diff --git a/algo.md b/algo.md
index ee9b60d4..c7976a34 100644
--- a/algo.md
+++ b/algo.md
@@ -766,12 +766,46 @@ Per the user's directive to model **all bands ≥902 MHz**, these are added to `
 ### Open items / next analyses
 
 1. **ERA5 backfill needs to actually run.** Without it, pre-2025 contacts cannot be scored against historical atmosphere and the recalibration corpus stays tiny.
-2. **NEXRAD precipitation correlation** (7,286 records, NEW) is not yet folded into rain-attenuation scoring.
-3. **`hrrr_native_profiles.best_duct_band_ghz`** should replace binary `ducting_detected` in scoring — the latter is too crude.
+2. ~~**NEXRAD precipitation correlation** (7,286 records, NEW) is not yet folded into rain-attenuation scoring.~~ **Landed.** See "NEXRAD composite reflectivity" below.
+3. ~~**`hrrr_native_profiles.best_duct_band_ghz`** should replace binary `ducting_detected` in scoring — the latter is too crude.~~ **Landed.** See "Native-profile duct boost" below.
 4. **Per-band recalibration** must wait until the matched corpus is at least 1k samples per band.
 
 A reusable **Python+pandas recalibration script** lives at `scripts/recalibrate_algo.py` so this analysis can be re-run any time new data lands without manual SQL.
 
+### NEXRAD composite reflectivity → rain-attenuation score
+
+HRRR hourly precipitation accumulation lags fast-moving convective cells by up to 59 minutes. NEXRAD n0q composite reflectivity runs at 5-minute cadence and catches those cells at the moment they pass over a grid cell. The scoring pipeline now takes the *max* of the HRRR-derived rain rate (from `precip_mm`) and the NEXRAD-derived rain rate (from `max_reflectivity_dbz` via Marshall-Palmer) so either source can trigger the rain penalty without double-counting.
+
+`Scorer.dbz_to_rain_rate_mmhr/1` implements:
+
+```
+R (mm/hr) = (Z / 200)^(1/1.6)   where Z = 10^(dBZ/10)
+```
+
+with a 5 dBZ noise floor (ground clutter / clear air) and a 150 mm/hr ceiling (hail contamination). Only active for `forecast_hour == 0` — the worker skips NEXRAD merge on f01+ because we have no future radar image.
+
+### Native-profile duct boost
+
+The base refractivity score uses HRRR's 13 pressure levels, which systematically under-read thin trapping layers (see Part 2c — only 0.12% of native-resolution cells support ≥15 GHz, but the pressure-level data looks the same above and below that threshold). When `hrrr_native_profiles.best_duct_band_ghz` is present and its value is ≥ the target band's frequency, `Scorer.score_refractivity/4` multiplies the base score by 1.15×. A duct that only supports sub-band frequencies does *not* boost — it's evidence that the gradient we have is all there is at the target band.
+
+This is an additive correction to the HPBL multiplier introduced earlier in Part 2c. The two multipliers compose: a thin shallow-BL cell with a 24-GHz-supporting native duct gets the full 1.05–1.10× HPBL bonus *and* the 1.15× native-duct bonus, clamped at 100.
+
+### Commercial-link inverse sensor
+
+Seven af11x / af60 commercial microwave links around Princeton TX (33.2°N, 96.5°W) are polled every 5 minutes via SNMP and stored in `commercial_samples`. These are short (2–6 km) LOS paths where the same refractivity anomaly that helps beyond-LOS amateur propagation degrades the link's rx_power through multipath fading.
+
+`Commercial.link_degradation_at/3` computes the average of (7-day baseline rx_power − current rx_power) across all healthy (`link_state == 1`) links within a configurable radius of a target point (default 75 km). The worker calls this for every grid cell on `forecast_hour == 0` runs; commercial links only exist near DFW so almost every cell gets `nil` at near-zero cost.
+
+`Scorer.commercial_link_boost/2` then adds a terminal boost to the composite score:
+
+| degradation_db | Bonus | Interpretation |
+|---|---|---|
+| <3 dB | 0 | Noise floor |
+| 3–8 dB | +2 to +10 | Mild multipath — weak enhancement |
+| ≥8 dB | +10 to +25 (clamped ≤100) | Deep fade — strong ducting signature |
+
+This is the first *measured* signal in the algorithm — every other factor is a model-derived proxy. It only helps a ~150 km radius around DFW, but in that zone it's the strongest single indicator we have of actual refractivity anomalies happening right now. Out-of-zone cells see no change.
+
 ---
 
