Live World weather: moving systems (lows, hurricanes & typhoons)

The base cloud/rain field relaxes to a static pattern under fixed forcing, so it looked
frozen. stepWeather now also runs a population of drifting WeatherSystem agents (world
objects, not cells; transient/not saved; separate sWeatherRng seeded from cfg.seed so
tectonic determinism is intact): spawn over warm tropical ocean (5-25 deg, SST gate) or a
mid-latitude ocean low (capped at weatherSystemMax); move along the steering wind (sWind at
the nearest cell) plus a poleward recurve at weatherSystemSpeed; intensify over warm sea,
decay and cull over land/cold; and stamp a Gaussian cloud/rain shield onto the grid so cloud
clusters travel and dissipate behind them. A tropical system past weatherHurricaneStr is a
hurricane/typhoon.

Render: an animated cyclonic spiral marker per system (red + eye for cyclones, blue lows;
spins with liveTime by hemisphere) in 3D + 2D, HUD system/cyclone counts, and a basin-named
storm list in the Sky & tides panel -- all under K. New weather* storm knobs. test_weather.cpp
adds: systems spawn, move between steps, thicken cloud, RNG isolation, determinism. All five
suites pass; GUI build clean. Docs updated.

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

BUILD.md

@@ -215,6 +215,14 @@ saved v10. Evaporate over warm seas -> advect along the wind -> condense -> rain
     weatherRainThresh   0.5    cloud cover above this rains
     weatherRainRate     0.5    /h rain rate from excess cloud
     weatherCloudDissip  0.12   /h cloud clearing
+    weatherSystemMax    8      max concurrent moving weather systems
+    weatherSpawnRate    0.06   /h genesis probability scale
+    weatherSystemSpeed  28     km/h drift speed of systems
+    weatherTropicalSST  26 C   min sea-surface temp for tropical genesis
+    weatherSystemRadius 0.16   rad angular radius of a system's cloud/rain shield
+    weatherSystemCloud  1.2    /h cloud stamped at a system core
+    weatherSystemRain   1.6    /h rain at a system core
+    weatherHurricaneStr 0.6    strength above which a tropical system is a hurricane/typhoon
 
 ## Headless logic test (no display)
 

CLAUDE.md

@@ -88,7 +88,13 @@ the fixed-grid Eulerian model + the climate fields are the groundwork for it.
   **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.
+- **Weather — moving systems (lows, hurricanes & typhoons)** *(done — see `PlanetWeather.cpp`)* —
+  drifting low-pressure **agents** (`WeatherSystem`) spawn over warm tropical seas / mid-latitude
+  oceans, travel with the steering wind (poleward recurve), intensify over warm water, decay over
+  land, and **stamp** travelling cloud/rain onto the grid — so the sky visibly evolves. The intense
+  tropical ones are **hurricanes/typhoons** (spin by hemisphere, eye + animated spiral marker).
+  Transient (not saved; respawn from the seed). This makes the weather visibly move (the base field
+  alone relaxes to a static pattern).
 
 ## Current state
 
@@ -392,6 +398,20 @@ Working and verified (logic tested headless):
   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.
+- **Live World — moving weather systems (lows / hurricanes / typhoons):** the base cloud/rain
+  field relaxes to a *static* pattern under fixed forcing, so `stepWeather` now also runs a
+  population of drifting **`WeatherSystem`** agents (PlanetTypes; transient, not saved; separate
+  `sWeatherRng` seeded from `cfg.seed` → tectonic determinism intact). Each step: **spawn** over
+  warm tropical ocean (5–25°, SST ≥ `weatherTropicalSST`) or a mid-latitude (30–62°) ocean low
+  (capped at `weatherSystemMax`, prob ∝ `weatherSpawnRate`); **move** along the steering wind
+  (`sWind` at the nearest cell) + a poleward recurve at `weatherSystemSpeed`; **intensify** over
+  warm sea / **decay+cull** over land/cold; **stamp** a Gaussian cloud/rain shield
+  (`weatherSystemCloud`/`Rain`, scaled by strength × local humidity) — so cloud clusters travel
+  and dissipate behind the system. A tropical system past `weatherHurricaneStr` is a
+  hurricane/typhoon. Render: an animated cyclonic **spiral marker** per system (red + eye for
+  cyclones, blue lows; spins with `liveTime`·hemisphere) in 3D + 2D, HUD system/cyclone counts,
+  and a storm list (basin-named) in the Sky & tides panel — all under `K`. `test_weather.cpp`
+  adds: systems spawn, move between steps, thicken cloud, RNG isolation, determinism.
 - 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
@@ -635,6 +655,11 @@ triangles (plates are fixed in phase 1).
   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.
+- **Weather systems (`weather*` storm knobs, `planet.cfg`):** `weatherSystemMax` (concurrent
+  cap), `weatherSpawnRate` (genesis frequency), `weatherSystemSpeed` (km/h drift),
+  `weatherTropicalSST` (min SST for tropical genesis), `weatherSystemRadius` (cloud-shield size),
+  `weatherSystemCloud`/`weatherSystemRain` (stamp strength), `weatherHurricaneStr` (strength to
+  count as a hurricane/typhoon). Markers show under `K`. Tune these for a stormier or calmer world.
 - 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`

