diff --git a/src/mudlib/weather.py b/src/mudlib/weather.py
new file mode 100644
index 0000000..7771639
--- /dev/null
+++ b/src/mudlib/weather.py
@@ -0,0 +1,223 @@
+"""Weather system with procedural transitions."""
+
+from __future__ import annotations
+
+import enum
+import random
+from dataclasses import dataclass
+
+
+class WeatherCondition(enum.Enum):
+    clear = "clear"
+    cloudy = "cloudy"
+    rain = "rain"
+    storm = "storm"
+    snow = "snow"
+    fog = "fog"
+
+
+@dataclass
+class WeatherState:
+    condition: WeatherCondition
+    intensity: float  # 0.0 to 1.0
+
+
+def get_weather_description(state: WeatherState) -> str:
+    """Return atmospheric text for the current weather."""
+    condition = state.condition
+    intensity = state.intensity
+
+    if condition == WeatherCondition.clear:
+        return "the sky is clear"
+
+    elif condition == WeatherCondition.cloudy:
+        if intensity < 0.4:
+            return "thin clouds drift overhead"
+        elif intensity < 0.7:
+            return "clouds fill the sky"
+        else:
+            return "heavy clouds loom darkly"
+
+    elif condition == WeatherCondition.rain:
+        if intensity < 0.3:
+            return "a light drizzle falls"
+        elif intensity < 0.6:
+            return "rain patters steadily"
+        else:
+            return "rain hammers down relentlessly"
+
+    elif condition == WeatherCondition.storm:
+        if intensity < 0.5:
+            return "thunder rumbles in the distance"
+        else:
+            return "lightning splits the sky as the storm rages"
+
+    elif condition == WeatherCondition.snow:
+        if intensity < 0.3:
+            return "light snow drifts down"
+        elif intensity < 0.6:
+            return "snow falls steadily"
+        else:
+            return "heavy snow blankets everything"
+
+    elif condition == WeatherCondition.fog:
+        if intensity < 0.4:
+            return "thin mist hangs in the air"
+        elif intensity < 0.7:
+            return "fog obscures the distance"
+        else:
+            return "thick fog shrouds everything"
+
+    return ""
+
+
+# Climate profiles: transition weights for each condition
+# Format: {from_condition: {to_condition: weight, ...}, ...}
+CLIMATE_PROFILES = {
+    "temperate": {
+        WeatherCondition.clear: {
+            "clear": 50,
+            "cloudy": 40,
+            "fog": 10,
+        },
+        WeatherCondition.cloudy: {
+            "clear": 30,
+            "cloudy": 35,
+            "rain": 20,
+            "snow": 10,
+            "fog": 5,
+        },
+        WeatherCondition.rain: {
+            "cloudy": 35,
+            "rain": 30,
+            "storm": 15,
+            "snow": 10,
+            "fog": 10,
+        },
+        WeatherCondition.storm: {
+            "rain": 60,
+            "cloudy": 40,
+        },
+        WeatherCondition.snow: {
+            "snow": 40,
+            "cloudy": 35,
+            "fog": 25,
+        },
+        WeatherCondition.fog: {
+            "fog": 30,
+            "cloudy": 40,
+            "clear": 30,
+        },
+    },
+    "arid": {
+        WeatherCondition.clear: {
+            "clear": 90,
+            "cloudy": 10,
+        },
+        WeatherCondition.cloudy: {
+            "clear": 70,
+            "cloudy": 20,
+            "rain": 5,
+            "fog": 5,
+        },
+        WeatherCondition.rain: {
+            "cloudy": 60,
+            "rain": 20,
+            "clear": 20,
+        },
+        WeatherCondition.storm: {
+            "rain": 50,
+            "cloudy": 50,
+        },
+        WeatherCondition.snow: {
+            "cloudy": 100,
+        },
+        WeatherCondition.fog: {
+            "clear": 60,
+            "fog": 20,
+            "cloudy": 20,
+        },
+    },
+    "arctic": {
+        WeatherCondition.clear: {
+            "cloudy": 50,
+            "fog": 30,
+            "clear": 20,
+        },
+        WeatherCondition.cloudy: {
+            "snow": 40,
+            "cloudy": 35,
+            "fog": 25,
+        },
+        WeatherCondition.rain: {
+            "snow": 40,
+            "cloudy": 40,
+            "fog": 20,
+        },
+        WeatherCondition.storm: {
+            "snow": 60,
+            "cloudy": 40,
+        },
+        WeatherCondition.snow: {
+            "snow": 50,
+            "cloudy": 30,
+            "fog": 20,
+        },
+        WeatherCondition.fog: {
+            "fog": 40,
+            "cloudy": 40,
+            "snow": 20,
+        },
+    },
+}
+
+
+def advance_weather(
+    current: WeatherState,
+    season: str = "summer",
+    rng: random.Random | None = None,
+    climate: str = "temperate",
+) -> WeatherState:
+    """Advance weather by one step (one game hour)."""