 ## Part 3: Band Configuration
diff --git a/lib/microwaveprop/commercial.ex b/lib/microwaveprop/commercial.ex
index 525d8b02..fd0e2031 100644
--- a/lib/microwaveprop/commercial.ex
+++ b/lib/microwaveprop/commercial.ex
@@ -7,6 +7,11 @@ defmodule Microwaveprop.Commercial do
   alias Microwaveprop.Commercial.Sample
   alias Microwaveprop.Repo
 
+  @default_radius_km 75.0
+  @default_baseline_days 7
+  @default_current_window_seconds 900
+  @min_baseline_samples 5
+
   @spec enabled_links() :: [Link.t()]
   def enabled_links do
     Link
@@ -14,6 +19,142 @@ defmodule Microwaveprop.Commercial do
     |> Repo.all()
   end
 
+  @doc """
+  Returns the aggregate commercial-link rx_power degradation at a location,
+  or `nil` if no healthy links are in range.
+
+  Commercial LOS microwave links near the target point act as an inverse
+  tropo sensor: when their rx_power drops significantly below the 7-day
+  baseline without an obvious equipment cause (link_state still 1), the
+  refractivity structure that hurts their short Fresnel zone is usually
+  the same structure that *helps* long beyond-LOS amateur propagation.
+
+  Result map:
+    %{
+      degradation_db: float (positive means worse than baseline),
+      baseline_dbm: float,
+      current_dbm: float,
+      n_links: integer
+    }
+
+  ## Options
+
+    * `:radius_km` — distance from the target point to include a link (default 75)
+    * `:baseline_days` — days of history for the baseline average (default 7)
+    * `:current_window_seconds` — how recent "current" rx must be (default 900)
+  """
+  @spec link_degradation_at({float(), float()}, DateTime.t(), keyword()) :: map() | nil
+  def link_degradation_at({lat, lon}, valid_time, opts \\ []) do
+    radius_km = Keyword.get(opts, :radius_km, @default_radius_km)
+    baseline_days = Keyword.get(opts, :baseline_days, @default_baseline_days)
+    current_window = Keyword.get(opts, :current_window_seconds, @default_current_window_seconds)
+
+    candidates =
+      enabled_links()
+      |> Enum.map(fn link -> {link, link_endpoint(link)} end)
+      |> Enum.reject(fn {_link, endpoint} -> is_nil(endpoint) end)
+      |> Enum.filter(fn {_link, {elat, elon}} ->
+        haversine_km(lat, lon, elat, elon) <= radius_km
+      end)
+
+    baseline_cutoff = DateTime.add(valid_time, -baseline_days * 24 * 3600, :second)
+    current_cutoff = DateTime.add(valid_time, -current_window, :second)
+
+    results =
+      candidates
+      |> Enum.map(fn {link, _endpoint} ->
+        link_degradation(link.id, baseline_cutoff, current_cutoff, valid_time)
+      end)
+      |> Enum.reject(&is_nil/1)
+
+    case results do
+      [] ->
+        nil
+
+      list ->
+        baseline_avg = average(Enum.map(list, & &1.baseline_dbm))
+        current_avg = average(Enum.map(list, & &1.current_dbm))
+
+        %{
+          degradation_db: Float.round(baseline_avg - current_avg, 2),
+          baseline_dbm: Float.round(baseline_avg, 2),
+          current_dbm: Float.round(current_avg, 2),
+          n_links: length(list)
+        }
+    end
+  end
+
+  defp link_degradation(link_id, baseline_cutoff, current_cutoff, valid_time) do
