Demo
Rain alerts for one exact spot
A replay of an invented stormy afternoon on a made-up coastline. Bulletins name the towns where rain is falling now; the app works out which of them lie upwind of Willow, the home town, how soon their rain could arrive, and whether that is worth an alert. Every number says where it came from and how sure it is.
This is a simulation. The coastline, the towns, the bulletins, the forecast runs, the warnings and the feedback are all invented, and the speeds are assumptions written into this page. No real place, weather feed or forecast is used, nothing you do leaves your browser or is kept, and reloading the page starts again.
1. Bulletins, the cone and the gates
Day 1
Change the wind to see the cone turn and the arrival range move. The bulletin on show is checked again.
No bulletin read yet: 6 arrive over the afternoon.
Latest bulletin
Nothing yet.
Press “Read the next bulletin” to play the first one.
- falling now rain falling there now
- expected rain only expected
- name not in the town table, left out
- Falling now
- None yet
- Expected
- None yet
- Not in the town table
- None
What the menu says
Waiting for the first bulletin.
- Rain falling now, all gates pass
- Rain falling now, blocked
- Rain only expected
- Willow, the home town
- Distance along the wind
The five gates, town by town
Each town the bulletin names is checked against five gates in order. An alert goes out only when one town passes all five; when none does, the menu says which gate stopped the most promising one.
The gate rules and the assumed speeds
- Falling now. The bulletin says rain is falling there now, not only expected.
- Upwind and in range. The town lies upwind, no more than 60 km out along the wind line, inside the cone: 4 km either side of the line plus 0.15 km for each km out.
- Bulletin fresh. The bulletin is no more than 45 minutes old.
- Early edge in the window. The early edge of the arrival range is between 10 and 60 minutes away: sooner is too late to help, later is too unsure.
- Not already announced. Rain from that town has not been announced already.
The arrival range uses two assumed speeds for a rain cell: the early edge at 100% of the steering wind, rounded down to 5 minutes, the late edge at 60%, rounded up. The steering wind is the average of the winds at two heights, taken as arrows, not as angles.
Bulletins read so far
None yet.
2. Four levels over one afternoon
Day 1
The same invented afternoon, as the menu would show it. Each level answers a different question from a different source, and says how sure it is. Once rain arrives the odds give way to “rain until”.
Level 1, hours ahead
Showing: Rain likely 15:00 to 18:00: 3 of 3 centres agree.
Level 2, official outlook
Showing: Issued 06:00: “Cloudy, with rain showers in the afternoon and evening.”
Level 3, minutes ahead
Quiet: Quiet: no bulletin passes all five gates.
Level 4, official warning
Quiet: No warning in force.
Once it is raining
Not raining in Willow yet.
- 12:00: 0.0 mm, forecast
- 13:00: 0.0 mm, forecast
- 14:00: 0.0 mm, forecast
- 15:00: 2.5 mm, forecast
- 16:00: 0.0 mm, forecast
- 17:00: 1.2 mm, forecast
- 18:00: 0.4 mm, forecast
- 19:00: 0.0 mm, forecast
- 20:00: 0.0 mm, forecast
- 21:00: 0.0 mm, forecast
- 22:00: 0.0 mm, forecast
Under 0.2 mm in an hour counts as dry. The rain is called over only after 2 dry hours in a row.
How the three centres vote
For each centre, the share of its runs that put rain on the ground in each three-hour block. A centre says rain at 50% or more, and the level warns only when at least 2 of the 3 agree. Pooling every run together would let the centre with the most runs decide.
| Block | Centre 1, 11 runs | Centre 2, 21 runs | Centre 3, 51 runs | Centres agreeing | All runs pooled |
|---|---|---|---|---|---|
| 09:00 to 12:00 | 1 of 11, 9% | 2 of 21, 10% | 5 of 51, 10% | 0 of 3: no warning | 10% |
| 12:00 to 15:00 | 2 of 11, 18% | 4 of 21, 19% | 40 of 51, 78% | 1 of 3: no warning | 55%, would warn |
| 15:00 to 18:00 | 8 of 11, 73% | 14 of 21, 67% | 30 of 51, 59% | 3 of 3: warns | 63% |
| 18:00 to 21:00 | 6 of 11, 55% | 12 of 21, 57% | 10 of 51, 20% | 2 of 3: warns | 34%, would miss it |
| 21:00 to 00:00 | 1 of 11, 9% | 3 of 21, 14% | 4 of 51, 8% | 0 of 3: no warning | 10% |
The share of runs is how much the centres agree, not a calibrated chance of rain.
- Willow, the home town: its exact point is tested
- Other towns
- The warning area in force
Warning area, tested against the exact point
No warning in force.
A warning covers an area. The app tests Willow’s own point against the area’s outline (a point in polygon test) and sounds its alarm only when the point is inside. 2 warning areas are issued this afternoon.
3. The shadow log
Every check the replay makes goes into this log, the silent ones too, with two feedback answers: did rain come within the hour, or did it stay dry? Answer the replay’s checks yourself, or add invented later days that were answered already. The app will not show an accuracy figure until it has 20 answers, and a bulletin checked again after a wind change counts once.
Accuracy
- No accuracy figure yet: 0 of the 20 answers needed.
- A figure from a handful of answers would look precise and mean little, so the app keeps it back.
- A bulletin checked again after a wind change counts once, by its latest answered check.
5 invented days left to add.
The log is empty. Read a bulletin above, or add a day.
What the demo shows
- Towns read out of plain sentences. The bulletins are prose. The parser separates “falling now” from “expected” sentence by sentence and looks each name up in an offline town table, so the alert path needs no network lookup; a name missing from the table is left out, never guessed.
- Upwind, inside a cone that narrows. A town counts only if it lies upwind, within 60 km, and close enough to the wind line: 4 km either side plus 0.15 km for each km out, so far towns must sit almost on the line. The travel distance is the part along the wind, not the straight line.
- A range, never a single minute. Two assumed speeds give the early and late edges of the arrival range, and the alert is sent only when the early edge falls between 10 and 60 minutes away.
- Gates that say why. Falling now, upwind and in range, bulletin fresh, early edge in the window, not already announced. When one blocks, the menu names it and gives the reason.
- Four levels, each with its source and certainty. Forecast runs that vote by centre, the official outlook relayed word for word, the approach alert, and official warnings tested against the town’s exact point. Once it rains, “rain until” replaces the odds and waits for 2 dry hours before calling the rain over.
- An app that earns its own accuracy figure. Every check, silent or not, is logged for feedback, and no figure is shown until 20 answers are in, one per bulletin. A check stopped only because its rain was announced already is not counted as a miss.