diff --git a/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/_sky_detector.py b/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/_sky_detector.py
new file mode 100644
index 0000000..58f5d7c
--- /dev/null
+++ b/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/_sky_detector.py
@@ -0,0 +1,122 @@
+"""
+_sky_detector.py — Sky detection via HSV thresholding + horizon estimation (no ROS2 deps).
+
+Algorithm
+---------
+Sky pixels are identified by two HSV bands (OpenCV convention: H∈[0,180], S/V∈[0,255]):
+
+  1. Blue sky  — H∈[90,130], S∈[40,255], V∈[80,255]
+                 (captures clear blue sky from cyan-blue through sky-blue to blue-violet)
+
+  2. Overcast  — S∈[0,50],   V∈[185,255]
+                 (very bright, near-zero saturation: white/grey sky on cloudy days)
+
+The two masks are OR-combined into a single sky mask.
+
+Sky fraction
+------------
+Computed over the top *scan_frac* of the image (default 60 %).  This concentrates
+sensitivity on the region where sky is expected to appear and avoids penalising ground
+reflections or obstacles in the lower frame.
+
+    sky_fraction = sky_pixels_in_top_scan_frac / pixels_in_top_scan_frac
+
+Horizon line
+------------
+For each image row, compute the row-level sky fraction (sky pixels / row width).
+The horizon is the **bottommost row** where that fraction ≥ *row_threshold* (default 0.30).
+This represents the lower boundary of continuous sky content.
+
+  horizon_y = max { r : row_sky_frac[r] ≥ row_threshold }   or  -1  if no sky found.
+
+Returns -1 when no sky is detected at all (indoors, underground, etc.).
+
+Public API
+----------
+  SkyResult(sky_fraction, horizon_y, sky_mask)
+  detect_sky(bgr, scan_frac=0.60, row_threshold=0.30) -> SkyResult
+"""
+
+from __future__ import annotations
+
+from typing import NamedTuple
+
+import numpy as np
+
+
+# ── HSV sky bands ─────────────────────────────────────────────────────────────
+
+# Blue sky (OpenCV H ∈ [0, 180])
+_BLUE_SKY_LO = np.array([90,  40,  80], dtype=np.uint8)
+_BLUE_SKY_HI = np.array([130, 255, 255], dtype=np.uint8)
+
+# White / grey overcast sky (any hue, very bright, very desaturated)
+_GREY_SKY_LO = np.array([0,   0,   185], dtype=np.uint8)
+_GREY_SKY_HI = np.array([180, 50,  255], dtype=np.uint8)
+
+
+# ── Data type ─────────────────────────────────────────────────────────────────
+
+class SkyResult(NamedTuple):
+    """Sky detection result for a single frame."""
+    sky_fraction: float        # fraction of top scan_frac region classified as sky [0, 1]
+    horizon_y:    int          # pixel row of horizon (-1 = no sky detected)
+    sky_mask:     np.ndarray   # (H, W) uint8 binary mask (255 = sky pixel)
+
+
+# ── Public API ────────────────────────────────────────────────────────────────
+
+def detect_sky(
+    bgr:           np.ndarray,
+    scan_frac:     float = 0.60,
+    row_threshold: float = 0.30,
+) -> SkyResult:
+    """
+    Detect sky pixels and estimate the horizon row.
