Live World weather: dynamic clouds & rain cycle (save v10)

A per-cell humidity/cloud/rain cycle advanced on the live clock (PlanetWeather.cpp,
raylib-free): evaporate over warm sunlit seas -> advect humidity & cloud along the prevailing
wind (upwind differencing) -> condense into cloud (saturation vs temperature + windward
orographic lift) -> rain out thick cloud -> dissipate. Bounded exp-rate forms keep it stable at
any timestep, so it runs cleanly from hours/sec up to a month/sec. initWeather() spins the
fields up from the moisture climatology; fully deterministic (no RNG).

Render: a translucent cloud shell over the 3D globe (white -> dark slate where it rains,
alpha = cover) plus a matching drawWeather2D layer on the 2D map (shared drawMapTris
rasterizer), toggled with K (default on). stepSim runs stepWeather each live frame at the
sim-hours added to liveTime (held when paused). Cell-info shows cloud/humidity/raining.

Saved as v10 (humidity/cloud/rain, flag-gated; pre-v10 saves spin weather up live). New
weather* config knobs. Reseed/regen now also drops out of Live World. test_weather.cpp:
fields in range, clouds form + rain falls, oceans moister than land, determinism, v10
round-trip; the other four suites still pass; GUI build clean. Docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

BUILD.md

@@ -51,6 +51,7 @@ the full ~2.8x speedup; the default uses all cores for no extra gain:
     N           toggle the day/night terminator (Live World)
     T           toggle the tide-coloured coastline (Live World)
     O           toggle ocean-current arrows (warm = poleward/red, cold = equatorward/blue)
+    K           toggle weather clouds/rain cover (Live World)
     SPACE       pause while forming / re-evolve once settled (or the on-screen button)
     [ / ]       drift speed (My/s) -- in Live World: live clock rate (hours/s, hour->month)
     S           single tectonic tick
@@ -71,9 +72,9 @@ CLI flags (applied before the first load/generate):
     planet.cfg   human-editable key=value config of every PlanetConfig parameter;
                  auto-created on first run, reload live with F2. Range-checked on
                  load; an invalid file reverts to safe defaults (not overwritten).
-    planet.save  binary snapshot (versioned, currently v9: +moons; v8 +Live World clock;
+    planet.save  binary snapshot (versioned, currently v10: +weather; v9 +moons; v8 +Live
-                 v7 +biota): seed + config + full planet state; F5 writes it, F9 reloads and
+                 World clock; v7 +biota): seed + config + full planet state; F5 writes it, F9
-                 resumes deterministically. As of v6
+                 reloads and resumes deterministically. As of v6
                  the config is stored as a self-describing key=value block (like
                  planet.cfg), so adding/removing config fields no longer breaks saves
                  (unknown keys ignored, missing keys default). v6 cannot load pre-v6
@@ -202,18 +203,31 @@ moving snow line. axialTilt (above) drives the seasonal declination. Press W to
 Moons (1-3, randomized in generateMoons() + saved v9): orbit on the live clock, raise the
 tides with the sun, and render as small lit spheres with phases, orbit rings and eclipses.
 
+Weather (PlanetConfig, Live World): a dynamic clouds/rain cycle on the live clock (key K),
+saved v10. Evaporate over warm seas -> advect along the wind -> condense -> rain -> dissipate.
+
+    weatherEvapRate     0.4    /h ocean evaporation toward marine saturation
+    weatherWindKmh      45     km/h wind speed advecting humidity/cloud
+    weatherSatBase      0.4    air saturation at 0 C (lower = cloudier)
+    weatherSatTempCoef  0.025  saturation rise per +1 C
+    weatherCondense     0.6    /h supersaturation -> cloud
+    weatherOrographic   0.0009 extra condensation per m of windward upslope
+    weatherRainThresh   0.5    cloud cover above this rains
+    weatherRainRate     0.5    /h rain rate from excess cloud
+    weatherCloudDissip  0.12   /h cloud clearing
+
 ## Headless logic test (no display)
 
     g++ -std=c++17 -O2 -Isrc/sim test_logic.cpp src/sim/IcoSphere.cpp \
         src/sim/Planet.cpp src/sim/PlanetTectonics.cpp src/sim/PlanetDrift.cpp \
         src/sim/PlanetErosion.cpp src/sim/PlanetHydrology.cpp \
         src/sim/PlanetBiomes.cpp src/sim/PlanetClimate.cpp src/sim/PlanetLive.cpp \
-        src/sim/PlanetOcean.cpp src/sim/PlanetBiota.cpp src/sim/PlanetFloraGen.cpp \
+        src/sim/PlanetOcean.cpp src/sim/PlanetWeather.cpp src/sim/PlanetBiota.cpp \
-        src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp src/sim/PlanetIO.cpp \
+        src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp \
-        -o /tmp/t && /tmp/t
+        src/sim/PlanetIO.cpp -o /tmp/t && /tmp/t
 
-    # Biota / Live World / Ocean suites: same source list, swap test_logic.cpp ->
+    # Biota / Live World / Ocean / Weather suites: same source list, swap test_logic.cpp ->
-    # test_biota.cpp, test_live.cpp or test_ocean.cpp
+    # test_biota.cpp, test_live.cpp, test_ocean.cpp or test_weather.cpp
 
 Verifies geometry, plate assignment, gradual non-saturating relief and
 determinism. Run after changing Planet::step().

CLAUDE.md

@@ -83,6 +83,12 @@ the fixed-grid Eulerian model + the climate fields are the groundwork for it.
   **warm (poleward) / cold (equatorward)** currents back into `sTemp` as a bounded coastal anomaly
   (`climateCurrentFactor`), so biomes shift naturally. Rendered as warm/cold **current arrows**
   over the sea (key `O`, 3D + 2D). This completes the Live World ocean/sky pass.
+- **Weather — dynamic clouds & rain** *(done — see `PlanetWeather.cpp`)* — a per-cell
+  humidity/cloud/rain cycle advanced on the live clock: **evaporate** over warm sunlit seas →
+  **advect** humidity & cloud along the prevailing wind → **condense** into cloud (extra on
+  windward upslopes) → **rain** out → **dissipate**. Rendered as a translucent moving cloud
+  shell (white → dark storm where it rains) over the globe + 2D map (key `K`). Saved (v10).
+  **Next:** tropical cyclones (hurricanes/typhoons) as moving vortex agents on top of this field.
 
 ## Current state
 
@@ -373,6 +379,19 @@ Working and verified (logic tested headless):
   in `refreshView`. New knob `climateCurrentFactor` (4 °C). `test_ocean.cpp` adds: currents
   tangent + zero on land + widespread, feedback bounded by the knob and produces both warming and
   cooling, deterministic. Live-World ocean/sky pass complete.
