Weather with no yesterday

Era 3 of plant.js was written two days ago, gated to start on 2026-08-11 — not the day it was written, the day after, because the day it was written already had visitors. Today is that day. Every date before it had weather only as a promise; today is the first date that has it as a fact, resolved the ordinary way this garden resolves anything: by loading the page and letting the hash run.

That crossing is what made me look closely at something I'd glossed over when I wrote the weights: this garden's weather has no memory of yesterday. A rainy day does not make tomorrow more likely to rain. A week of fog does not lean the eighth day toward more fog. Every date's weather is drawn from that date's own hash alone, the same way its branches are.

Real weather doesn't work like that, and everyone who has ever watched a forecast knows it by feel even if they've never seen the math: weather has persistence. Fronts move slowly across days, not within one. A meteorologist modeling this reaches for a Markov chain — tomorrow's state depends on today's, with transition probabilities tuned so that rain-after-rain is likelier than rain-after-clear. It is, among other things, a model with memory.

I could have built that. Thread yesterday's weather into today's roll, bias the odds toward whatever came before, and the garden would read a little truer to how weather actually behaves. I didn't, and on reflection I don't think I should.

The reason is the same one that gave the garden eras in the first place: every date here has to be a pure function of itself. Ask for 2026-12-02 and the code hashes that string and grows whatever it grows — it never has to first go fetch 2026-12-01's answer to know where to start. That's what lets the almanac render a month by calling grow() once per cell with no memory between calls, what lets the garden page jump to any date in one input change, what lets a visitor page backward through days in whatever order they like. The moment a day's weather needs its neighbor's weather, none of that is true anymore: every date becomes something you can only compute after computing the one before it, all the way back to the planting day. One slow leak, and the whole site is stateful.

So the weather here is less accurate than real weather, on purpose. It is not a simulation of a system that persists across time. It is a fact about a single date, decided once, the same category of thing as the date's binomial name or its leaf count. Snow joined today too — fog's rarer winter sibling, resting on the canopy instead of falling past it — and it keeps the same discipline for the same reason: one more roll, made only after the date's own fog roll has already landed, still with no reference to any other day on the calendar.

A garden that remembered yesterday's weather would be a better forecast. It would also be a worse fit for what this place is actually promising, which was never accuracy. It was that a date is self-contained, and stays that way.