docs/design-notes.md

@@ -190,8 +190,17 @@ frame from `Viewer::stepSim` with dt = the sim-hours added to `liveTime` (0 when
 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
+flag-gated; pre-v10 saves spin weather up on entering Live World).
-(moving vortex agents) layered on this field.
+
+**Moving weather systems** (same `stepWeather`): the base field above relaxes to a *static*
+pattern under fixed forcing, so a population of drifting `WeatherSystem` **agents** (world objects,
+not cells — like moons; transient/not saved; separate `sWeatherRng` seeded from `cfg.seed`)
+provides the motion. Each step they **spawn** over warm tropical ocean (5–25°) or a mid-latitude
+(30–62°) ocean low, **move** along the steering wind (`sWind` at the nearest cell) + a poleward
+recurve (`weatherSystemSpeed`), **intensify** over warm sea / **decay+cull** over land/cold, and
+**stamp** a Gaussian cloud/rain shield onto the grid — so cloud clusters travel and dissipate
+behind them. Tropical systems past `weatherHurricaneStr` are hurricanes/typhoons; rendered as
+animated cyclonic spiral markers (eye for cyclones) spinning by hemisphere, in 3D + 2D, under `K`.
 
 ## Headless testing
 

src/render/ViewerRender.cpp

@@ -128,6 +128,37 @@ void Viewer::renderGlobe3D() {
         }
         rlEnd();
     }
+    // Live World storm markers: an animated cyclonic spiral per weather system (hurricanes red
+    // with an eye; lows blue), spinning with the live clock by the system's hemisphere sense.
+    if (liveWorld && showClouds && !planet.storms().empty()) {
+        const float SR = visBase + 0.05f;
+        for (const auto& ws : planet.storms()) {
+            Vec3 p{ ws.pos.x, ws.pos.y, ws.pos.z };
+            Vec3 u = p.cross(Vec3{0, 1, 0}); if (u.length() < 1e-6) u = p.cross(Vec3{1, 0, 0});
+            u = u.normalized(); Vec3 v = p.cross(u).normalized();
+            bool hur = ws.tropical && ws.strength >= planet.cfg.weatherHurricaneStr;
+            unsigned char cR = hur ? 240 : 150, cG = hur ? 60 : 200, cB = hur ? 60 : 235;
+            unsigned char A = (unsigned char)(110 + 140 * std::clamp(ws.strength, 0.0, 1.0));
+            double rmax = 0.04 + 0.10 * ws.strength;
+            double phase = liveTime * ws.spin * 0.4;
+            rlSetLineWidth(2.0f); rlBegin(RL_LINES); rlColor4ub(cR, cG, cB, A);
+            const int N = 36; const double turns = 2.2;
+            for (int arm = 0; arm < 2; ++arm) {
+                double a0 = phase + arm * M_PI; Vec3 prev{};
+                for (int k = 0; k <= N; ++k) {
+                    double t = (double)k / N;
+                    double a = a0 + t * turns * 2.0 * M_PI * ws.spin;
+                    Vec3 dir = u * std::cos(a) + v * std::sin(a);
+                    Vec3 wp = (p + dir * (rmax * t)).normalized() * (double)SR;
+                    if (k > 0) { rlVertex3f((float)prev.x, (float)prev.y, (float)prev.z);
+                                 rlVertex3f((float)wp.x, (float)wp.y, (float)wp.z); }
+                    prev = wp;
+                }
+            }
+            rlEnd(); rlSetLineWidth(1.0f);
+            if (hur) { Vec3 e = p * (double)SR; DrawSphere(Vector3{(float)e.x,(float)e.y,(float)e.z}, 0.02f, Color{255,240,200,255}); }
+        }
+    }
     if (showGrat) drawGraticule3D(graticule, gratR);