+- **Live World — dynamic weather (clouds & rain):** `Planet::stepWeather(dtHours)`
+  (PlanetWeather.cpp) advances a per-cell humidity/cloud/rain cycle on the live clock:
+  **evaporate** over warm sunlit ocean (uses `sInsolation`+`sTemp`), **advect** humidity & cloud
+  downwind (upwind differencing along `sWind`/`sUpwind`, `weatherWindKmh`), **condense** the
+  supersaturated air into cloud — saturation `weatherSatBase + weatherSatTempCoef·T`, plus
+  windward **orographic** lift — **rain** out cloud above `weatherRainThresh`, then **dissipate**.
+  `initWeather()` spins the fields up from the moisture climatology; bounded exponential rate
+  forms keep it stable at any timestep. Runs each live frame in `stepSim` (dt = the same sim-hours
+  added to `liveTime`; held when paused). Render: a translucent **cloud shell** (white → dark
+  storm where it rains, alpha = cover) over the 3D globe + a `drawWeather2D` layer on the 2D map,
+  key `K` (default on); cell-info adds cloud/humidity/raining. **Saved v10** (humidity/cloud/rain,
+  flag-gated; older saves spin weather up live). Deterministic (no RNG). `test_weather.cpp`:
+  fields in range, clouds form + rain falls, oceans moister than land, determinism, v10 round-trip.
 - Mouse hover (in either view) shows per-cell info. Clicking a tile opens a
   right-side detail panel: tile info header + the tile's subgrid drawn as a
   flat hoverable grid of subtiles (neighbor-owned subtiles dimmed). A high-res
@@ -405,7 +424,8 @@ src/
     PlanetHydrology.cpp  routeFlow/computeHydrology/hydrology (Phase 3)
     PlanetClimate.cpp    computeClimate() (temperature + orographic precipitation)
     PlanetLive.cpp       computeInsolation/computeLiveSeason (Live World: day/night + live seasons)
-    PlanetOcean.cpp      moons (generate/orbit) + computeTides (Live World sky & tides)
+    PlanetOcean.cpp      moons (generate/orbit) + computeTides + computeOceanCurrents
+    PlanetWeather.cpp    stepWeather (Live World dynamic clouds & rain cycle)
     PlanetBiomes.cpp     classifyBiomes() (per-cell Cell.biome from elev + climate)
     PlanetBiota.hpp      BiotaKind/SizeClass/EcoRole/Organism/CellBiota + archetype table decls
     PlanetBiota.cpp      archetype library + slot/point draw + generateBiota/computeBiotaDensity
@@ -471,12 +491,12 @@ g++ -std=c++17 -O2 -Isrc/sim test_logic.cpp src/sim/IcoSphere.cpp \
     src/sim/Planet.cpp src/sim/PlanetTectonics.cpp src/sim/PlanetDrift.cpp \
     src/sim/PlanetErosion.cpp src/sim/PlanetHydrology.cpp \
     src/sim/PlanetBiomes.cpp src/sim/PlanetClimate.cpp src/sim/PlanetLive.cpp \
-    src/sim/PlanetOcean.cpp src/sim/PlanetBiota.cpp src/sim/PlanetFloraGen.cpp \
+    src/sim/PlanetOcean.cpp src/sim/PlanetWeather.cpp src/sim/PlanetBiota.cpp \
-    src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp src/sim/PlanetIO.cpp \
+    src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp src/sim/PlanetFungiGen.cpp \
-    -o /tmp/t && /tmp/t
+    src/sim/PlanetIO.cpp -o /tmp/t && /tmp/t
 ```
-(Swap `test_logic.cpp` for `test_biota.cpp`, `test_live.cpp` or `test_ocean.cpp` to run the
+(Swap `test_logic.cpp` for `test_biota.cpp`, `test_live.cpp`, `test_ocean.cpp` or
-Biota / Live World / Ocean suites — same source list.)
+`test_weather.cpp` to run the Biota / Live World / Ocean / Weather suites — same source list.)
 
 Use this to verify tectonics after changing `Planet::step()` without launching
 the window (the engine lives in `src/sim` and is raylib-free, so it links without
@@ -507,7 +527,8 @@ elevation/plate/age/crust-type/biome/temperature/precipitation/flora/fauna/funga
 active mode shown top-center of the globe) ·
 `B` plate borders · `D` drift vectors · `G` lat/lon grid · `J` rivers (Phase 3,
 all in 3D + 2D) · `N` day/night terminator (Live World) · `T` tide-coloured coastline (Live World) ·
-`O` ocean-current arrows (warm/cold) · `SPACE` or on-screen button pause ·
+`O` ocean-current arrows (warm/cold) · `K` weather clouds/rain (Live World) ·
+`SPACE` or on-screen button pause ·
 `[`/`]` drift speed (My/sec) — in **Live World** the live-clock rate (hours/sec, hour→month) ·
 `S` single tick · `F` fast-forward Phase-1 forming to settled ·
 `H` toggle Phase 3 (hydrology) · `L` generate biota population (flora/fauna/funga,
@@ -541,9 +562,11 @@ save header is versioned (currently **9**; v2 adds the `[`/`]` drift rate, v3 a
 `phase3` flag, v4 a per-cell biome byte, v6 stores config as a **self-describing
 key=value text block** instead of a raw POD dump, v7 appends the **biota population**
 block — three Organism lists per cell, gated by a flag byte, v8 appends the **Live World**
-clock — a flag byte + `liveTime`, v9 appends the **moons** block); newer-than-supported is
+clock — a flag byte + `liveTime`, v9 appends the **moons** block, v10 appends the **weather** block —
+humidity/cloud/rain, flag-gated); newer-than-supported is
 rejected. Older saves (no biota block) load fine with an empty population (press `L`);
-pre-v8 saves load with Live World off; pre-v9 saves synthesize moons from the seed.
+pre-v8 saves load with Live World off; pre-v9 saves synthesize moons from the seed; pre-v10
+saves spin weather up live.
 **As of v6, adding/removing PlanetConfig fields no longer breaks saves** — the saved
 config is parsed like `planet.cfg` (unknown keys ignored, missing keys keep defaults),
 written at `precision(17)` so doubles round-trip exactly. (v6 cannot load pre-v6 saves —
@@ -605,6 +628,13 @@ triangles (plates are fixed in phase 1).