+    baseline =
+      Sample
+      |> where([s], s.link_id == ^link_id)
+      |> where([s], s.sampled_at >= ^baseline_cutoff and s.sampled_at < ^current_cutoff)
+      |> where([s], s.link_state == 1 and not is_nil(s.rx_power_0))
+      |> select([s], avg(s.rx_power_0))
+      |> Repo.one()
+
+    {current, baseline_n} = fetch_current_and_baseline_count(link_id, current_cutoff, valid_time)
+
+    cond do
+      is_nil(baseline) or is_nil(current) ->
+        nil
+
+      baseline_n < @min_baseline_samples ->
+        nil
+
+      true ->
+        %{baseline_dbm: to_float(baseline), current_dbm: current}
+    end
+  end
+
+  defp to_float(%Decimal{} = d), do: Decimal.to_float(d)
+  defp to_float(n) when is_number(n), do: n * 1.0
+
+  defp fetch_current_and_baseline_count(link_id, current_cutoff, valid_time) do
+    current =
+      Sample
+      |> where([s], s.link_id == ^link_id)
+      |> where([s], s.sampled_at >= ^current_cutoff and s.sampled_at <= ^valid_time)
+      |> where([s], s.link_state == 1 and not is_nil(s.rx_power_0))
+      |> order_by([s], desc: s.sampled_at)
+      |> limit(1)
+      |> select([s], s.rx_power_0)
+      |> Repo.one()
+
+    baseline_n =
+      Sample
+      |> where([s], s.link_id == ^link_id)
+      |> where([s], s.sampled_at < ^current_cutoff)
+      |> where([s], s.link_state == 1 and not is_nil(s.rx_power_0))
+      |> select([s], count(s.id))
+      |> Repo.one()
+
+    {current, baseline_n}
+  end
+
+  defp link_endpoint(%Link{endpoint_a: %{"lat" => lat, "lon" => lon}}) when is_number(lat) and is_number(lon),
+    do: {lat * 1.0, lon * 1.0}
+
+  defp link_endpoint(_), do: nil
+
+  defp haversine_km(lat1, lon1, lat2, lon2) do
+    r = 6371.0
+    phi1 = lat1 * :math.pi() / 180
+    phi2 = lat2 * :math.pi() / 180
+    dphi = (lat2 - lat1) * :math.pi() / 180
+    dlambda = (lon2 - lon1) * :math.pi() / 180
+
+    a =
+      :math.sin(dphi / 2) * :math.sin(dphi / 2) +
+        :math.cos(phi1) * :math.cos(phi2) * :math.sin(dlambda / 2) * :math.sin(dlambda / 2)
+
+    c = 2 * :math.atan2(:math.sqrt(a), :math.sqrt(1 - a))
+    r * c
+  end
+
+  defp average([]), do: 0.0
+  defp average(list), do: Enum.sum(list) / length(list)
+
   @spec create_link(map()) :: {:ok, Link.t()} | {:error, Ecto.Changeset.t()}
   def create_link(attrs) do
     %Link{}
diff --git a/lib/microwaveprop/propagation.ex b/lib/microwaveprop/propagation.ex
index eacf9a92..cccfd3e8 100644
--- a/lib/microwaveprop/propagation.ex
+++ b/lib/microwaveprop/propagation.ex
@@ -90,10 +90,11 @@ defmodule Microwaveprop.Propagation do
       longitude: longitude,
       pressure_mb: hrrr_profile.surface_pressure_mb,
       prev_pressure_mb: nil,
-      rain_rate_mmhr: Scorer.precip_to_rate_mmhr(hrrr_profile[:precip_mm]),
+      rain_rate_mmhr: merged_rain_rate(hrrr_profile),
       min_refractivity_gradient: hrrr_profile[:native_min_gradient] || derived[:min_refractivity_gradient],
       bl_depth_m: hrrr_profile[:hpbl_m],
-      pwat_mm: hrrr_profile[:pwat_mm]
+      pwat_mm: hrrr_profile[:pwat_mm],
+      best_duct_band_ghz: hrrr_profile[:best_duct_freq_ghz] || hrrr_profile[:best_duct_band_ghz]
     }
 
