?

Because the model is a clear sky and yours probably is not. Cloud flattens everything to grey, haze and dust redden the light hours before sunset, and a city’s own lights lift the night. What the tool gives you is the colour the sunlight alone would make; the weather is the part it cannot know.

Can I put the current sky on my own page?

Yes. Under the answer there is a snippet holding an image address that redraws itself every ten minutes, so a page carrying it shows the sky over that city turning gold in the evening and going dark at night, on its own. The address carries the city and nothing else, and the picture loads no script of ours.

Is my location sent anywhere?

No. The city you pick is matched against a copy of the GeoNames database held on our own server, and the coordinates and the moment are used for the computation and then dropped. There is no account and no cookie.

Sources and further reading

• US Naval Observatory — rise, set and twilight definitions, the source of the civil, nautical and astronomical boundaries in the table above.
• NOAA Solar Calculator, to check the sun’s height and bearing for any place and any moment.
• NASA — visible light, on scattering and why the sky has the colours it has.