   `climateCurrentFactor` (4 °C) is the max coastal warming/cooling from ocean currents (0 = off;
   ocean-current arrows toggle with `O`). Current deflection angle + smoothing passes are
   constants in `computeOceanCurrents()` (PlanetOcean.cpp), not config.
+- **Weather (`weather*` in PlanetConfig / `planet.cfg`):** the Live World clouds/rain cycle —
+  `weatherEvapRate` (ocean evaporation speed), `weatherWindKmh` (advection speed of humidity/cloud),
+  `weatherSatBase`/`weatherSatTempCoef` (how much moisture the air holds vs temperature — lower
+  base = cloudier), `weatherCondense` (supersaturation→cloud rate), `weatherOrographic` (windward
+  rain on mountains), `weatherRainThresh`/`weatherRainRate` (when/how fast thick cloud rains),
+  `weatherCloudDissip` (cloud clearing). Toggle the overlay with `K`. Cloud render colours
+  (white→storm, alpha) are constants in ViewerRender/Map2D.
 - Seasons (`season*` + `axialTilt` + `biomeSeasonWeight`, `planet.cfg`) — `axialTilt` is the
   master driver (0 = no seasons); `seasonAmpMax` (18 °C max seasonal half-range at full
   tilt/lat/interior), `seasonLatExp` (1.2, push swing toward poles), `seasonContinentRings`

CMakeLists.txt

@@ -28,6 +28,7 @@ add_executable(planetsim
     src/sim/PlanetClimate.cpp
     src/sim/PlanetLive.cpp
     src/sim/PlanetOcean.cpp
+    src/sim/PlanetWeather.cpp
     src/sim/PlanetBiota.cpp
     src/sim/PlanetFloraGen.cpp
     src/sim/PlanetFaunaGen.cpp

docs/design-notes.md

@@ -34,7 +34,10 @@ include path, so includes stay flat (`#include "Planet.hpp"`, `"Viewer.hpp"`).
 - `PlanetLive.cpp` — `computeInsolation()`/`computeLiveSeason()` (Live World: day/night + live
   seasonal temperature; derived, not saved).
 - `PlanetOcean.cpp` — moons (`generateMoons`, `moonDirection`/`sunDirection`/`moonOrbitNormal`) +
-  `computeTides()` (Live World sky & equilibrium tides). Moons are saved (v9); tides derived.
+  `computeTides()` + `computeOceanCurrents()` (Live World sky, tides & currents). Moons saved
+  (v9); tides/currents derived.
+- `PlanetWeather.cpp` — `initWeather`/`stepWeather` (Live World dynamic humidity/cloud/rain cycle;
+  saved v10).
 - `PlanetBiomes.cpp` — `classifyBiomes()` (per-cell `Cell.biome` from elevation + climate).
 - `PlanetBiota.{hpp,cpp}` — Biota types + archetype table + slot/point draw +
   `computeBiotaDensity()`/`generateBiota()` (flora/fauna/funga).
@@ -171,6 +174,25 @@ back into `sTemp` as a bounded coastal anomaly (`climateCurrentFactor`, smoothed
 applied before seasons → biomes shift with it). Rendered as warm/cold arrows over the sea
 (`buildCurrents`, key `O`). Currents/feedback are derived (not saved).
 
+## Weather (Live World dynamic clouds & rain)
+
+`PlanetWeather.cpp` advances a per-cell **humidity / cloud / rain** cycle on the live clock
+(`stepWeather(dtHours)`), time-varying unlike the static climate. One step: **evaporate** over
+warm sunlit ocean (relax humidity toward a marine target scaled by `sTemp` warmth + `sInsolation`
+daytime), **advect** humidity & cloud downwind (upwind differencing along `sWind`/`sUpwind`, speed
+`weatherWindKmh`), **condense** the supersaturated air into cloud (saturation
+`weatherSatBase + weatherSatTempCoef·T`, plus windward orographic lift), **rain** out cloud above
+`weatherRainThresh`, then **dissipate** (half returns to humidity). All rate terms use bounded
+`1−exp(−rate·dt)` forms so it's stable at any timestep (the clock can run hours→months/sec).
+`initWeather()` seeds it from the moisture climatology. Deterministic (no RNG). Driven each live
+frame from `Viewer::stepSim` with dt = the sim-hours added to `liveTime` (0 when paused).
+
+Render: a translucent **cloud shell** over the 3D globe (white → dark slate where it rains, alpha
+= cover, a second triangle layer at `visBase+0.03`) and a matching `drawWeather2D` layer on the
+2D map (shared `drawMapTris` rasterizer), toggled with `K`. Saved as **v10** (humidity/cloud/rain,
+flag-gated; pre-v10 saves spin weather up on entering Live World). Future: tropical cyclones
+(moving vortex agents) layered on this field.
+
 ## Headless testing
 
 Engine is raylib-free, so logic is tested without a display. Build/run:
@@ -178,7 +200,7 @@ Engine is raylib-free, so logic is tested without a display. Build/run:
 g++ -std=c++17 -O2 -Isrc/sim test_logic.cpp src/sim/IcoSphere.cpp src/sim/Planet.cpp \
     src/sim/PlanetTectonics.cpp src/sim/PlanetDrift.cpp src/sim/PlanetErosion.cpp \
     src/sim/PlanetHydrology.cpp src/sim/PlanetBiomes.cpp src/sim/PlanetClimate.cpp \
-    src/sim/PlanetLive.cpp src/sim/PlanetOcean.cpp \
+    src/sim/PlanetLive.cpp src/sim/PlanetOcean.cpp src/sim/PlanetWeather.cpp \
     src/sim/PlanetBiota.cpp src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp \
     src/sim/PlanetFungiGen.cpp src/sim/PlanetIO.cpp -o /tmp/t && /tmp/t
 # test_biota.cpp uses the same source list (Biota suite).

src/render/Map2D.cpp

@@ -29,8 +29,9 @@ Vector2 mapScreen(const Map2D& m, int idx, Rectangle r, double lonOffset) {
     return projLonLat(m.lon[idx], m.lat[idx], lonOffset, r);
 }
 
-void drawMap2D(const Planet& p, const std::vector<Color>& vc,
+// Shared triangle rasterizer for the 2D map: calls colorAt(cellIndex) -> Color per vertex.