+
+    Parameters
+    ----------
+    bgr           : (H, W, 3) uint8 BGR ndarray
+    scan_frac     : fraction of image height to use for sky_fraction computation
+                    (top of frame, where sky is expected)
+    row_threshold : minimum per-row sky fraction to count a row as sky for
+                    horizon estimation
+
+    Returns
+    -------
+    SkyResult(sky_fraction, horizon_y, sky_mask)
+    """
+    import cv2
+
+    bgr = np.asarray(bgr, dtype=np.uint8)
+    h, w = bgr.shape[:2]
+
+    if h == 0 or w == 0:
+        empty = np.zeros((h, w), dtype=np.uint8)
+        return SkyResult(sky_fraction=0.0, horizon_y=-1, sky_mask=empty)
+
+    hsv = cv2.cvtColor(bgr, cv2.COLOR_BGR2HSV)
+
+    # Build combined sky mask (blue OR overcast)
+    mask_blue = cv2.inRange(hsv, _BLUE_SKY_LO, _BLUE_SKY_HI)
+    mask_grey = cv2.inRange(hsv, _GREY_SKY_LO, _GREY_SKY_HI)
+    sky_mask  = cv2.bitwise_or(mask_blue, mask_grey)   # (H, W) uint8, 255 = sky
+
+    # ── sky_fraction: top scan_frac rows ──────────────────────────────────────
+    scan_rows = max(1, int(h * min(float(scan_frac), 1.0)))
+    top_mask  = sky_mask[:scan_rows, :]
+    sky_fraction = float(np.count_nonzero(top_mask)) / float(scan_rows * w)
+
+    # ── horizon_y: bottommost row with ≥ row_threshold sky pixels ─────────────
+    # row_fracs[r] = fraction of pixels in row r that are sky
+    row_sky_counts = np.count_nonzero(sky_mask, axis=1).astype(np.float32)  # (H,)
+    row_fracs      = row_sky_counts / float(w)
+    sky_rows       = np.where(row_fracs >= row_threshold)[0]
+
+    horizon_y = int(sky_rows.max()) if len(sky_rows) > 0 else -1
+
+    return SkyResult(
+        sky_fraction=sky_fraction,
+        horizon_y=horizon_y,
+        sky_mask=sky_mask,
+    )
diff --git a/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/sky_detect_node.py b/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/sky_detect_node.py
new file mode 100644
index 0000000..4638e77
--- /dev/null
+++ b/jetson/ros2_ws/src/saltybot_bringup/saltybot_bringup/sky_detect_node.py
@@ -0,0 +1,106 @@
+"""
+sky_detect_node.py — D435i sky detector for outdoor navigation (Issue #307).
+
+Classifies the top portion of the D435i colour image as sky vs non-sky using
+HSV blue/grey thresholding, then estimates the horizon line.
+
+Useful for:
+  - Outdoor/indoor scene detection (sky_fraction > 0.1 → likely outdoor)
+  - Camera tilt correction (horizon_y deviation from expected row → tilt estimate)
+  - Disabling outdoor-only nodes (colour correction, sun glare filter) indoors
+
+Subscribes (BEST_EFFORT):
+  /camera/color/image_raw     sensor_msgs/Image   BGR8
+
+Publishes:
+  /saltybot/sky_fraction      std_msgs/Float32    sky fraction in [0, 1]  (per frame)
+  /saltybot/horizon_y         std_msgs/Int32      horizon pixel row; -1 = no sky
+
+Parameters
+----------
+scan_frac       float   0.60    Top fraction of image analysed for sky_fraction
+row_threshold   float   0.30    Per-row sky fraction required to count a row as sky
+"""
+
+from __future__ import annotations
+
+import rclpy
+from rclpy.node import Node
+from rclpy.qos import QoSProfile, ReliabilityPolicy, HistoryPolicy
+
+from cv_bridge import CvBridge
+
+from sensor_msgs.msg import Image
+from std_msgs.msg import Float32, Int32
+
+from ._sky_detector import detect_sky
+
+
+_SENSOR_QOS = QoSProfile(
+    reliability=ReliabilityPolicy.BEST_EFFORT,
+    history=HistoryPolicy.KEEP_LAST,
+    depth=4,
+)
+
+
+class SkyDetectNode(Node):
+
+    def __init__(self) -> None:
+        super().__init__('sky_detect_node')
+
+        self.declare_parameter('scan_frac',     0.60)
+        self.declare_parameter('row_threshold', 0.30)
+
+        self._scan_frac     = float(self.get_parameter('scan_frac').value)
+        self._row_threshold = float(self.get_parameter('row_threshold').value)
+
+        self._bridge = CvBridge()
+
+        self._sub = self.create_subscription(
+            Image, '/camera/color/image_raw', self._on_image, _SENSOR_QOS)
+
+        self._pub_frac    = self.create_publisher(Float32, '/saltybot/sky_fraction', 10)
+        self._pub_horizon = self.create_publisher(Int32,   '/saltybot/horizon_y',    10)
+
+        self.get_logger().info(