     // Markers: selected (orange), hovered cell (yellow), hovered subcell (white).
     if (selectedCell >= 0) {
@@ -221,6 +252,18 @@ void Viewer::renderMap2D() {
     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 (liveWorld && showClouds && !planet.storms().empty()) {
+        for (const auto& ws : planet.storms()) {
+            double lon, lat; dirToLonLat(Vec3{ws.pos.x, ws.pos.y, ws.pos.z}, lon, lat);
+            Vector2 sp = projLonLat(lon, lat, mapLon, mapRect);
+            bool hur = ws.tropical && ws.strength >= planet.cfg.weatherHurricaneStr;
+            Color c = hur ? Color{240, 60, 60, 255} : Color{150, 200, 235, 255};
+            float rad = 5.0f + 10.0f * (float)ws.strength;
+            DrawCircleLines((int)sp.x, (int)sp.y, rad, c);
+            if (hur) DrawCircleLines((int)sp.x, (int)sp.y, rad * 0.55f, c);
+            DrawCircleV(sp, 2.0f, c);
+        }
+    }
     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);
@@ -330,6 +373,24 @@ void Viewer::renderLiveInfo() {
     } else {
         DrawText("click a coastal tile", x, y, 15, Color{150, 155, 170, 255});
     }
+
+    // Active weather systems (lows / tropical cyclones), named by basin.
+    y += 12;
+    DrawText("Weather systems", x, y, 18, Color{200, 205, 220, 255}); y += 26;
+    const auto& storms = planet.storms();
+    if (storms.empty()) DrawText("(calm — none active)", x, y, 15, Color{150, 155, 170, 255});
+    int shown = 0;
+    for (const auto& ws : storms) {
+        if (shown >= 6 || y > (int)(r.y + r.height) - 22) break;
+        double lon, lat; dirToLonLat(Vec3{ws.pos.x, ws.pos.y, ws.pos.z}, lon, lat);
+        bool hur = ws.tropical && ws.strength >= planet.cfg.weatherHurricaneStr;
+        const char* kind = hur ? (lon > -0.5 && lon < 2.4 ? "Typhoon" : "Hurricane")  // W Pacific vs rest
+                         : ws.tropical ? "Tropical low" : "Low";
+        Color c = hur ? Color{240, 90, 80, 255} : Color{170, 200, 230, 255};
+        DrawText(TextFormat("%s  %.0f%%  @ %+.0f,%+.0f", kind, ws.strength * 100.0,
+                            lat * 180.0 / M_PI, lon * 180.0 / M_PI), x, y, 15, c);
+        y += 21; ++shown;
+    }
 }
 
 // Right column: hover/selection info (top) + detail panel or world stats (bottom).
@@ -382,6 +443,9 @@ void Viewer::renderHUD() {
         else                         { rl = "h/s";  rv = liveRate; }
         line(TextFormat("rate %.1f %s   day/night %s   ([ / ] speed, N toggle, W exit)",
                         rv, rl, dayNightOn ? "on" : "off"));
+        int nStorm = 0, nHur = 0;
+        for (const auto& ws : planet.storms()) { ++nStorm; if (ws.tropical && ws.strength >= planet.cfg.weatherHurricaneStr) ++nHur; }
+        line(TextFormat("weather systems: %d   tropical cyclones: %d", nStorm, nHur));
     }
     else {
         line(TextFormat("%s  %.1f My elapsed   %.1f My/s%s",

src/sim/Planet.cpp

@@ -52,7 +52,7 @@ 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;
+    sHasWeather = false; sStorms.clear();
 }
 
 void Planet::assignPlates() {

src/sim/Planet.hpp

@@ -99,6 +99,7 @@ public:
     const std::vector<double>& humidity() const { return sHumidity; }
     const std::vector<double>& cloud()    const { return sCloud; }
     const std::vector<double>& rain()     const { return sRain; }
+    const std::vector<WeatherSystem>& storms() const { return sStorms; }
 