-               const Map2D& m, Rectangle r, double lonOffset) {
+template <typename ColorFn>
+static void drawMapTris(const Planet& p, const Map2D& m, Rectangle r, double lonOffset, ColorFn colorAt) {
     double hw = EqualEarth::halfWidth();
     auto px = [&](double lon, double lat) -> float {
         double x, y; EqualEarth::forward(lon, lat, x, y);
@@ -47,7 +48,8 @@ void drawMap2D(const Planet& p, const std::vector<Color>& vc,
         double mx = std::max({lo[0], lo[1], lo[2]});
         if (mx - mn <= M_PI) {                              // fast path (no wrap)
             for (int t = 0; t < 3; ++t) {
-                rlColor4ub(vc[v[t]].r, vc[v[t]].g, vc[v[t]].b, 255);
+                Color c = colorAt(v[t]);
+                rlColor4ub(c.r, c.g, c.b, c.a);
                 rlVertex2f(px(lo[t], m.lat[v[t]]), m.pos[v[t]].y);
             }
         } else {                                            // antimeridian seam
@@ -60,10 +62,28 @@ void drawMap2D(const Planet& p, const std::vector<Color>& vc,
             const double shift[3] = { 0.0, 2 * M_PI, -2 * M_PI };  // both edges; scissor clips
             for (double sh : shift)
                 for (int t = 0; t < 3; ++t) {
-                    rlColor4ub(vc[v[t]].r, vc[v[t]].g, vc[v[t]].b, 255);
+                    Color c = colorAt(v[t]);
+                    rlColor4ub(c.r, c.g, c.b, c.a);
                     rlVertex2f(px(ul[t] + sh, m.lat[v[t]]), m.pos[v[t]].y);
                 }
         }
     }
     rlEnd();
 }
+
+void drawMap2D(const Planet& p, const std::vector<Color>& vc,
+               const Map2D& m, Rectangle r, double lonOffset) {
+    drawMapTris(p, m, r, lonOffset, [&](int i) { return Color{ vc[i].r, vc[i].g, vc[i].b, 255 }; });
+}
+
+void drawWeather2D(const Planet& p, const std::vector<double>& cloud, const std::vector<double>& rain,
+                   const Map2D& m, Rectangle r, double lonOffset) {
+    if (cloud.empty()) return;
+    double maxR = 1e-6; for (double v : rain) maxR = std::max(maxR, v);
+    drawMapTris(p, m, r, lonOffset, [&](int i) -> Color {
+        double c = std::clamp(cloud[i], 0.0, 1.0);
+        double rain01 = std::clamp(rain[i] / maxR, 0.0, 1.0);
+        return Color{ (unsigned char)(245 - 150 * rain01), (unsigned char)(245 - 130 * rain01),
+                      (unsigned char)(250 - 95 * rain01), (unsigned char)(c * 205.0) };
+    });
+}

src/render/Map2D.hpp

@@ -21,3 +21,8 @@ Vector2 mapScreen(const Map2D& m, int idx, Rectangle r, double lonOffset);
 // lonOffset pans the map east/west (radians); y is unchanged by the pan.
 void drawMap2D(const Planet& p, const std::vector<Color>& vc,
                const Map2D& m, Rectangle r, double lonOffset);
+
+// Translucent Live World cloud/rain layer over the 2D map (white -> dark storm where it rains;
+// alpha = cloud cover). Same triangle iteration as drawMap2D but blended on top.
+void drawWeather2D(const Planet& p, const std::vector<double>& cloud, const std::vector<double>& rain,
+                   const Map2D& m, Rectangle r, double lonOffset);

src/render/Panels.cpp

@@ -64,6 +64,11 @@ static std::vector<std::string> cellInfo(const Planet& p, int i, double elev, do
     if (sized(p.tide()))
         L.push_back(std::string(TextFormat("tide %+.2f m (%s)", p.tide()[i],
                                            p.tide()[i] >= 0.0 ? "high" : "low")));
+    if (sized(p.cloud()))
+        L.push_back(std::string(TextFormat("weather: cloud %.0f%%  humidity %.0f%%%s",
+                                           p.cloud()[i] * 100.0,
+                                           sized(p.humidity()) ? p.humidity()[i] * 100.0 : 0.0,
+                                           (sized(p.rain()) && p.rain()[i] > 0.02) ? "  raining" : "")));
     L.push_back(std::string(TextFormat("geoAge %.0f My   neighbors %d", age, (int)c.neighbors.size())));
     // Hydrology (derived; present once routeFlow()/hydrology() has run).
     if (sized(p.discharge()) && p.discharge()[i] > p.cfg.riverThreshold)

src/render/Viewer.cpp

@@ -227,6 +227,7 @@ void Viewer::regenWorld() {                 // after generate(): geometry change
     buildMap2D(planet, mapRect, map2D);
     selectedCell = -1; subgrids.clear();
     settled = false; settleRun = 0; formAccum = 0.0; stepCount = 0; paused = false;
+    liveWorld = false;                        // reseed/regen drops back to World Creation
     planet.drifting = false;                  // Phase 1: original forming behavior
     phase3 = false; phase3Prompt = false; phase3PromptAt = planet.cfg.phase3AfterMy;
     rivers.clear(); bigRivers.clear();
@@ -278,7 +279,7 @@ void Viewer::loadGame(const char* path) {
     if (ver >= 8) { is.read(reinterpret_cast<char*>(&lw), sizeof lw);
                     is.read(reinterpret_cast<char*>(&lh), sizeof lh); }   // v8: Live World clock
     if (!is || std::memcmp(magic, "PLSV", 4) != 0 || ver > SAVE_VERSION) { setStatus("Load failed: bad file"); return; }
-    if (!planet.readState(is, ver >= 4, ver >= 7, ver >= 9)) { setStatus("Load failed: corrupt/mismatch"); return; }   // v4: biome, v7: biota, v9: moons
+    if (!planet.readState(is, ver >= 4, ver >= 7, ver >= 9, ver >= 10)) { setStatus("Load failed: corrupt/mismatch"); return; }   // v4 biome, v7 biota, v9 moons, v10 weather
     cfg = planet.cfg;                          // adopt the loaded config
     elapsedMy = em; settled = (st != 0);
     planet.drifting = settled;                 // resume drift boosts iff mid-drift
@@ -302,7 +303,8 @@ void Viewer::loadGame(const char* path) {
 void Viewer::stepSim() {
     if (liveWorld) {
         // --- Live World: advance the slow clock; geology is frozen --------
-        if (!paused) liveTime += liveRate * GetFrameTime();   // hours
+        double dtH = (!paused) ? liveRate * GetFrameTime() : 0.0;   // simulated hours this frame
+        liveTime += dtH;
         double days        = liveTime / planet.cfg.dayLengthHours;
         double dayOfYear01 = days / planet.cfg.yearLengthDays;
         dayOfYear01 -= std::floor(dayOfYear01);
@@ -319,6 +321,7 @@ void Viewer::stepSim() {
             moonDirs.push_back(Vector3{ (float)md.x, (float)md.y, (float)md.z });
             moonNormals.push_back(Vector3{ (float)mn.x, (float)mn.y, (float)mn.z });
         }
+        planet.stepWeather(dtH);                 // dynamic clouds & rain on the live clock
         rebuildLiveOverlay();
         return;
     }

src/render/Viewer.hpp

@@ -15,7 +15,7 @@
 // ViewerInput.cpp (input/picking/keys) and ViewerRender.cpp (drawing).