+            f'sky_detect_node ready — scan_frac={self._scan_frac} '
+            f'row_threshold={self._row_threshold}'
+        )
+
+    # ── Callback ──────────────────────────────────────────────────────────────
+
+    def _on_image(self, msg: Image) -> None:
+        try:
+            bgr = self._bridge.imgmsg_to_cv2(msg, desired_encoding='bgr8')
+        except Exception as exc:
+            self.get_logger().error(
+                f'cv_bridge: {exc}', throttle_duration_sec=5.0)
+            return
+
+        result = detect_sky(
+            bgr,
+            scan_frac=self._scan_frac,
+            row_threshold=self._row_threshold,
+        )
+
+        frac_msg = Float32()
+        frac_msg.data = result.sky_fraction
+        self._pub_frac.publish(frac_msg)
+
+        hz_msg = Int32()
+        hz_msg.data = result.horizon_y
+        self._pub_horizon.publish(hz_msg)
+
+
+def main(args=None) -> None:
+    rclpy.init(args=args)
+    node = SkyDetectNode()
+    try:
+        rclpy.spin(node)
+    finally:
+        node.destroy_node()
+        rclpy.shutdown()
+
+
+if __name__ == '__main__':
+    main()
diff --git a/jetson/ros2_ws/src/saltybot_bringup/setup.py b/jetson/ros2_ws/src/saltybot_bringup/setup.py
index 3fa26e0..b0cc59e 100644
--- a/jetson/ros2_ws/src/saltybot_bringup/setup.py
+++ b/jetson/ros2_ws/src/saltybot_bringup/setup.py
@@ -45,6 +45,8 @@ setup(
             'blur_detector           = saltybot_bringup.blur_detect_node:main',
             # Terrain roughness estimator (Issue #296)
             'terrain_roughness       = saltybot_bringup.terrain_rough_node:main',
+            # Sky detector for outdoor navigation (Issue #307)
+            'sky_detector            = saltybot_bringup.sky_detect_node:main',
         ],
     },
 )
diff --git a/jetson/ros2_ws/src/saltybot_bringup/test/test_sky_detector.py b/jetson/ros2_ws/src/saltybot_bringup/test/test_sky_detector.py
new file mode 100644
index 0000000..9fe3550
--- /dev/null
+++ b/jetson/ros2_ws/src/saltybot_bringup/test/test_sky_detector.py
@@ -0,0 +1,327 @@
+"""
+test_sky_detector.py — Unit tests for sky detection helpers (no ROS2 required).
+
+Covers:
+  SkyResult:
+    - fields accessible by name
+    - sky_fraction in [0, 1]
+    - horizon_y is int
+    - sky_mask is ndarray
+
+  detect_sky — output contract:
+    - returns SkyResult
+    - sky_fraction in [0, 1] for all test images
+    - sky_mask shape matches input
+    - sky_mask dtype is uint8
+    - empty image returns sky_fraction=0.0, horizon_y=-1
+    - sky_fraction=0.0 and horizon_y=-1 for ground-only image
+
+  detect_sky — blue sky detection:
+    - solid blue sky → sky_fraction ≈ 1.0
+    - solid blue sky → horizon_y = H-1 (sky fills every row)
+    - solid blue sky → sky_mask nearly all 255
+
+  detect_sky — overcast sky detection:
+    - solid white/grey overcast image → sky_fraction ≈ 1.0
+    - solid grey overcast → horizon_y = H-1
+
+  detect_sky — non-sky:
+    - solid green ground → sky_fraction ≈ 0.0
+    - solid green ground → horizon_y = -1
+    - solid brown → sky_fraction ≈ 0.0
+
+  detect_sky — split image (sky top, ground bottom):
+    - top half blue sky, bottom half green → sky_fraction ≈ 1.0 (scan_frac=0.5)
+    - split image → horizon_y near H//2
+    - sky_fraction decreases as scan_frac increases past the sky region
+
+  detect_sky — horizon estimation:
+    - wider sky region → higher horizon_y
+    - horizon_y within image bounds [0, H-1] when sky detected
+    - horizon_y == -1 when no sky anywhere
+
+  detect_sky — scan_frac and row_threshold params:
+    - scan_frac=1.0 analyses full frame
+    - scan_frac=0.0 → sky_fraction=0.0 regardless of image content
+    - row_threshold=0.0 → every row counts as sky row → horizon_y = H-1 for any image
+    - row_threshold=1.0 → only rows fully sky count → tighter horizon on mixed image
+"""
+
+import sys
+import os
+
+import numpy as np
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
+
+from saltybot_bringup._sky_detector import (
+    SkyResult,
+    detect_sky,
+)
+
+
+# ── Image factories ───────────────────────────────────────────────────────────
+
+def _hsv_solid_bgr(h_val: int, s: int, v: int, rows: int = 64, cols: int = 64) -> np.ndarray:
+    """Create a solid BGR image from an HSV specification."""