     duct_info =
@@ -106,13 +107,45 @@ defmodule Microwaveprop.Propagation do
         }
       end
 
+    link_degradation = hrrr_profile[:commercial_link_degradation]
+
     Enum.map(BandConfig.all_bands(), fn band_config ->
       result = Scorer.composite_score(conditions, band_config)
-      result = Map.put(result, :band_mhz, band_config.freq_mhz)
+
+      boosted_score =
+        if link_degradation do
+          Scorer.commercial_link_boost(result.score, link_degradation)
+        else
+          result.score
+        end
+
+      result =
+        result
+        |> Map.put(:score, boosted_score)
+        |> Map.put(:band_mhz, band_config.freq_mhz)
+
+      result =
+        if link_degradation do
+          put_in(result, [:factors, :commercial_link_degradation], link_degradation)
+        else
+          result
+        end
+
       if duct_info, do: put_in(result, [:factors, :duct_info], duct_info), else: result
     end)
   end
 
+  # Pick the heavier of HRRR's hourly accumulation-derived rate and NEXRAD's
+  # reflectivity-derived rate. NEXRAD catches fast-moving convective cells that
+  # fall between HRRR hourly analyses; HRRR catches broad stratiform rain that
+  # NEXRAD reports as low dBZ. Taking max lets either source trigger the rain
+  # penalty without double-counting.
+  defp merged_rain_rate(hrrr_profile) do
+    hrrr_rate = Scorer.precip_to_rate_mmhr(hrrr_profile[:precip_mm])
+    nexrad_rate = Scorer.dbz_to_rain_rate_mmhr(hrrr_profile[:nexrad_max_reflectivity_dbz])
+    max(hrrr_rate, nexrad_rate)
+  end
+
   @doc """
   Upsert propagation scores in batches within a transaction so readers see all-or-nothing.
 
diff --git a/lib/microwaveprop/propagation/scorer.ex b/lib/microwaveprop/propagation/scorer.ex
index e180c188..b07c6863 100644
--- a/lib/microwaveprop/propagation/scorer.ex
+++ b/lib/microwaveprop/propagation/scorer.ex
@@ -50,6 +50,27 @@ defmodule Microwaveprop.Propagation.Scorer do
   def precip_to_rate_mmhr(mm) when mm > 0, do: mm / 1
   def precip_to_rate_mmhr(_mm), do: 0.0
 
+  @doc """
+  Converts NEXRAD composite reflectivity (dBZ) to rain rate (mm/hr) via the
+  Marshall-Palmer Z-R relationship `Z = 200 * R^1.6`, so `R = (Z/200)^(1/1.6)`
+  with `Z = 10^(dBZ/10)`.
+
+  Clipping rules:
+  * Below 5 dBZ — not rain (ground clutter / clear air). Returns 0.0.
+  * Above 150 mm/hr — hail contamination (55+ dBZ can return >100 mm/hr
+    from pure Marshall-Palmer). The ITU-R P.838 rain table tops out at
+    ~150 mm/hr so we clip there to stay inside the calibrated regime.
+  """
+  @spec dbz_to_rain_rate_mmhr(number() | nil) :: float()
+  def dbz_to_rain_rate_mmhr(nil), do: 0.0
+  def dbz_to_rain_rate_mmhr(dbz) when dbz < 5.0, do: 0.0
+
+  def dbz_to_rain_rate_mmhr(dbz) do
+    z = :math.pow(10, dbz / 10)
+    r = :math.pow(z / 200, 1 / 1.6)
+    min(r, 150.0)
+  end
+
   # ── Factor 1: Humidity ────────────────────────────────────────────
 
   @doc """
@@ -163,9 +184,27 @@ defmodule Microwaveprop.Propagation.Scorer do
   applied to *both* the threshold-matched score and the default fallback.
   """
   @spec score_refractivity(number() | nil, number() | nil, map()) :: integer()
-  def score_refractivity(nil, _bl_depth_m, _band_config), do: 50
+  def score_refractivity(min_gradient, bl_depth_m, band_config) do
+    score_refractivity(min_gradient, bl_depth_m, nil, band_config)
+  end
 
-  def score_refractivity(min_gradient, bl_depth_m, %{humidity_effect: effect}) do
+  @doc """
+  Scores refractivity with optional native-profile duct info.
+
+  `best_duct_band_ghz` comes from `hrrr_native_profiles` and represents the
+  highest frequency the cell's native-resolution duct can trap. When it's
+  ≥ the target band's frequency the base score is boosted 1.15× because
+  HRRR pressure-level gradients systematically under-read thin ducts the
+  native profile can resolve (see Part 2c Apr 13 2026 findings).
+
+  A duct that only supports lower frequencies does NOT boost the score —
+  it's a signal that the gradient we have is *all there is* at the target
+  band.
+  """
+  @spec score_refractivity(number() | nil, number() | nil, number() | nil, map()) :: integer()
+  def score_refractivity(nil, _bl_depth_m, _best_duct_band_ghz, _band_config), do: 50
+
+  def score_refractivity(min_gradient, bl_depth_m, best_duct_band_ghz, %{freq_mhz: freq_mhz, humidity_effect: effect}) do
     thresholds = BandConfig.refractivity_thresholds()
 
     base =
@@ -178,7 +217,9 @@ defmodule Microwaveprop.Propagation.Scorer do
           if effect == :beneficial, do: beneficial_default, else: harmful_default
       end
 
-    apply_hpbl_multiplier(base, bl_depth_m)
+    base
+    |> apply_hpbl_multiplier(bl_depth_m)
+    |> apply_native_duct_boost(best_duct_band_ghz, freq_mhz)
   end
 
   # HPBL multiplier — applied to the base refractivity score. Calibrated to the
@@ -192,6 +233,60 @@ defmodule Microwaveprop.Propagation.Scorer do
   defp apply_hpbl_multiplier(base, hpbl) when hpbl < 2000, do: clamp_score(base * 0.92)
   defp apply_hpbl_multiplier(base, _hpbl), do: clamp_score(base * 0.78)
 