+    if rng is None:
+        rng = random.Random()
+
+    # Get climate profile
+    profile = CLIMATE_PROFILES.get(climate, CLIMATE_PROFILES["temperate"])
+
+    # Get transition weights for current condition
+    transitions = profile.get(current.condition, {})
+
+    # Filter out snow if not winter/autumn
+    if season not in ("winter", "autumn"):
+        transitions = {k: v for k, v in transitions.items() if k != "snow"}
+
+    # If no valid transitions, stay in current condition
+    if not transitions:
+        return WeatherState(
+            condition=current.condition, intensity=rng.uniform(0.0, 1.0)
+        )
+
+    # Weighted random choice
+    conditions = list(transitions.keys())
+    weights = list(transitions.values())
+
+    # Convert string keys to WeatherCondition enums
+    resolved_conditions = []
+    for cond in conditions:
+        if isinstance(cond, str):
+            resolved_conditions.append(WeatherCondition[cond])
+        else:
+            resolved_conditions.append(cond)
+
+    new_condition = rng.choices(resolved_conditions, weights=weights, k=1)[0]
+
+    # Generate appropriate intensity for the new condition
+    if new_condition == WeatherCondition.clear:
+        new_intensity = 0.5  # Clear has no meaningful intensity variation
+    elif new_condition == WeatherCondition.storm:
+        new_intensity = rng.uniform(0.5, 1.0)  # Storms are always intense
+    else:
+        new_intensity = rng.uniform(0.0, 1.0)
+
+    return WeatherState(condition=new_condition, intensity=new_intensity)
diff --git a/tests/test_weather.py b/tests/test_weather.py
new file mode 100644
index 0000000..f75c034
--- /dev/null
+++ b/tests/test_weather.py
@@ -0,0 +1,208 @@
+"""Tests for weather system."""
+
+import random
+
+from mudlib.weather import (
+    WeatherCondition,
+    WeatherState,
+    advance_weather,
+    get_weather_description,
+)
+
+
+def test_weather_condition_enum():
+    assert WeatherCondition.clear.value == "clear"
+    assert WeatherCondition.cloudy.value == "cloudy"
+    assert WeatherCondition.rain.value == "rain"
+    assert WeatherCondition.storm.value == "storm"
+    assert WeatherCondition.snow.value == "snow"
+    assert WeatherCondition.fog.value == "fog"
+
+
+def test_weather_state_dataclass():
+    state = WeatherState(condition=WeatherCondition.rain, intensity=0.5)
+    assert state.condition == WeatherCondition.rain
+    assert state.intensity == 0.5
+
+
+def test_get_weather_description_returns_non_empty():
+    state = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    description = get_weather_description(state)
+    assert isinstance(description, str)
+    assert len(description) > 0
+
+
+def test_get_weather_description_varies_by_condition():
+    clear = get_weather_description(
+        WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    )
+    rain = get_weather_description(
+        WeatherState(condition=WeatherCondition.rain, intensity=0.5)
+    )
+    snow = get_weather_description(
+        WeatherState(condition=WeatherCondition.snow, intensity=0.5)
+    )
+
+    # Different conditions should produce different descriptions
+    assert clear != rain
+    assert rain != snow
+    assert clear != snow
+
+
+def test_get_weather_description_varies_by_intensity():
+    light_rain = get_weather_description(
+        WeatherState(condition=WeatherCondition.rain, intensity=0.2)
+    )
+    heavy_rain = get_weather_description(
+        WeatherState(condition=WeatherCondition.rain, intensity=0.9)
+    )
+
+    # Different intensities should produce different descriptions
+    assert light_rain != heavy_rain
+
+
+def test_advance_weather_returns_new_state():
+    current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    rng = random.Random(42)
+    new_state = advance_weather(current, season="summer", rng=rng)
+
+    assert isinstance(new_state, WeatherState)
+    assert isinstance(new_state.condition, WeatherCondition)
+    assert 0.0 <= new_state.intensity <= 1.0
+
+
+def test_advance_weather_is_deterministic_with_seed():
+    current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+
+    rng1 = random.Random(42)
+    state1 = advance_weather(current, season="summer", rng=rng1)
+
+    rng2 = random.Random(42)
+    state2 = advance_weather(current, season="summer", rng=rng2)
+
+    assert state1.condition == state2.condition
+    assert state1.intensity == state2.intensity
+
+
+def test_advance_weather_transitions_naturally():