 struct Viewer {
     // ---- Files / save format ------------------------------------------------
-    static constexpr uint32_t SAVE_VERSION = 9;   // v9: +moons; v8: +Live World clock; v7: +biota population; v6: self-describing config; v4: +biome; v3: +phase3
+    static constexpr uint32_t SAVE_VERSION = 10;  // v10: +weather; v9: +moons; v8: +Live World clock; v7: +biota; v6: self-describing config; v4: +biome; v3: +phase3
     const char* CONFIG_PATH = "planet.cfg";
     const char* SAVE_PATH   = "planet.save";
     std::string configPath  = "planet.cfg";       // initial config (--config overrides)
@@ -90,6 +90,7 @@ struct Viewer {
     bool showTides = false;         // colour the coastline by the live tide level (key T)
     std::vector<Vector3> currentSegs; std::vector<Color> currentCols;  // ocean-current arrows
     bool showCurrents = false;      // ocean current arrows, warm/cold (key O)
+    bool showClouds = true;         // Live World cloud/rain cover overlay (key K)
 
     // Selection + subgrid (phase 4/5 preview).
     int selectedCell = -1;

src/render/ViewerInput.cpp

@@ -145,6 +145,7 @@ void Viewer::handleInput() {
     if (IsKeyPressed(KEY_N))     dayNightOn = !dayNightOn;   // toggle the day/night terminator
     if (IsKeyPressed(KEY_T))     showTides  = !showTides;    // toggle tide-coloured coastline
     if (IsKeyPressed(KEY_O))     showCurrents = !showCurrents; // toggle ocean current arrows
+    if (IsKeyPressed(KEY_K))     showClouds = !showClouds;     // toggle weather cloud/rain cover
     if (IsKeyPressed(KEY_C))     { selectedCell = -1; subgrids.clear(); }
     if (IsKeyPressed(KEY_R))     { cfg.seed = (uint32_t)(GetTime() * 100000) | 1; regen(); }
     if (IsKeyPressed(KEY_S))     { stepOnce(); refreshView(); }   // one tick (handy while paused/settled)

src/render/ViewerRender.cpp

@@ -103,6 +103,31 @@ void Viewer::renderGlobe3D() {
         }
         rlEnd(); rlSetLineWidth(1.0f);
     }
+    // Live World weather: a translucent cloud shell over the globe (white -> dark storm where it
+    // rains), alpha = cloud cover. Drawn as a second triangle layer just above the terrain.
+    if (liveWorld && showClouds && !planet.cloud().empty()) {
+        const std::vector<double>& cl = planet.cloud();
+        const std::vector<double>& rn = planet.rain();
+        double maxR = 1e-6; for (double r : rn) maxR = std::max(maxR, r);
+        const std::vector<int>& ctri = planet.triIndices();
+        const float cr = visBase + 0.03f;
+        rlBegin(RL_TRIANGLES);
+        for (size_t k = 0; k + 2 < ctri.size(); k += 3) {
+            for (int j = 0; j < 3; ++j) {
+                int idx = ctri[k + j];
+                double c = std::clamp(cl[idx], 0.0, 1.0);
+                double rain01 = std::clamp(rn[idx] / maxR, 0.0, 1.0);
+                unsigned char R = (unsigned char)(245 - 150 * rain01);   // white -> slate
+                unsigned char G = (unsigned char)(245 - 130 * rain01);
+                unsigned char B = (unsigned char)(250 - 95  * rain01);
+                unsigned char A = (unsigned char)(std::clamp(c, 0.0, 1.0) * 205.0);
+                const Vec3& u = planet.cells[idx].unit;
+                rlColor4ub(R, G, B, A);
+                rlVertex3f((float)(u.x * cr), (float)(u.y * cr), (float)(u.z * cr));
+            }
+        }
+        rlEnd();
+    }
     if (showGrat) drawGraticule3D(graticule, gratR);
     // Markers: selected (orange), hovered cell (yellow), hovered subcell (white).
     if (selectedCell >= 0) {
@@ -195,6 +220,7 @@ void Viewer::renderMap2D() {
     if (showDrift   && !driftArrows.empty()) drawSegments2D(driftArrows, Color{90, 230, 255, 255}, 2.0f, mapRect, mapLon);
     if (liveWorld && showTides && !coastCols.empty()) drawColoredSegments2D(coast, coastCols, 2.0f, mapRect, mapLon);
     if (showCurrents && !currentCols.empty()) drawColoredSegments2D(currentSegs, currentCols, 1.6f, mapRect, mapLon);
+    if (liveWorld && showClouds && !planet.cloud().empty()) drawWeather2D(planet, planet.cloud(), planet.rain(), map2D, mapRect, mapLon);
     if (phase3 && showRivers) {
         drawSegments2D(rivers,    Color{80, 170, 235, 255}, 1.5f, mapRect, mapLon);
         drawSegments2D(bigRivers, Color{80, 170, 235, 255}, 3.0f, mapRect, mapLon);
@@ -374,8 +400,8 @@ void Viewer::renderHUD() {
     y += 8;
     line("hover: cell info | click tile: open detail panel | C close");
     line("1 elev  2 plates  3 age  4 crust  5 biome  6 temp* 7 precip  8 flora  9 fauna  0 funga  (*6 cycles mean/summer/winter/season)");
-    line(TextFormat("B borders [%s] | D vectors [%s] | G grid [%s] | J rivers [%s] | N day/night [%s] | T tides [%s] | O currents [%s]",
+    line(TextFormat("B borders [%s] | D vectors [%s] | G grid [%s] | J rivers [%s] | N day/night [%s] | T tides [%s] | O currents [%s] | K clouds [%s]",
-                    showBorders ? "on" : "off", showDrift ? "on" : "off", showGrat ? "on" : "off", showRivers ? "on" : "off", dayNightOn ? "on" : "off", showTides ? "on" : "off", showCurrents ? "on" : "off"));
+                    showBorders ? "on" : "off", showDrift ? "on" : "off", showGrat ? "on" : "off", showRivers ? "on" : "off", dayNightOn ? "on" : "off", showTides ? "on" : "off", showCurrents ? "on" : "off", showClouds ? "on" : "off"));
     line(TextFormat("SPACE pause | [ / ] speed | S step | F fast-fwd | H hydrology [%s] | L biota [%s] | W live [%s] | R reseed | +/-",
                     phase3 ? "on" : "off", planet.biotaPopulated() ? "on" : "off", liveWorld ? "on" : "off"));
     line("F5 save | F9 load | F12 screenshot | F2 reload planet.cfg");

src/sim/Planet.cpp

@@ -51,6 +51,8 @@ void Planet::buildGeometry() {
     }
     sBiota.assign(cells.size(), {});   // empty biota population until generateBiota()
     sHasBiota = false;
+    sHumidity.clear(); sCloud.clear(); sRain.clear();   // weather spins up on entering Live World
+    sHasWeather = false;
 }
 
 void Planet::assignPlates() {

src/sim/Planet.hpp

@@ -89,6 +89,17 @@ public:
     void   computeOceanCurrents();
     const std::vector<Vec3>& current() const { return sCurrent; }
 
+    // Weather (Live World): dynamic per-cell humidity / cloud cover / rain advanced on the live
+    // clock. initWeather() spins the fields up from the climatology; stepWeather(dtHours) runs
+    // one cycle (evaporate over warm seas -> advect along the wind -> condense into cloud, with
+    // orographic lift -> rain out -> dissipate). Reads sInsolation/sTemp/sWind/sUpwind/sMoist
+    // (computeClimate + computeInsolation set those). Saved (v10).