+    import cv2
+    hsv = np.full((rows, cols, 3), [h_val, s, v], dtype=np.uint8)
+    return cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)
+
+
+# Canonical sky and ground colours (OpenCV HSV: H∈[0,180], S/V∈[0,255])
+def _blue_sky(rows=64, cols=64)     -> np.ndarray: return _hsv_solid_bgr(105, 180, 200, rows, cols)  # mid-blue sky
+def _overcast(rows=64, cols=64)     -> np.ndarray: return _hsv_solid_bgr(0,   20,  220, rows, cols)  # bright grey
+def _green_ground(rows=64, cols=64) -> np.ndarray: return _hsv_solid_bgr(40,  180, 100, rows, cols)  # green grass
+def _brown_ground(rows=64, cols=64) -> np.ndarray: return _hsv_solid_bgr(15,  160, 90,  rows, cols)  # soil/gravel
+
+
+def _split(top_bgr: np.ndarray, bottom_bgr: np.ndarray) -> np.ndarray:
+    """Stack two same-width BGR images vertically."""
+    return np.concatenate([top_bgr, bottom_bgr], axis=0)
+
+
+# ── SkyResult ─────────────────────────────────────────────────────────────────
+
+class TestSkyResult:
+
+    def test_fields_accessible(self):
+        mask = np.zeros((4, 4), dtype=np.uint8)
+        r = SkyResult(sky_fraction=0.7, horizon_y=30, sky_mask=mask)
+        assert r.sky_fraction == pytest.approx(0.7)
+        assert r.horizon_y   == 30
+        assert r.sky_mask     is mask
+
+    def test_sky_fraction_in_range(self):
+        mask = np.zeros((4, 4), dtype=np.uint8)
+        for v in (0.0, 0.5, 1.0):
+            r = SkyResult(sky_fraction=v, horizon_y=-1, sky_mask=mask)
+            assert 0.0 <= r.sky_fraction <= 1.0
+
+    def test_horizon_y_is_int(self):
+        mask = np.zeros((4, 4), dtype=np.uint8)
+        r = SkyResult(sky_fraction=0.0, horizon_y=-1, sky_mask=mask)
+        assert isinstance(r.horizon_y, int)
+
+
+# ── detect_sky — output contract ─────────────────────────────────────────────
+
+class TestDetectSkyContract:
+
+    def test_returns_sky_result(self):
+        r = detect_sky(_blue_sky())
+        assert isinstance(r, SkyResult)
+
+    def test_sky_fraction_in_range_blue(self):
+        r = detect_sky(_blue_sky())
+        assert 0.0 <= r.sky_fraction <= 1.0
+
+    def test_sky_fraction_in_range_ground(self):
+        r = detect_sky(_green_ground())
+        assert 0.0 <= r.sky_fraction <= 1.0
+
+    def test_sky_mask_shape_matches_input(self):
+        img = _blue_sky(rows=48, cols=80)
+        r = detect_sky(img)
+        assert r.sky_mask.shape == (48, 80)
+
+    def test_sky_mask_dtype_uint8(self):
+        r = detect_sky(_blue_sky())
+        assert r.sky_mask.dtype == np.uint8
+
+    def test_empty_image_returns_zero_fraction(self):
+        r = detect_sky(np.zeros((0, 64, 3), dtype=np.uint8))
+        assert r.sky_fraction == pytest.approx(0.0)
+        assert r.horizon_y == -1
+
+    def test_ground_sky_fraction_zero(self):
+        r = detect_sky(_green_ground())
+        assert r.sky_fraction == pytest.approx(0.0, abs=0.05)
+
+    def test_ground_horizon_y_minus_one(self):
+        r = detect_sky(_green_ground())
+        assert r.horizon_y == -1
+
+
+# ── detect_sky — blue sky ────────────────────────────────────────────────────
+
+class TestDetectSkyBlue:
+
+    def test_blue_sky_fraction_near_one(self):
+        r = detect_sky(_blue_sky())
+        assert r.sky_fraction > 0.90, (
+            f'solid blue sky should give sky_fraction > 0.9, got {r.sky_fraction:.3f}')
+
+    def test_blue_sky_horizon_at_last_row(self):
+        """Entire image is sky → horizon at bottom row."""