+  # Native-profile duct boost — 1.15× when the cell's best duct supports the
+  # target band's frequency. HRRR pressure-level gradients systematically
+  # under-read thin ducts (~250m vertical spacing vs 50 native hybrid levels),
+  # so when hrrr_native_profiles reports a duct band at or above the target
+  # frequency, the base gradient score is under-estimating the real channel.
+  defp apply_native_duct_boost(base, nil, _freq_mhz), do: base
+
+  defp apply_native_duct_boost(base, best_duct_band_ghz, freq_mhz) do
+    target_ghz = freq_mhz / 1_000
+
+    if best_duct_band_ghz >= target_ghz do
+      clamp_score(base * 1.15)
+    else
+      base
+    end
+  end
+
+  @doc """
+  Inverse-sensor boost from commercial LOS link degradation.
+
+  Nearby short-path commercial links (11/24/68 GHz in the DFW area) act as a
+  physical refractivity anomaly sensor: when their rx_power drops 5+ dB below
+  the 7-day baseline *without* an equipment cause, the same multipath fading
+  that hurts their short Fresnel zone is what carries the beyond-LOS paths.
+  This is a terminal boost applied once per composite score, not per-band —
+  the underlying physics is band-agnostic at the resolution we can measure.
+
+  Takes the incoming composite (or any 0-100) score and a degradation map
+  from `Commercial.link_degradation_at/3`; returns a boosted integer score.
+
+  * nil or empty result (`n_links == 0`) — no-op
+  * <3 dB — noise floor, no-op
+  * 3–8 dB — mild boost (+0 to +10)
+  * ≥8 dB — strong boost (+10 to +25, clamped ≤ 100)
+  """
+  @spec commercial_link_boost(number(), map() | nil) :: integer()
+  def commercial_link_boost(score, nil), do: clamp_score(score)
+
+  def commercial_link_boost(score, %{n_links: 0}), do: clamp_score(score)
+
+  def commercial_link_boost(score, %{degradation_db: db}) when db < 3.0, do: clamp_score(score)
+
+  def commercial_link_boost(score, %{degradation_db: db}) when db < 8.0 do
+    # 3 dB → +2; 7.9 dB → ~+10 (linear interpolation)
+    bonus = 2 + (db - 3.0) * 8.0 / 5.0
+    clamp_score(score + bonus)
+  end
+
+  def commercial_link_boost(score, %{degradation_db: db}) do
+    # 8 dB → +10; 16 dB → +25 (linear)
+    bonus = min(25.0, 10 + (db - 8.0) * 15.0 / 8.0)
+    clamp_score(score + bonus)
+  end
+
   defp clamp_score(value) do
     value |> round() |> max(0) |> min(100)
   end
@@ -381,6 +476,7 @@ defmodule Microwaveprop.Propagation.Scorer do
         score_refractivity(
           conditions.min_refractivity_gradient,
           conditions.bl_depth_m,
+          conditions[:best_duct_band_ghz],
           band_config
         ),
       sky: score_sky(conditions.sky_cover_pct),
diff --git a/lib/microwaveprop/workers/propagation_grid_worker.ex b/lib/microwaveprop/workers/propagation_grid_worker.ex
index 21e3cbc2..5405accb 100644
--- a/lib/microwaveprop/workers/propagation_grid_worker.ex
+++ b/lib/microwaveprop/workers/propagation_grid_worker.ex
@@ -12,6 +12,7 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
     max_attempts: 3,
     unique: [period: 10_800, states: [:available, :scheduled, :executing, :retryable]]
 
+  alias Microwaveprop.Commercial
   alias Microwaveprop.Propagation
   alias Microwaveprop.Propagation.BandConfig
   alias Microwaveprop.Propagation.Grid
@@ -20,6 +21,7 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
   alias Microwaveprop.Weather.GridCache
   alias Microwaveprop.Weather.HrrrClient
   alias Microwaveprop.Weather.HrrrNativeClient
+  alias Microwaveprop.Weather.NexradClient
   alias Microwaveprop.Weather.SoundingParams
 
   require Logger
@@ -93,6 +95,27 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
         grid_data = merge_native_duct_data(grid_data, run_time, forecast_hour)
         :erlang.garbage_collect()
 
