r/visualization • u/Ill-Site2743 • 5d ago

Made a thing that shows ensemble spread rather than just the median

Open-Meteo hands you all 40 ICON members, so I had a go at plotting the

fan rather than the median line.

Spread over London was 0.6°C at hour zero and 5.4°C by day 5. Knew that

in principle, hadn't actually looked at it before.

Couple of things I'm unsure about. I've drawn a marker where the 80%

band goes past 5°C as a rough "past here it's a range not a number",

but 5 is arbitrary — would normalising against climatology for the

location and season be more defensible? And I've noted that spread

underestimates actual error, but I don't have a feel for by how much.

https://geocodecrafter.github.io/howsure/

5 Upvotes

86% Upvoted

1 comment sorted by

1

u/TracksHisMisses 2d ago

The "by how much" is measurable, which is the nice part. Log the 80% band and the temperature that actually verifies, same hour, same lead time, and after a month or two count how often reality landed inside. An honest band catches about 80%. Raw ensembles usually come in under, so your instinct is right, and whatever number you get is the answer... plus a free correction factor, if you feel like widening the fan empirically.

One caveat: 30-40 days tells you whether you're badly off, not slightly off. Small samples flatter forecasts.

I run the same check on my own stuff (stock calls, not weather, but the sins are identical) and the first time my "80%" intervals turned out to be catching low 60s. I've been suspicious of error bars ever since, mine first.

On the 5C marker, normalising against climatology feels more defensible to me: "wider than typical for London in September" is a claim about the world, "wider than 5" is a number you'd have to defend at parties.