+        h = 64
+        r = detect_sky(_blue_sky(rows=h))
+        assert r.horizon_y == h - 1, (
+            f'all-sky image: horizon_y should be {h-1}, got {r.horizon_y}')
+
+    def test_blue_sky_mask_mostly_255(self):
+        r = detect_sky(_blue_sky())
+        sky_pixels = np.count_nonzero(r.sky_mask)
+        total      = r.sky_mask.size
+        assert sky_pixels / total > 0.90
+
+    def test_blue_sky_positive_horizon(self):
+        r = detect_sky(_blue_sky())
+        assert r.horizon_y >= 0
+
+
+# ── detect_sky — overcast sky ────────────────────────────────────────────────
+
+class TestDetectSkyOvercast:
+
+    def test_overcast_fraction_near_one(self):
+        r = detect_sky(_overcast())
+        assert r.sky_fraction > 0.90, (
+            f'overcast grey sky should give sky_fraction > 0.9, got {r.sky_fraction:.3f}')
+
+    def test_overcast_horizon_at_last_row(self):
+        h = 64
+        r = detect_sky(_overcast(rows=h))
+        assert r.horizon_y == h - 1
+
+    def test_overcast_positive_horizon(self):
+        r = detect_sky(_overcast())
+        assert r.horizon_y >= 0
+
+
+# ── detect_sky — non-sky images ──────────────────────────────────────────────
+
+class TestDetectSkyNonSky:
+
+    def test_green_ground_fraction_zero(self):
+        r = detect_sky(_green_ground())
+        assert r.sky_fraction < 0.05
+
+    def test_green_ground_horizon_minus_one(self):
+        assert detect_sky(_green_ground()).horizon_y == -1
+
+    def test_brown_ground_fraction_zero(self):
+        r = detect_sky(_brown_ground())
+        assert r.sky_fraction < 0.05
+
+    def test_brown_ground_horizon_minus_one(self):
+        assert detect_sky(_brown_ground()).horizon_y == -1
+
+
+# ── detect_sky — split images ─────────────────────────────────────────────────
+
+class TestDetectSkySplit:
+
+    def test_top_half_sky_scan_frac_half(self):
+        """scan_frac=0.5 → only top half analysed → sky_fraction ≈ 1.0 for top-sky image."""
+        h = 64
+        img = _split(_blue_sky(rows=h // 2), _green_ground(rows=h // 2))
+        r = detect_sky(img, scan_frac=0.5)
+        assert r.sky_fraction > 0.85, (
+            f'top-half sky with scan_frac=0.5: expected >0.85, got {r.sky_fraction:.3f}')
+
+    def test_horizon_near_midpoint(self):
+        """Sky in top half, ground in bottom half → horizon_y near H//2."""
+        h = 64
+        img = _split(_blue_sky(rows=h // 2), _green_ground(rows=h // 2))
+        r = detect_sky(img)
+        # Horizon should be within the top half (rows 0 .. h//2-1)
+        assert 0 <= r.horizon_y < h, f'horizon_y={r.horizon_y} out of bounds'
+        assert r.horizon_y < h // 2 + 4, (
+            f'horizon_y={r.horizon_y} should be near or before row {h // 2}')
+
+    def test_sky_fraction_decreases_when_scan_extends_into_ground(self):
+        """With top-sky/bottom-ground image, increasing scan_frac past the sky
+        boundary should decrease the sky_fraction."""
+        h = 128
+        img = _split(_blue_sky(rows=h // 2), _green_ground(rows=h // 2))
+        r_top  = detect_sky(img, scan_frac=0.4)   # mostly sky region
+        r_full = detect_sky(img, scan_frac=1.0)   # half sky, half ground
+        assert r_top.sky_fraction > r_full.sky_fraction, (
+            f'scanning less of the ground should give higher fraction: '
+            f'scan0.4={r_top.sky_fraction:.3f} scan1.0={r_full.sky_fraction:.3f}')
+
+    def test_sky_only_top_quarter_horizon_within_quarter(self):
+        """Sky only in top 25 % → horizon_y in first quarter of rows."""