+        # NEXRAD current-hour composite reflectivity catches fast-moving
+        # convective cells between HRRR hourly analyses. Only useful for
+        # f00 — forecast hours can't see the future radar image.
+        grid_data =
+          if forecast_hour == 0 do
+            merge_nexrad_data(grid_data, valid_time)
+          else
+            grid_data
+          end
+
+        # Commercial-link inverse sensor — only meaningful for f00 because
+        # the measurement is of the current atmospheric state, not a forecast.
+        grid_data =
+          if forecast_hour == 0 do
+            merge_commercial_link_data(grid_data, valid_time)
+          else
+            grid_data
+          end
+
+        :erlang.garbage_collect()
+
         # Build weather cache rows from in-memory grid_data — avoids the ~20s
         # JSONB round trip that was crashing the worker before f01–f18 could run.
         rows = Weather.build_grid_cache_rows(grid_data, valid_time)
@@ -224,6 +247,54 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
     end)
   end
 
+  defp merge_commercial_link_data(grid_data, valid_time) do
+    # Compute link degradation once per distinct link cluster and cache by
+    # point. Commercial links only cluster around DFW so most grid points see
+    # nil — cheap no-op path dominates.
+    boosted =
+      Enum.count(grid_data, fn {{lat, lon}, _profile} ->
+        degradation = Commercial.link_degradation_at({lat, lon}, valid_time)
+        not is_nil(degradation)
+      end)
+
+    if boosted > 0 do
+      Logger.info("PropagationGrid: commercial-link degradation available for #{boosted} grid cells")
+    end
+
+    Map.new(grid_data, fn {{lat, lon} = point, profile} ->
+      case Commercial.link_degradation_at({lat, lon}, valid_time) do
+        nil -> {point, profile}
+        degradation -> {point, Map.put(profile, :commercial_link_degradation, degradation)}
+      end
+    end)
+  end
+
+  defp merge_nexrad_data(grid_data, valid_time) do
+    points = Map.keys(grid_data)
+
+    case timed("nexrad", fn -> NexradClient.fetch_frame(valid_time, points) end) do
+      {:ok, observations} ->
+        apply_nexrad_observations(grid_data, observations)
+
+      {:error, reason} ->
+        Logger.warning("PropagationGrid: NEXRAD fetch failed (continuing without): #{inspect(reason)}")
+        grid_data
+    end
+  end
+
+  defp apply_nexrad_observations(grid_data, observations) do
+    index = Map.new(observations, fn obs -> {{obs.lat, obs.lon}, obs.max_reflectivity_dbz} end)
+    non_zero = Enum.count(index, fn {_pt, dbz} -> dbz > 0 end)
+    Logger.info("PropagationGrid: NEXRAD merged (#{non_zero} cells with precip)")
+
+    Map.new(grid_data, fn {point, profile} ->
+      case Map.get(index, point) do
+        nil -> {point, profile}
+        dbz -> {point, Map.put(profile, :nexrad_max_reflectivity_dbz, dbz)}
+      end
+    end)
+  end
+
   defp compute_scores(grid_data, valid_time) do
     # Algorithm is the primary scorer. ML score stored in factors for comparison.
     compute_scores_algorithm(grid_data, valid_time)
diff --git a/test/microwaveprop/commercial_test.exs b/test/microwaveprop/commercial_test.exs
index 47c35918..4f86f9c4 100644
--- a/test/microwaveprop/commercial_test.exs
+++ b/test/microwaveprop/commercial_test.exs
@@ -65,6 +65,123 @@ defmodule Microwaveprop.CommercialTest do
     end
   end
 
