From b2612ab1ca6afd6bd1b882353ab5f5c116e34689 Mon Sep 17 00:00:00 2001
From: Emi Griffith <me@emigriffith.dev>
Date: Sun, 19 Jul 2026 12:10:24 -0700
Subject: [PATCH] Show rain in centimetres rather than millimetres on metric
 pages (#191)
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Precipitation on a °C page now reads "4.90 cm" where it read "49.0 mm".

Both units keep two decimals. cm and inches are only 2.54x apart, so cm needs the
same resolution inches have — at one decimal every light-rain day would collapse
to 0.0 or 0.1 cm, and the source itself is hundredths of an inch.

Ingest is untouched: climate.py still converts the upstream mm to inches, which
remains the stored unit and the value carried in data-precip-in.

Claude-Session: https://claude.ai/code/session_013dRZmX9D3JEntfMKWMTWZ8
---
 static/app.js    |  4 ++--
 static/shared.js |  4 ++--
 static/units.js  | 19 ++++++++++---------
 3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/static/app.js b/static/app.js
index 9d6704d..a2b41d7 100644
--- a/static/app.js
+++ b/static/app.js
@@ -188,8 +188,8 @@ async function runGrade() {
 const cTemp = fmtTemp;   // active unit, bare degree
 const cHum = (v) => (v == null ? "—" : `${Math.round(v)}%`);   // relative %, unit-invariant
 const cWind = (v) => (v == null ? "—" : `${Math.round(toWind(v))}`);
-// fmtPrecip carries the precision: two decimals for inches, one for mm (0.04" and
-// 1.0mm are the same amount, and "1.02 mm" would imply precision we don't have).
+// fmtPrecip carries the precision — two decimals in either unit, since cm and
+// inches are close enough in scale to need the same resolution.
 const cRain = (v) => (v == null ? "—" : v > 0 ? fmtPrecip(v, false) : "·");
 
 // Monochrome inline icons (currentColor) so the UI chrome matches the dark theme
diff --git a/static/shared.js b/static/shared.js
index 0c221f5..e124d65 100644
--- a/static/shared.js
+++ b/static/shared.js
@@ -121,8 +121,8 @@ function fmtMetricVal(unit, v) {
   if (unit === "temp") return fmtTemp(v);
   if (unit === "humid") return fmtHumid(v);
   if (unit === "wind") return fmtWind(v);
-  // The tier bar is tight on width, so precip keeps the ″ glyph over " mm"/" in".
-  if (unit === "precip") return `${fmtPrecip(v, false)}${getUnit() === "C" ? "mm" : "″"}`;
+  // The tier bar is tight on width, so precip keeps the ″ glyph over " cm"/" in".
+  if (unit === "precip") return `${fmtPrecip(v, false)}${getUnit() === "C" ? "cm" : "″"}`;
   if (unit === "dsr") return `${Math.round(v)}d`;
   return `${Math.round(v)}`;
 }
diff --git a/static/units.js b/static/units.js
index 49ee291..aefa0f1 100644
--- a/static/units.js
+++ b/static/units.js
@@ -4,7 +4,7 @@
 // `onUnitChange`, so stored and plotted values stay imperial and only the numbers
 // shown flip.
 //
-// One flag drives all of them: picking °C also gives mm and km/h. A reader who
+// One flag drives all of them: picking °C also gives cm and km/h. A reader who
 // wants Celsius wants the rest metric too, and a per-measure control would be
 // three toggles in a header that has room for one. The button still shows °F/°C
 // because temperature is what people look for.
@@ -67,24 +67,25 @@ export function fmtDelta(dF, withLetter) {
 }
 
 // --- precipitation (stored in inches) ----------------------------------------
-const MM_PER_IN = 25.4;
+const CM_PER_IN = 2.54;
 
 // Inches as a number in the active unit.
-export function toPrecip(vIn) { return _unit === "C" ? vIn * MM_PER_IN : vIn; }
+export function toPrecip(vIn) { return _unit === "C" ? vIn * CM_PER_IN : vIn; }
 
-// Millimetres get one decimal and inches two: 0.04 in and 1.0 mm are the same
-// amount, and rendering "1.02 mm" would imply a precision the source lacks.
-// The suffix is " in"/" mm" to match what content.py renders server-side, so a
+// Both units get two decimals. They are only 2.54x apart, so cm needs the same
+// resolution inches have — at one decimal every light-rain day would collapse to
+// 0.0 or 0.1 cm, and the source itself is hundredths of an inch.
+// The suffix is " in"/" cm" to match what content.py renders server-side, so a
 // climate page's repaint reproduces the text it shipped with rather than swapping
 // the glyph. Compact surfaces (the tier bar) pass withUnit=false and add their own.
 export function fmtPrecip(vIn, withUnit = true) {
   if (vIn == null || isNaN(vIn)) return "—";
   return _unit === "C"
-    ? `${(vIn * MM_PER_IN).toFixed(1)}${withUnit ? " mm" : ""}`
+    ? `${(vIn * CM_PER_IN).toFixed(2)}${withUnit ? " cm" : ""}`
     : `${vIn.toFixed(2)}${withUnit ? " in" : ""}`;
 }
 
-export const precipUnit = () => (_unit === "C" ? "mm" : "in");
+export const precipUnit = () => (_unit === "C" ? "cm" : "in");
 
 // --- wind (stored in mph) ----------------------------------------------------
 const KMH_PER_MPH = 1.609344;
@@ -116,7 +117,7 @@ export function onUnitChange(cb) { _unitCbs.push(cb); }
 // known. Runs on load and on every toggle, wherever units.js is imported.
 const UNIT_LABEL = {
   temp: () => `°${_unit}`,
-  precip: () => (_unit === "C" ? "mm" : "inches"),
+  precip: () => (_unit === "C" ? "cm" : "inches"),
   wind: () => windUnit(),
   humid: () => "g/m³",
 };