+    void   initWeather();
+    void   stepWeather(double dtHours);
+    const std::vector<double>& humidity() const { return sHumidity; }
+    const std::vector<double>& cloud()    const { return sCloud; }
+    const std::vector<double>& rain()     const { return sRain; }
+
     // Phase 3 (biomes): classify every cell into a Biome from elevation + the climate
     // fields (temperature + normalized precipitation). Derived + written back into
     // cell.biome (saved). Assumes computeClimate() ran this tick. Re-run as terrain evolves.
@@ -124,8 +135,10 @@ public:
     // older saves (v3) pass false -- biomes are reclassified after the cells load.
     // hasBiota: whether the stream carries the biota population block (save v7+).
     // hasMoons: whether the stream carries the moons block (save v9+); older saves
-    // synthesize moons from the seed instead.
+    // synthesize moons from the seed instead. hasWeather: the weather block (save v10+);
-    bool readState(std::istream& is, bool hasBiome = true, bool hasBiota = true, bool hasMoons = true);
+    // older saves leave weather to spin up on entering Live World.
+    bool readState(std::istream& is, bool hasBiome = true, bool hasBiota = true,
+                   bool hasMoons = true, bool hasWeather = true);
 
     // Helpers for rendering / info.
     double cellWidthMeters() const;     // approx lateral cell spacing
@@ -199,6 +212,9 @@ private:
     // sTide is the equilibrium tidal height (m) from the moons + sun.
     std::vector<double>  sInsolation, sLiveTemp, sTide;
     std::vector<Vec3>    sCurrent;   // ocean surface current velocity (tangent; zero on land)
+    // Weather (Live World; saved v10). sHasWeather latches once spun up/loaded.
+    std::vector<double>  sHumidity, sCloud, sRain;
+    bool                 sHasWeather = false;
 
     // Biota: derived density scalars (0..1; recomputed each tick, not saved) and the
     // on-demand discrete population (saved). sHasBiota latches once generated/loaded.

src/sim/PlanetIO.cpp

@@ -36,6 +36,9 @@
     D(bioFungaTempMin) D(bioRegionBonus) \
     D(dayLengthHours) D(yearLengthDays) D(snowTemp) D(seaIceTemp) \
     D(tideAmplitude) D(tideSunFactor) \
+    D(weatherEvapRate) D(weatherWindKmh) D(weatherSatBase) D(weatherSatTempCoef) \
+    D(weatherCondense) D(weatherOrographic) D(weatherRainThresh) D(weatherRainRate) \
+    D(weatherCloudDissip) \
     I(subdivisions) I(plateCount) I(beltWidth) I(splitCheckEvery) I(stalemateWindows) \
     I(miniPlateCells) I(fuseMinPlates) I(babyMinCells) I(seaLevelEvery) \
     I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) \
@@ -191,6 +194,15 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(rng(cfg.seaIceTemp, -60.0, 20.0, "seaIceTemp"));
     E(rng(cfg.tideAmplitude, 0.0, 100.0, "tideAmplitude"));
     E(rng(cfg.tideSunFactor, 0.0, 5.0, "tideSunFactor"));
+    E(rng(cfg.weatherEvapRate, 0.0, 50.0, "weatherEvapRate"));
+    E(rng(cfg.weatherWindKmh, 0.0, 1000.0, "weatherWindKmh"));
+    E(rng(cfg.weatherSatBase, 0.01, 5.0, "weatherSatBase"));
+    E(rng(cfg.weatherSatTempCoef, 0.0, 1.0, "weatherSatTempCoef"));
+    E(rng(cfg.weatherCondense, 0.0, 50.0, "weatherCondense"));
+    E(rng(cfg.weatherOrographic, 0.0, 1.0, "weatherOrographic"));
+    E(rng(cfg.weatherRainThresh, 0.0, 1.5, "weatherRainThresh"));
+    E(rng(cfg.weatherRainRate, 0.0, 50.0, "weatherRainRate"));
+    E(rng(cfg.weatherCloudDissip, 0.0, 50.0, "weatherCloudDissip"));
     E(irng(cfg.subdivisions, 0, 7, "subdivisions"));
     E(irng(cfg.plateCount, 1, 100, "plateCount"));
     E(irng(cfg.beltWidth, 1, 12, "beltWidth"));
@@ -279,9 +291,12 @@ void Planet::writeState(std::ostream& os) const {
             writeVec(os, cb.flora); writeVec(os, cb.fauna); writeVec(os, cb.funga);
         }
     }
+    // v10: Live World weather (humidity/cloud/rain). Flag-gated like biota.
+    uint8_t hasWx = (sHasWeather && sHumidity.size() == cells.size()) ? 1 : 0; writePod(os, hasWx);
+    if (hasWx) { writeVec(os, sHumidity); writeVec(os, sCloud); writeVec(os, sRain); }
 }
 
-bool Planet::readState(std::istream& is, bool hasBiome, bool hasBiota, bool hasMoons) {
+bool Planet::readState(std::istream& is, bool hasBiome, bool hasBiota, bool hasMoons, bool hasWeather) {
     // Read the length-prefixed key=value config block (see writeState). A default
     // PlanetConfig is parsed over, so fields absent from an older save keep their
     // current defaults. The length guard rejects pre-v6 (raw-POD-config) saves.