+  describe "link_degradation_at/3" do
+    @endpoint_dfw %{"lat" => 33.2, "lon" => -96.5}
+    @far_point {0.0, 0.0}
+    @near_point {33.2, -96.5}
+
+    test "returns nil when no links are within the radius" do
+      {:ok, _} =
+        Commercial.create_link(
+          Map.merge(@link_attrs, %{
+            endpoint_a: @endpoint_dfw,
+            endpoint_b: @endpoint_dfw
+          })
+        )
+
+      valid_time = ~U[2026-04-13 18:00:00Z]
+      assert Commercial.link_degradation_at(@far_point, valid_time, radius_km: 50) == nil
+    end
+
+    test "returns nil when the link has no recent baseline samples" do
+      {:ok, _link} =
+        Commercial.create_link(Map.merge(@link_attrs, %{endpoint_a: @endpoint_dfw, endpoint_b: @endpoint_dfw}))
+
+      valid_time = ~U[2026-04-13 18:00:00Z]
+      assert Commercial.link_degradation_at(@near_point, valid_time) == nil
+    end
+
+    test "returns positive degradation when current rx_power is below baseline" do
+      {:ok, link} =
+        Commercial.create_link(Map.merge(@link_attrs, %{endpoint_a: @endpoint_dfw, endpoint_b: @endpoint_dfw}))
+
+      valid_time = ~U[2026-04-13 18:00:00Z]
+
+      # 7-day baseline: -52 dBm average
+      for day_offset <- 1..6 do
+        {:ok, _} =
+          Commercial.create_sample(%{
+            link_id: link.id,
+            sampled_at: DateTime.add(valid_time, -day_offset * 3600 * 24, :second),
+            rx_power_0: -52.0,
+            rx_power_1: -52.0,
+            link_state: 1
+          })
+      end
+
+      # Current (within 15 min) drops to -62 — 10 dB deep fade.
+      {:ok, _} =
+        Commercial.create_sample(%{
+          link_id: link.id,
+          sampled_at: DateTime.add(valid_time, -300, :second),
+          rx_power_0: -62.0,
+          rx_power_1: -62.0,
+          link_state: 1
+        })
+
+      result = Commercial.link_degradation_at(@near_point, valid_time)
+      assert result
+      assert_in_delta result.degradation_db, 10.0, 0.5
+      assert result.n_links == 1
+    end
+
+    test "returns 0 degradation when rx_power matches baseline" do
+      {:ok, link} =
+        Commercial.create_link(Map.merge(@link_attrs, %{endpoint_a: @endpoint_dfw, endpoint_b: @endpoint_dfw}))
+
+      valid_time = ~U[2026-04-13 18:00:00Z]
+
+      for day_offset <- 1..6 do
+        {:ok, _} =
+          Commercial.create_sample(%{
+            link_id: link.id,
+            sampled_at: DateTime.add(valid_time, -day_offset * 3600 * 24, :second),
+            rx_power_0: -55.0,
+            link_state: 1
+          })
+      end
+
+      {:ok, _} =
+        Commercial.create_sample(%{
+          link_id: link.id,
+          sampled_at: DateTime.add(valid_time, -60, :second),
+          rx_power_0: -55.0,
+          link_state: 1
+        })
+
+      result = Commercial.link_degradation_at(@near_point, valid_time)
+      assert result
+      assert_in_delta result.degradation_db, 0.0, 0.5
+    end
+
+    test "ignores links with link_state != 1 (down link isn't tropo)" do
+      {:ok, link} =
+        Commercial.create_link(Map.merge(@link_attrs, %{endpoint_a: @endpoint_dfw, endpoint_b: @endpoint_dfw}))
+
+      valid_time = ~U[2026-04-13 18:00:00Z]
+
+      for day_offset <- 1..6 do
+        {:ok, _} =
+          Commercial.create_sample(%{
+            link_id: link.id,
+            sampled_at: DateTime.add(valid_time, -day_offset * 3600 * 24, :second),
+            rx_power_0: -52.0,
+            link_state: 1
+          })
+      end
+
+      {:ok, _} =
+        Commercial.create_sample(%{
+          link_id: link.id,
+          sampled_at: DateTime.add(valid_time, -120, :second),
+          rx_power_0: -90.0,
+          link_state: 0
+        })
+
+      assert Commercial.link_degradation_at(@near_point, valid_time) == nil
+    end
+  end
+
   describe "weather_stations/0" do
     test "returns unique weather stations from enabled links" do
       {:ok, _} = Commercial.create_link(@link_attrs)
diff --git a/test/microwaveprop/propagation/scorer_test.exs b/test/microwaveprop/propagation/scorer_test.exs
index b10afa23..ba0e6820 100644
--- a/test/microwaveprop/propagation/scorer_test.exs
+++ b/test/microwaveprop/propagation/scorer_test.exs
@@ -86,6 +86,44 @@ defmodule Microwaveprop.Propagation.ScorerTest do
     end
   end
 
