How the globe reads the weather

A guide to what you are looking at

City In A Snowglobe takes an ordinary weather observation — the sort of reading a forecast turns into a number and an icon — and builds it into a small world instead. Nothing in the globe is decorative. Every cloud, every falling thing and every shadow comes from a field in that observation. This page says which field does what, so you can read the globe back.

Where the reading comes from

The weather is OpenWeather's current-conditions report for the city you typed, fetched through this site's own endpoint so the API key stays on the server. A report is cached for fifteen minutes; going back to a city you have already looked at costs nothing and shows the same reading rather than a slightly different one.

The report carries rather more than a temperature: a condition code, a written description, cloud cover as a percentage, wind speed and bearing, humidity, pressure, visibility, the time of the observation, the city's offset from UTC, and its latitude and longitude. All of it is used.

The sky

Clouds

Cloud cover arrives as a percentage, and it sets how many clouds there are and how tightly they are packed. A clear reading gives a handful drifting near the glass; an overcast one fills the dome. Their colour comes from the condition rather than the cover: rain and thunder darken a cloud to slate, snow only dulls it, and anything else leaves it white.

They drift. The direction is the wind bearing from the report and the speed follows the wind speed, so on a still day they barely move and in a gale they cross the dome. When one reaches the edge it wraps round to the other side, carrying on rather than piling up — which is a thing it used to do, all of them stacking into one bank on the downwind side.

Rain, snow and lightning

What falls is decided by the condition code: rain and drizzle give rain, snow and sleet give snow, a thunderstorm gives both cloud-to-ground bolts and the dark cloud they come out of.

All of it falls out of the clouds, which is less obvious than it sounds. Each emitter used to have its own patch of sky and its own fixed height, so it rained over the city while the clouds sat somewhere else entirely. Now the clouds report where they are on every frame — they are drifting, so it cannot be worked out once — and every drop, flake and bolt starts inside the footprint of an actual cloud and just below its underside.

A drop stops when it meets something. The city is sampled into a coarse map of what is highest at each point — a roof, a canopy, a bench, the water in the fountain, the street — and the ring it leaves appears at that height rather than on the ground beneath whatever it hit. Snow uses the same map to settle: a flake that reaches the surface height for its column is copied into a second, still point cloud, and the blanket builds up over a few minutes.

Wind

Beyond the clouds, the wind bends the trees. Every canopy sways on its own phase, leaning with the bearing and harder as the speed rises, and the flowers in the beds bend with it too — less, because a flower is a stalk rather than a branch.

The light

Day, night, and the hour in between

The globe runs on the city's clock, not yours. The observation carries the city's offset from UTC, and that is what decides the hour: look up Tokyo at breakfast in London and you get Tokyo's evening. Sunrise and sunset come with the report, so the switch from day to night happens when it happens there.

Where the sun and the moon actually are

They are not stuck in the corner of the sky. Both are placed from the city's latitude and longitude and the current time, using the standard solar and lunar position formulas: the sun's declination and hour angle give its elevation and bearing, and the moon gets the same treatment along with its phase. So the sun rises in the right part of the sky for the place, tracks across at the right height for the season, and sets on the correct side — and at night the moon is where the real one is, lit the way the real one is lit.

Everything else follows from that. The direction of the shadows across the park, the colour of the light on the buildings, how far the street lamps have to work — all of it comes from the sun's elevation rather than from a switch.

Stars

They come out when the sun is far enough below the horizon, and how many of them appear depends on the cloud cover: an overcast night gets very few, because the sky over the city is not actually clear.

The city itself

The skyline is generated per city from the city's own name, so the same name always builds the same city: the same towers in the same places, every visit. A handful of cities have hand-placed landmarks on top of that. The park in the middle is the same park everywhere — a fountain, four paths, benches, lamps, beds of flowers and a couple of hundred trees and bushes — planted in one pass so that nothing ends up growing through a bench.

What it is not

It is a reading, not a forecast. The globe shows the conditions reported for the last observation, which may be up to an hour old, and it does not predict anything. The six-day and forty-eight-hour panels in the side drawer are OpenWeather's forecast, shown as numbers, and the globe does not animate them.