+    # Clear can become cloudy
+    rng = random.Random(42)
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+        new_state = advance_weather(current, season="summer", rng=rng)
+        if new_state.condition == WeatherCondition.cloudy:
+            break
+    else:
+        raise AssertionError("clear never transitioned to cloudy in 100 iterations")
+
+    # Cloudy can become rain or clear
+    rng = random.Random(43)
+    found_rain = False
+    found_clear = False
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.cloudy, intensity=0.5)
+        new_state = advance_weather(current, season="summer", rng=rng)
+        if new_state.condition == WeatherCondition.rain:
+            found_rain = True
+        if new_state.condition == WeatherCondition.clear:
+            found_clear = True
+        if found_rain and found_clear:
+            break
+    assert found_rain or found_clear
+
+
+def test_storm_transitions_to_rain_or_cloudy():
+    # Storm should always transition away (doesn't last)
+    rng = random.Random(44)
+    found_non_storm = False
+    for _ in range(50):
+        current = WeatherState(condition=WeatherCondition.storm, intensity=0.8)
+        new_state = advance_weather(current, season="summer", rng=rng)
+        if new_state.condition in (WeatherCondition.rain, WeatherCondition.cloudy):
+            found_non_storm = True
+            break
+    assert found_non_storm, "storm should transition to rain or cloudy"
+
+
+def test_snow_only_in_winter_autumn():
+    # Snow in winter
+    rng = random.Random(45)
+    found_snow = False
+    for _ in range(200):
+        current = WeatherState(condition=WeatherCondition.cloudy, intensity=0.5)
+        new_state = advance_weather(current, season="winter", rng=rng)
+        if new_state.condition == WeatherCondition.snow:
+            found_snow = True
+            break
+    assert found_snow, "snow should be possible in winter"
+
+    # Snow should be rare or impossible in summer
+    rng = random.Random(46)
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.cloudy, intensity=0.5)
+        new_state = advance_weather(current, season="summer", rng=rng)
+        # Should not produce snow in summer
+        assert new_state.condition != WeatherCondition.snow
+
+
+def test_climate_temperate_default():
+    current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    rng = random.Random(47)
+
+    # Should work without climate parameter (defaults to temperate)
+    new_state = advance_weather(current, season="summer", rng=rng)
+    assert isinstance(new_state, WeatherState)
+
+
+def test_climate_arid_favors_clear():
+    # Arid should heavily favor clear weather
+    rng = random.Random(48)
+    clear_count = 0
+
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+        new_state = advance_weather(current, season="summer", rng=rng, climate="arid")
+        if new_state.condition == WeatherCondition.clear:
+            clear_count += 1
+
+    # Arid should stay clear most of the time
+    assert clear_count > 70, f"arid should favor clear, got {clear_count}/100"
+
+
+def test_climate_arid_no_snow():
+    # Arid should never produce snow
+    rng = random.Random(49)
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.cloudy, intensity=0.5)
+        new_state = advance_weather(current, season="winter", rng=rng, climate="arid")
+        assert new_state.condition != WeatherCondition.snow
+
+
+def test_climate_arctic_favors_snow_fog_cloudy():
+    # Arctic should produce snow, fog, or cloudy frequently
+    rng = random.Random(50)
+    arctic_conditions = 0
+
+    for _ in range(100):
+        current = WeatherState(condition=WeatherCondition.cloudy, intensity=0.5)
+        new_state = advance_weather(current, season="winter", rng=rng, climate="arctic")
+        if new_state.condition in (
+            WeatherCondition.snow,
+            WeatherCondition.fog,
+            WeatherCondition.cloudy,
+        ):
+            arctic_conditions += 1
+
+    # Arctic should heavily favor snow/fog/cloudy
+    assert arctic_conditions > 70, (
+        f"arctic should favor snow/fog/cloudy, got {arctic_conditions}/100"
+    )
+
+
+def test_advance_weather_accepts_all_seasons():
+    current = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    rng = random.Random(51)
+
+    for season in ["spring", "summer", "autumn", "winter"]:
+        new_state = advance_weather(current, season=season, rng=rng)
+        assert isinstance(new_state, WeatherState)