@@ -327,6 +342,16 @@ bool Planet::readState(std::istream& is, bool hasBiome, bool hasBiota, bool hasM
             }
         }
     }
+    // v10: Live World weather. Older saves leave it to spin up on entering Live World.
+    sHasWeather = false; sHumidity.clear(); sCloud.clear(); sRain.clear();
+    if (hasWeather) {
+        uint8_t hasWx = 0; readPod(is, hasWx);
+        if (hasWx) {
+            readVec(is, sHumidity); readVec(is, sCloud); readVec(is, sRain);
+            if (!is || (int)sHumidity.size() != (int)cells.size()) return false;
+            sHasWeather = true;
+        }
+    }
     computeBiotaDensity();                  // derived density scalars for the colour views
     return (bool)is;
 }

src/sim/PlanetTypes.hpp

@@ -257,4 +257,18 @@ struct PlanetConfig {
     double seaIceTemp     = -2.0;    // C: ocean below the live temperature shows sea ice
     double tideAmplitude  = 0.6;     // m: equilibrium-tide scale per unit tide-raising weight
     double tideSunFactor  = 0.46;    // sun's tide weight relative to a unit moon (Earth ~0.46)
+
+    // --- Weather (Live World dynamic clouds & rain) -- see PlanetWeather.cpp -----
+    // A per-cell humidity/cloud/rain cycle advanced on the live clock: evaporate over warm
+    // sunlit seas, advect along the prevailing wind, condense into cloud (more on windward
+    // upslopes), rain out, and dissipate. Rates are per simulated hour.
+    double weatherEvapRate    = 0.4;    // /h: ocean evaporation toward marine saturation
+    double weatherWindKmh     = 45.0;   // km/h: prevailing wind speed for advecting humidity/cloud
+    double weatherSatBase     = 0.4;    // air saturation humidity at 0 C (warmer air holds more)
+    double weatherSatTempCoef = 0.025;  // saturation rise per +1 C
+    double weatherCondense    = 0.6;    // /h: fraction of supersaturation that becomes cloud
+    double weatherOrographic  = 0.0009; // extra condensation per m of windward upslope
+    double weatherRainThresh  = 0.5;    // cloud cover above this precipitates
+    double weatherRainRate    = 0.5;    // /h: rain rate from excess cloud
+    double weatherCloudDissip = 0.12;   // /h: cloud clearing (half returns to humidity)
 };

src/sim/PlanetWeather.cpp

@@ -0,0 +1,96 @@
+#include "Planet.hpp"
+#include <algorithm>
+#include <cmath>
+#include <vector>
+
+// Live World weather: a dynamic per-cell humidity / cloud / rain cycle advanced on the live
+// clock (geometry fixed -- these are fields flowed over the grid, like climate, but time-varying).
+// One step: evaporate over warm sunlit seas -> advect humidity & cloud along the prevailing wind
+// -> condense the supersaturated air into cloud (extra on windward upslopes) -> rain out the
+// thick cloud -> dissipate. Reads the static climate scaffolding (sTemp/sWind/sUpwind/sMoist set
+// by computeClimate) and the live sInsolation (computeInsolation). Deterministic; saved (v10).
+
+void Planet::initWeather() {
+    const int n = (int)cells.size();
+    sHumidity.assign(n, 0.0);
+    sCloud.assign(n, 0.0);
+    sRain.assign(n, 0.0);
+    const double sea = cfg.seaLevel;
+    const bool haveM = ((int)sMoist.size() == n);
+    for (int i = 0; i < n; ++i) {
+        if (cells[i].elevation <= sea) sHumidity[i] = 0.9;                       // saturated marine air
+        else sHumidity[i] = haveM ? (0.2 + 0.5 * sMoist[i]) : 0.3;              // land: from climatology
+    }
+    sHasWeather = true;
+}
+
+void Planet::stepWeather(double dtHours) {
+    const int n = (int)cells.size();
+    if (!sHasWeather || (int)sHumidity.size() != n || (int)sCloud.size() != n || (int)sRain.size() != n)
+        initWeather();
+    if (dtHours <= 0.0) return;                 // paused: hold the current sky
+    if ((int)sTemp.size() != n) return;         // need the climate fields
+
+    const double sea = cfg.seaLevel;
+    auto isOcean = [&](int i) { return cells[i].elevation <= sea; };
+    const double cw = std::max(1.0, cellWidthMeters());
+    double advFrac = std::clamp(cfg.weatherWindKmh * 1000.0 * dtHours / cw, 0.0, 1.0);
+    const bool haveSun = ((int)sInsolation.size() == n);
+    const bool haveUp  = ((int)sUpwind.size() == n);
+
+    // 1. Advect humidity downwind (upwind differencing) + evaporate over warm sunlit ocean.
+    std::vector<double> nh(n);
+    for (int i = 0; i < n; ++i) {
+        double hUp = (haveUp && sUpwind[i] >= 0) ? sHumidity[sUpwind[i]] : sHumidity[i];
+        double h = sHumidity[i] * (1.0 - advFrac) + hUp * advFrac;
+        if (isOcean(i)) {
+            double tf  = std::clamp((sTemp[i] + 2.0) / 30.0, 0.0, 1.0);          // warm seas evaporate more
+            double sun = haveSun ? (0.5 + 0.5 * sInsolation[i]) : 0.7;           // daytime boost
+            double target = 0.55 + 0.45 * tf;                                    // marine humidity target
+            double rate = 1.0 - std::exp(-cfg.weatherEvapRate * sun * dtHours);
+            if (target > h) h += (target - h) * rate;
+        }
+        nh[i] = h;
+    }
+    sHumidity.swap(nh);
+
+    // 2. Advect cloud (it drifts with the wind too).
+    std::vector<double> nc(n);
+    for (int i = 0; i < n; ++i) {
+        double cUp = (haveUp && sUpwind[i] >= 0) ? sCloud[sUpwind[i]] : sCloud[i];
+        nc[i] = sCloud[i] * (1.0 - advFrac) + cUp * advFrac;
+    }
+    sCloud.swap(nc);
+
+    // 3. Condense (saturation + orographic lift) -> rain -> dissipate, per cell.