     // Phase 3 (biomes): classify every cell into a Biome from elevation + the climate
     // fields (temperature + normalized precipitation). Derived + written back into
@@ -215,6 +216,10 @@ private:
     // Weather (Live World; saved v10). sHasWeather latches once spun up/loaded.
     std::vector<double>  sHumidity, sCloud, sRain;
     bool                 sHasWeather = false;
+    // Moving weather systems (transient agents; not saved). Separate RNG keeps tectonic
+    // determinism intact (seeded from cfg.seed in initWeather).
+    std::vector<WeatherSystem> sStorms;
+    uint32_t                   sWeatherRng = 1;
 
     // 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

@@ -39,9 +39,11 @@
     D(weatherEvapRate) D(weatherWindKmh) D(weatherSatBase) D(weatherSatTempCoef) \
     D(weatherCondense) D(weatherOrographic) D(weatherRainThresh) D(weatherRainRate) \
     D(weatherCloudDissip) \
+    D(weatherSpawnRate) D(weatherSystemSpeed) D(weatherTropicalSST) D(weatherSystemRadius) \
+    D(weatherSystemCloud) D(weatherSystemRain) D(weatherHurricaneStr) \
     I(subdivisions) I(plateCount) I(beltWidth) I(splitCheckEvery) I(stalemateWindows) \
     I(miniPlateCells) I(fuseMinPlates) I(babyMinCells) I(seaLevelEvery) \
-    I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) \
+    I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) I(weatherSystemMax) \
     I(bioFloraSlots) I(bioFaunaSlots) I(bioFungaSlots) \
     I(bioFloraPoints) I(bioFaunaPoints) I(bioFungaPoints) \
     U(seed)
@@ -203,6 +205,13 @@ std::string validateConfig(const PlanetConfig& cfg) {
     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(rng(cfg.weatherSpawnRate, 0.0, 10.0, "weatherSpawnRate"));
+    E(rng(cfg.weatherSystemSpeed, 0.0, 500.0, "weatherSystemSpeed"));
+    E(rng(cfg.weatherTropicalSST, -10.0, 40.0, "weatherTropicalSST"));
+    E(rng(cfg.weatherSystemRadius, 0.01, 1.5, "weatherSystemRadius"));
+    E(rng(cfg.weatherSystemCloud, 0.0, 20.0, "weatherSystemCloud"));
+    E(rng(cfg.weatherSystemRain, 0.0, 20.0, "weatherSystemRain"));
+    E(rng(cfg.weatherHurricaneStr, 0.0, 1.0, "weatherHurricaneStr"));
     E(irng(cfg.subdivisions, 0, 7, "subdivisions"));
     E(irng(cfg.plateCount, 1, 100, "plateCount"));
     E(irng(cfg.beltWidth, 1, 12, "beltWidth"));
@@ -215,6 +224,7 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(irng(cfg.climateWindPasses, 1, 1000, "climateWindPasses"));
     E(irng(cfg.climateMoistureSmooth, 0, 100, "climateMoistureSmooth"));
     E(irng(cfg.seasonContinentRings, 1, 100, "seasonContinentRings"));
+    E(irng(cfg.weatherSystemMax, 0, 1000, "weatherSystemMax"));
     E(irng(cfg.bioFloraSlots, 1, 1000, "bioFloraSlots"));
     E(irng(cfg.bioFaunaSlots, 1, 1000, "bioFaunaSlots"));
     E(irng(cfg.bioFungaSlots, 1, 1000, "bioFungaSlots"));
@@ -342,8 +352,10 @@ 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.
+    // v10: Live World weather. Older saves leave it to spin up on entering Live World. The moving
+    // weather systems are transient (not saved): clear them + reseed the weather RNG from the seed.
     sHasWeather = false; sHumidity.clear(); sCloud.clear(); sRain.clear();
+    sStorms.clear(); sWeatherRng = cfg.seed ? (cfg.seed ^ 0x5701A123u) : 0x5701A123u;
     if (hasWeather) {
         uint8_t hasWx = 0; readPod(is, hasWx);
         if (hasWx) {

src/sim/PlanetTypes.hpp

@@ -25,6 +25,20 @@ struct SubGrid {
 
 enum class PlateType { Oceanic, Continental };
 
