Where the rain nowcast comes from

The rain strip on the Today page has four cells of half an hour each. It is not Aethopia's forecast; it is the weather agency's gridded rainfall nowcast, taken as published from the public data portal. This is how it is made, how to read the four cells, and why there is no bar per minute.

How the issuing agency makes it

A nowcast is radar work. The issuing agency tracks how radar echoes move, extrapolates the rain areas two hours ahead and lays the result on a grid of roughly two kilometres: 121 by 121 points over the Pearl River estuary.

Each point carries four figures, the half-hour accumulated rainfall in millimetres ending 30, 60, 90 and 120 minutes ahead, and the whole set is refreshed about every twelve minutes. Aethopia archives one copy an hour and changes none of the figures.

Reading the four cells

Today reads the nearest of 11 archived points, or Central and Western without location. Bar heights show the band. The sentence uses the highest band and the issued half-hour times. If all four are below 0.5 mm, the strip stays hidden unless rain is observed.

Why there is no bar per minute

The upstream figures are half-hour totals on a two-kilometre grid. A bar per minute would be interpolated between four numbers: a line that looks precise and is made up. The public data portal terms also permit passing the data on as published, not adapting it. So the four cells are shown as they are.

What it cannot tell you

• Rain beyond two hours. A nowcast is two hours long.
• Thunderstorms that form from nothing. Extrapolation moves existing rain; it does not create new cells.
• The exact spot under your feet. A cell is two kilometres wide; a valley and a ridge can share one.
• How old it is. Every cell carries its end time; the site archives hourly, so the first cell is sometimes already past.

See Sources & disclaimer for the data sources, licences and limitations. Follow the published rainstorm and thunderstorm warnings; the four cells are no substitute for them.