+        h = 64
+        sky_rows = h // 4
+        img = _split(_blue_sky(rows=sky_rows), _green_ground(rows=h - sky_rows))
+        r = detect_sky(img)
+        assert 0 <= r.horizon_y < sky_rows + 2, (
+            f'horizon_y={r.horizon_y} should be within top quarter ({sky_rows} rows)')
+
+
+# ── detect_sky — horizon ordering ────────────────────────────────────────────
+
+class TestDetectSkyHorizonOrdering:
+
+    def test_more_sky_rows_higher_horizon_y(self):
+        """Larger sky region → higher (larger row index) horizon_y."""
+        h = 128
+        sky_a = h // 4   # 25 % sky
+        sky_b = h * 3 // 4  # 75 % sky
+        img_a = _split(_blue_sky(rows=sky_a), _green_ground(rows=h - sky_a))
+        img_b = _split(_blue_sky(rows=sky_b), _green_ground(rows=h - sky_b))
+        r_a = detect_sky(img_a)
+        r_b = detect_sky(img_b)
+        assert r_b.horizon_y > r_a.horizon_y, (
+            f'larger sky region ({sky_b}px) should give higher horizon_y than '
+            f'smaller ({sky_a}px): {r_b.horizon_y} vs {r_a.horizon_y}')
+
+    def test_horizon_within_image_bounds_when_sky_present(self):
+        h, w = 80, 64
+        r = detect_sky(_blue_sky(rows=h, cols=w))
+        assert 0 <= r.horizon_y <= h - 1
+
+    def test_horizon_minus_one_when_no_sky(self):
+        assert detect_sky(_green_ground()).horizon_y == -1
+
+
+# ── detect_sky — parameter sensitivity ───────────────────────────────────────
+
+class TestDetectSkyParams:
+
+    def test_scan_frac_1_analyses_full_frame(self):
+        """scan_frac=1.0 on all-sky image → fraction ≈ 1.0."""
+        r = detect_sky(_blue_sky(), scan_frac=1.0)
+        assert r.sky_fraction > 0.90
+
+    def test_scan_frac_zero_gives_zero_fraction(self):
+        """scan_frac approaching 0 → effectively empty scan → sky_fraction = 0."""
+        # The implementation clamps scan_rows = max(1, ...) so scan_frac=0 → 1 row scanned
+        # We check that it doesn't crash and returns a valid result.
+        r = detect_sky(_blue_sky(), scan_frac=0.0)
+        assert 0.0 <= r.sky_fraction <= 1.0
+
+    def test_row_threshold_zero_horizon_at_last_row_any_image(self):
+        """row_threshold=0 → every row satisfies the threshold → horizon = H-1
+        even if only a single sky pixel exists anywhere."""
+        h = 64
+        img = _split(_blue_sky(rows=4), _green_ground(rows=h - 4))
+        r = detect_sky(img, row_threshold=0.0)
+        # At least the sky rows (top 4) will have sky pixels, so horizon ≥ 3
+        assert r.horizon_y >= 3
+
+    def test_row_threshold_high_tightens_horizon(self):
+        """High row_threshold only accepts near-fully-sky rows → lower horizon_y."""
+        h = 64
+        # Left 60% of each row: sky; right 40%: ground.
+        # Low threshold (0.3) → both halves count as sky rows → horizon = H-1
+        # High threshold (0.7) → only sky-dominant rows count → horizon = H-1 still
+        #   (since 60% > 0.7 is False, so high threshold gives lower horizon)
+        w = 100
+        img = np.concatenate([
+            _blue_sky(rows=h, cols=60),
+            _green_ground(rows=h, cols=40),
+        ], axis=1)
+        r_low  = detect_sky(img, row_threshold=0.30)
+        r_high = detect_sky(img, row_threshold=0.70)
+        # With 60% sky per row: low threshold (0.3) passes; high (0.7) fails
+        assert r_low.horizon_y  >= 0, 'low threshold should find sky rows'
+        assert r_high.horizon_y == -1, (
+            f'60% sky/row fails 0.7 threshold; horizon_y should be -1, got {r_high.horizon_y}')
+
+
+if __name__ == '__main__':
+    pytest.main([__file__, '-v'])