+// A moving weather system (Live World): a drifting low-pressure disturbance that travels with the
+// steering wind and stamps clouds & rain onto the weather fields. Geometry is fixed, so this is a
+// world-object agent (a point on the sphere, like a moon), not a cell. The intense tropical ones
+// (strength past weatherHurricaneStrength) are hurricanes/typhoons. Transient -- not saved.
+struct WeatherSystem {
+    Vec3   pos;              // unit position on the sphere
+    double strength = 0.0;   // intensity 0..1 (drives cloud/rain boost + marker size)
+    double radius   = 0.15;  // angular radius (radians)
+    double age      = 0.0;   // hours alive
+    double life     = 120.0; // total lifetime (hours)
+    double spin     = 1.0;   // cyclonic sense: +1 CCW (N hemisphere) / -1 CW (S)
+    bool   tropical = false; // warm-core tropical (can become a cyclone) vs extratropical low
+};
+
 // Phase-3 (climate & biomes) classification of a cell, derived from elevation,
 // latitude (temperature) and hydrology/coast (moisture). Stored per cell (uint8,
 // serialized) so Phase-4 civilization can read it. Keep Ocean == 0 so a default-
@@ -271,4 +285,16 @@ struct PlanetConfig {
     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)
+
+    // --- Weather systems (moving lows / hurricanes / typhoons) -- PlanetWeather.cpp ---
+    // Drifting low-pressure disturbances travel with the steering wind and stamp cloud/rain onto
+    // the grid, so the sky visibly evolves; the intense tropical ones become tropical cyclones.
+    int    weatherSystemMax   = 8;      // max concurrent weather systems
+    double weatherSpawnRate   = 0.06;   // /h: genesis probability scale (when below the cap)
+    double weatherSystemSpeed = 28.0;   // km/h: steering speed at which systems drift
+    double weatherTropicalSST = 26.0;   // C: min sea-surface temp for tropical genesis
+    double weatherSystemRadius= 0.16;   // rad: angular radius of a system's cloud/rain shield
+    double weatherSystemCloud = 1.2;    // /h: cloud stamped at a system's core (scaled by strength)
+    double weatherSystemRain  = 1.6;    // /h: rain intensity at a system's core
+    double weatherHurricaneStr= 0.6;    // strength above which a tropical system is a hurricane/typhoon
 };

src/sim/PlanetWeather.cpp

@@ -21,6 +21,8 @@ void Planet::initWeather() {
         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
     }
+    sStorms.clear();
+    sWeatherRng = cfg.seed ? (cfg.seed ^ 0x5701A123u) : 0x5701A123u;            // separate RNG
     sHasWeather = true;
 }
 