+    const double condR = 1.0 - std::exp(-cfg.weatherCondense * dtHours);
+    const double rainR = 1.0 - std::exp(-cfg.weatherRainRate * dtHours);
+    const double dissR = 1.0 - std::exp(-cfg.weatherCloudDissip * dtHours);
+    const double invDt = 1.0 / dtHours;
+    for (int i = 0; i < n; ++i) {
+        double sat = std::max(0.05, cfg.weatherSatBase + cfg.weatherSatTempCoef * std::max(0.0, sTemp[i]));
+        double cond = 0.0;
+        double excess = sHumidity[i] - sat;
+        if (excess > 0.0) cond += excess * condR;                               // convective/thermal
+        if (haveUp && sUpwind[i] >= 0) {                                        // orographic (windward)
+            double up = cells[i].elevation - cells[sUpwind[i]].elevation;
+            if (up > 0.0) cond += sHumidity[i] * std::min(1.0, up * cfg.weatherOrographic) * condR;
+        }
+        cond = std::min(cond, sHumidity[i]);
+        sHumidity[i] -= cond;
+        sCloud[i]    += cond;
+
+        double rain = 0.0;
+        if (sCloud[i] > cfg.weatherRainThresh) {
+            rain = (sCloud[i] - cfg.weatherRainThresh) * rainR;
+            sCloud[i] -= rain;
+        }
+        double diss = sCloud[i] * dissR;
+        sCloud[i]    -= diss;
+        sHumidity[i] += diss * 0.5;                                             // half re-evaporates
+
+        sRain[i] = rain * invDt;                                                // intensity (per hour)
+        if (sHumidity[i] < 0.0) sHumidity[i] = 0.0;
+        sCloud[i] = std::clamp(sCloud[i], 0.0, 1.5);
+    }
+}

test_weather.cpp

@@ -0,0 +1,92 @@
+// Headless test for the Live World weather cycle (humidity / cloud / rain). No display needed.
+//
+//   g++ -std=c++17 -O2 -Isrc/sim test_weather.cpp src/sim/IcoSphere.cpp src/sim/Planet.cpp \
+//       src/sim/PlanetTectonics.cpp src/sim/PlanetDrift.cpp src/sim/PlanetErosion.cpp \
+//       src/sim/PlanetHydrology.cpp src/sim/PlanetBiomes.cpp src/sim/PlanetClimate.cpp \
+//       src/sim/PlanetLive.cpp src/sim/PlanetOcean.cpp src/sim/PlanetWeather.cpp \
+//       src/sim/PlanetBiota.cpp src/sim/PlanetFloraGen.cpp src/sim/PlanetFaunaGen.cpp \
+//       src/sim/PlanetFungiGen.cpp src/sim/PlanetIO.cpp -o /tmp/tw && /tmp/tw
+//
+// Verifies: fields stay in range; oceans (the evaporation source) end up moister than land;
+// clouds form and rain falls somewhere; the cycle is deterministic; and save v10 round-trips it.
+
+#include "Planet.hpp"
+#include <cstdio>
+#include <cmath>
+#include <algorithm>
+#include <sstream>
+
+static int failures = 0;
+static void check(bool cond, const char* what) {
+    std::printf("  [%s] %s\n", cond ? "PASS" : "FAIL", what);
+    if (!cond) ++failures;
+}
+
+// Run a fixed weather sequence on a planet (returns whether rain ever fell, max cloud seen).
+static void runWeather(Planet& p, bool& everRained, double& maxCloud) {
+    p.initWeather();
+    everRained = false; maxCloud = 0.0;
+    for (int k = 0; k < 200; ++k) {
+        p.computeInsolation(0.25, std::fmod(0.3 + 0.01 * k, 1.0));   // sun advances
+        p.stepWeather(1.0);                                         // 1-hour steps
+        const std::vector<double>& rn = p.rain();
+        const std::vector<double>& cl = p.cloud();
+        for (size_t i = 0; i < rn.size(); ++i) {
+            if (rn[i] > 0.0) everRained = true;
+            maxCloud = std::max(maxCloud, cl[i]);
+        }
+    }
+}
+
+int main() {
+    PlanetConfig cfg; cfg.subdivisions = 5; cfg.seed = 1337;
+    Planet p; p.generate(cfg);
+    const int n = (int)p.cells.size();
+
+    std::printf("Weather: cycle\n");
+    bool rained = false; double maxCloud = 0.0;
+    runWeather(p, rained, maxCloud);
+
+    bool inRange = true;
+    for (int i = 0; i < n; ++i) {
+        if (p.humidity()[i] < -1e-9) inRange = false;
+        if (p.cloud()[i] < -1e-9 || p.cloud()[i] > 1.5 + 1e-9) inRange = false;
+        if (p.rain()[i] < -1e-9) inRange = false;
+    }
+    check(inRange, "humidity/cloud/rain stay in range");
+    check(maxCloud > 0.05, "clouds form");
+    check(rained, "rain falls somewhere");
+
+    // Oceans are the moisture source -> moister than land on average.
+    double oh = 0, lh = 0; int oc = 0, lc = 0;
+    for (int i = 0; i < n; ++i) {
+        if (p.cells[i].elevation <= cfg.seaLevel) { oh += p.humidity()[i]; ++oc; }
+        else { lh += p.humidity()[i]; ++lc; }
+    }
+    oh /= std::max(1, oc); lh /= std::max(1, lc);
+    std::printf("    mean humidity: ocean %.3f, land %.3f\n", oh, lh);
+    check(oh > lh, "oceans end up moister than land");
+
+    std::printf("Weather: determinism\n");
+    Planet p2; p2.generate(cfg);
+    bool r2; double mc2; runWeather(p2, r2, mc2);
+    bool same = true;
+    for (int i = 0; i < n; ++i)
+        if (p2.humidity()[i] != p.humidity()[i] || p2.cloud()[i] != p.cloud()[i]
+            || p2.rain()[i] != p.rain()[i]) same = false;
+    check(same, "same seed + sequence -> identical weather");
+
+    std::printf("Weather: save v10\n");
+    std::stringstream ss(std::ios::in | std::ios::out | std::ios::binary);
+    p.writeState(ss);
+    Planet q;
+    bool ok = q.readState(ss, true, true, true, true);
+    bool rt = ok && (int)q.cloud().size() == n;
+    for (int i = 0; i < n && rt; ++i)
+        if (q.humidity()[i] != p.humidity()[i] || q.cloud()[i] != p.cloud()[i] || q.rain()[i] != p.rain()[i])
+            rt = false;
+    check(rt, "save v10 round-trips the weather state");
+
+    std::printf(failures ? "\nSOME WEATHER CHECKS FAILED (%d)\n" : "\nALL WEATHER CHECKS PASSED\n", failures);
+    return failures ? 1 : 0;
+}