@@ -93,4 +95,92 @@ void Planet::stepWeather(double dtHours) {
         if (sHumidity[i] < 0.0) sHumidity[i] = 0.0;
         sCloud[i] = std::clamp(sCloud[i], 0.0, 1.5);
     }
+
+    // 4. Moving weather systems (lows / hurricanes / typhoons). Drifting agents that travel with
+    //    the steering wind and stamp cloud/rain onto the grid, so the sky visibly evolves.
+    const bool haveWind = ((int)sWind.size() == n);
+    auto wrnd = [&]() { uint32_t x = sWeatherRng; x ^= x << 13; x ^= x >> 17; x ^= x << 5; sWeatherRng = x; return x; };
+    auto wrf  = [&]() { return (wrnd() & 0xFFFFFFu) / double(0x1000000); };
+    const Vec3 worldUp{0, 1, 0};
+    const double D2R = M_PI / 180.0;
+
+    // 4a. Genesis: over warm tropical ocean (5..25 deg) or a mid-latitude (30..62 deg) ocean low.
+    if ((int)sStorms.size() < cfg.weatherSystemMax) {
+        double pSpawn = 1.0 - std::exp(-cfg.weatherSpawnRate * dtHours);
+        if (wrf() < pSpawn) {
+            int bestIdx = -1; double bestScore = 0.0; bool bestTrop = false;
+            for (int t = 0; t < 8; ++t) {
+                int ci = (int)(wrnd() % (uint32_t)n);
+                if (cells[ci].elevation > sea) continue;
+                double absdeg = std::fabs(std::asin(std::clamp(cells[ci].unit.y, -1.0, 1.0))) / D2R;
+                double score = 0.0; bool trop = false;
+                if (absdeg > 5.0 && absdeg < 25.0 && sTemp[ci] >= cfg.weatherTropicalSST) { score = 0.6 + 0.4 * wrf(); trop = true; }
+                else if (absdeg >= 30.0 && absdeg <= 62.0) { score = 0.3 + 0.3 * wrf(); }
+                if (score > bestScore) { bestScore = score; bestIdx = ci; bestTrop = trop; }
+            }
+            if (bestIdx >= 0) {
+                WeatherSystem ws;
+                ws.pos = cells[bestIdx].unit;
+                ws.strength = 0.15;
+                ws.radius = cfg.weatherSystemRadius * (bestTrop ? 0.8 : 1.25);
+                ws.life = bestTrop ? (120.0 + 180.0 * wrf()) : (60.0 + 90.0 * wrf());
+                ws.spin = (cells[bestIdx].unit.y >= 0.0) ? 1.0 : -1.0;
+                ws.tropical = bestTrop;
+                sStorms.push_back(ws);
+            }
+        }
+    }
+
+    // 4b. Move, intensify and cull each system.
+    for (size_t s = 0; s < sStorms.size(); ) {
+        WeatherSystem& ws = sStorms[s];
+        int nc = 0; double nd = -2.0;                          // nearest cell to the system
+        for (int i = 0; i < n; ++i) { double d = cells[i].unit.dot(ws.pos); if (d > nd) { nd = d; nc = i; } }
+        const Vec3& nrm = ws.pos;
+        Vec3 steer = (haveWind && sWind[nc].length() > 1e-9) ? sWind[nc].normalized() : Vec3{0, 0, 0};
+        Vec3 northT = worldUp - nrm * worldUp.dot(nrm); double nl = northT.length();
+        if (nl > 1e-9) northT = northT * (1.0 / nl);
+        double poleSign = (nrm.y >= 0.0) ? 1.0 : -1.0;
+        Vec3 vel = steer + northT * (poleSign * 0.35);         // steering + poleward recurve
+        vel = vel - nrm * vel.dot(nrm);                        // keep tangent
+        double vl = vel.length();
+        if (vl > 1e-9) {
+            double dAng = cfg.weatherSystemSpeed * 1000.0 * dtHours / std::max(1.0, cfg.radius);
+            Vec3 vdir = vel * (1.0 / vl);
+            ws.pos = (nrm * std::cos(dAng) + vdir * std::sin(dAng)).normalized();
+        }
+        bool overWarmSea = (cells[nc].elevation <= sea) && (sTemp[nc] >= cfg.weatherTropicalSST - 4.0);
+        if (ws.tropical) {
+            if (overWarmSea) ws.strength += (1.0 - ws.strength) * (1.0 - std::exp(-0.05 * dtHours));
+            else             ws.strength -= ws.strength * (1.0 - std::exp(-0.15 * dtHours));
+        } else {
+            double frac = std::min(1.0, ws.age / std::max(1.0, ws.life));
+            ws.strength = 0.2 + 0.6 * std::sin(frac * M_PI);   // rise then fade
+            if (cells[nc].elevation > sea) ws.strength *= 0.7; // weaker over land
+        }
+        ws.strength = std::clamp(ws.strength, 0.0, 1.0);
+        ws.age += dtHours;
+        if (ws.age > ws.life || (ws.tropical && ws.strength < 0.05 && cells[nc].elevation > sea)) {
+            sStorms[s] = sStorms.back(); sStorms.pop_back();   // swap-remove dead system
+        } else ++s;
+    }
+
+    // 4c. Stamp each system's cloud/rain shield onto the grid (Gaussian-ish core falloff).
+    if (!sStorms.empty()) {
+        std::vector<double> cosR(sStorms.size());
+        for (size_t s = 0; s < sStorms.size(); ++s) cosR[s] = std::cos(std::min(M_PI, sStorms[s].radius));
+        for (int i = 0; i < n; ++i) {
+            double moist = 0.3 + 0.7 * std::clamp(sHumidity[i], 0.0, 1.0);
+            for (size_t s = 0; s < sStorms.size(); ++s) {
+                double dot = cells[i].unit.dot(sStorms[s].pos);
+                if (dot < cosR[s]) continue;                   // outside the system radius
+                double d = std::acos(std::clamp(dot, -1.0, 1.0));
+                double fall = 1.0 - d / sStorms[s].radius; fall *= fall;
+                double st = sStorms[s].strength;
+                sCloud[i] += st * fall * cfg.weatherSystemCloud * dtHours * moist;
+                sRain[i]  += st * fall * cfg.weatherSystemRain * moist;
+            }
+            sCloud[i] = std::clamp(sCloud[i], 0.0, 1.5);
+        }
+    }
 }

test_weather.cpp

@@ -22,11 +22,11 @@ static void check(bool cond, const char* what) {
     if (!cond) ++failures;
 }
 
-// Run a fixed weather sequence on a planet (returns whether rain ever fell, max cloud seen).
+// Run a fixed weather sequence on a planet (returns whether rain ever fell, max cloud + storms).
-static void runWeather(Planet& p, bool& everRained, double& maxCloud) {
+static void runWeather(Planet& p, bool& everRained, double& maxCloud, int& maxStorms) {
     p.initWeather();
-    everRained = false; maxCloud = 0.0;
+    everRained = false; maxCloud = 0.0; maxStorms = 0;
-    for (int k = 0; k < 200; ++k) {
+    for (int k = 0; k < 300; ++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();
@@ -35,6 +35,7 @@ static void runWeather(Planet& p, bool& everRained, double& maxCloud) {
             if (rn[i] > 0.0) everRained = true;
             maxCloud = std::max(maxCloud, cl[i]);
         }
+        maxStorms = std::max(maxStorms, (int)p.storms().size());
     }
 }
 
@@ -44,8 +45,8 @@ int main() {
     const int n = (int)p.cells.size();
 
     std::printf("Weather: cycle\n");
-    bool rained = false; double maxCloud = 0.0;
+    bool rained = false; double maxCloud = 0.0; int maxStorms = 0;
-    runWeather(p, rained, maxCloud);
+    runWeather(p, rained, maxCloud, maxStorms);
 
     bool inRange = true;
     for (int i = 0; i < n; ++i) {
@@ -67,14 +68,45 @@ int main() {
     std::printf("    mean humidity: ocean %.3f, land %.3f\n", oh, lh);
     check(oh > lh, "oceans end up moister than land");
 
+    std::printf("Weather: moving systems\n");
+    std::printf("    max concurrent systems: %d\n", maxStorms);
+    check(maxStorms > 0, "weather systems spawn over a run");
+    if (!p.storms().empty()) {                          // cloud shield (no mutation of p)
+        const auto& ws = p.storms()[0];
+        double inSum = 0, allSum = 0; int inN = 0;
+        for (int i = 0; i < n; ++i) {
+            allSum += p.cloud()[i];
+            double d = std::acos(std::clamp(p.cells[i].unit.dot(ws.pos), -1.0, 1.0));
+            if (d < ws.radius) { inSum += p.cloud()[i]; ++inN; }
+        }
+        check(inN > 0 && inSum / inN > allSum / n, "cloud is thicker inside a weather system");
+    }
+
+    std::printf("Weather: RNG isolation\n");
+    Planet z; z.generate(cfg);
+    std::vector<double> elev0(n); for (int i = 0; i < n; ++i) elev0[i] = z.cells[i].elevation;
+    z.initWeather();
+    for (int k = 0; k < 60; ++k) { z.computeInsolation(0.25, std::fmod(0.3 + 0.01 * k, 1.0)); z.stepWeather(1.0); }
+    bool terrainSame = true; for (int i = 0; i < n; ++i) if (z.cells[i].elevation != elev0[i]) terrainSame = false;
+    check(terrainSame, "weather + storm RNG never perturb the terrain");
+
     std::printf("Weather: determinism\n");
     Planet p2; p2.generate(cfg);
-    bool r2; double mc2; runWeather(p2, r2, mc2);
+    bool r2; double mc2; int ms2; runWeather(p2, r2, mc2, ms2);   // p and p2 both at 300 steps
-    bool same = true;
+    bool same = ((int)p2.storms().size() == (int)p.storms().size());
     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");
+    check(same, "same seed + sequence -> identical weather + systems");
+
+    std::printf("Weather: systems move\n");
+    if (!p.storms().empty()) {                          // one more step -> a system shifts position
+        Vec3 before = p.storms()[0].pos;
+        p.computeInsolation(0.25, 0.61); p.stepWeather(1.0);
+        double best = -2.0; for (const auto& ws : p.storms()) best = std::max(best, before.dot(ws.pos));
+        double ang = std::acos(std::clamp(best, -1.0, 1.0));
+        check(ang > 1e-4 && ang < 0.3, "a weather system moves between steps");
+    }
 
     std::printf("Weather: save v10\n");
     std::stringstream ss(std::ios::in | std::ios::out | std::ios::binary);