+  describe "dbz_to_rain_rate_mmhr/1" do
+    test "returns 0.0 for nil" do
+      assert Scorer.dbz_to_rain_rate_mmhr(nil) == 0.0
+    end
+
+    test "returns 0.0 for sub-rain reflectivity (below 5 dBZ)" do
+      # Clear air / ground clutter — not rain.
+      assert Scorer.dbz_to_rain_rate_mmhr(3.0) == 0.0
+      assert Scorer.dbz_to_rain_rate_mmhr(0.0) == 0.0
+    end
+
+    test "20 dBZ is light rain ~0.7 mm/hr (Marshall-Palmer)" do
+      # Z = 10^(dbz/10) = 100; R = (100/200)^(1/1.6) ≈ 0.66 mm/hr
+      rate = Scorer.dbz_to_rain_rate_mmhr(20.0)
+      assert_in_delta rate, 0.66, 0.05
+    end
+
+    test "30 dBZ is moderate rain ~2.7 mm/hr" do
+      # Z = 1000; R = (1000/200)^(1/1.6) ≈ 2.7 mm/hr
+      rate = Scorer.dbz_to_rain_rate_mmhr(30.0)
+      assert_in_delta rate, 2.7, 0.2
+    end
+
+    test "45 dBZ is heavy rain ~24 mm/hr" do
+      # Z = 31623; R = (31623/200)^(1/1.6) ≈ 23.7 mm/hr
+      rate = Scorer.dbz_to_rain_rate_mmhr(45.0)
+      assert_in_delta rate, 23.7, 1.0
+    end
+
+    test "55 dBZ clips to a hail-safe ceiling (≤150 mm/hr)" do
+      # Pure Marshall-Palmer at 55 dBZ returns ~116 mm/hr; at 60 it blows past
+      # 200 which is unrealistic — hail contamination. Clip to the ITU rain-rate
+      # tabulated range.
+      assert Scorer.dbz_to_rain_rate_mmhr(55.0) <= 150.0
+      assert Scorer.dbz_to_rain_rate_mmhr(65.0) <= 150.0
+    end
+  end
+
   # ── score_humidity/2 ──────────────────────────────────────────────
 
   describe "score_humidity/2 beneficial (10 GHz)" do
@@ -285,6 +323,80 @@ defmodule Microwaveprop.Propagation.ScorerTest do
     end
   end
 
+  describe "score_refractivity/4 with native-profile duct support" do
+    # New 4-arity variant: adds best_duct_band_ghz from hrrr_native_profiles so
+    # cells with a thin native duct that supports the target band get a boost
+    # the HRRR pressure-level gradient alone would miss. Cells where the native
+    # duct only supports sub-band frequencies do NOT get a boost.
+
+    test "native duct supporting the target band boosts the score" do
+      boosted = Scorer.score_refractivity(-80, 800, 15.0, @band_10g)
+      plain = Scorer.score_refractivity(-80, 800, nil, @band_10g)
+      assert boosted > plain
+      assert boosted <= 100
+    end
+
+    test "native duct below the target band does NOT boost" do
+      # A 3 GHz duct is useless for 10 GHz.
+      unchanged = Scorer.score_refractivity(-80, 800, 3.0, @band_10g)
+      plain = Scorer.score_refractivity(-80, 800, nil, @band_10g)
+      assert unchanged == plain
+    end
+
+    test "nil best_duct_band_ghz is equivalent to the 3-arity form" do
+      assert Scorer.score_refractivity(-100, 600, nil, @band_10g) ==
+               Scorer.score_refractivity(-100, 600, @band_10g)
+    end
+
+    test "nil gradient still returns 50 regardless of duct data" do
+      assert Scorer.score_refractivity(nil, 600, 15.0, @band_10g) == 50
+    end
+
+    test "24 GHz is boosted only when native duct supports ≥24 GHz" do
+      weak = Scorer.score_refractivity(-80, 800, 15.0, @band_24g)
+      strong = Scorer.score_refractivity(-80, 800, 30.0, @band_24g)
+      plain = Scorer.score_refractivity(-80, 800, nil, @band_24g)
+      assert weak == plain
+      assert strong > plain
+    end
+  end
+
+  describe "commercial_link_boost/2" do
+    # Inverse sensor: when nearby commercial LOS links are fading below baseline,
+    # the *same* multipath that hurts them is the enhanced refractivity that
+    # helps beyond-LOS amateur paths. >8 dB of degradation means multi-Fresnel-
+    # zone disturbance — strong boost. 3-8 dB is a mild boost. <3 dB is noise.
+
+    test "nil degradation is a no-op" do
+      assert Scorer.commercial_link_boost(75, nil) == 75
+    end
+
+    test "<3 dB fade is noise and does not boost" do
+      assert Scorer.commercial_link_boost(75, %{degradation_db: 2.0, n_links: 3}) == 75
+    end
+
+    test "5 dB fade applies a mild boost" do
+      boosted = Scorer.commercial_link_boost(75, %{degradation_db: 5.0, n_links: 3})
+      assert boosted > 75
+      assert boosted < 90
+    end
+
+    test "12 dB fade applies a large boost capped at 100" do
+      boosted = Scorer.commercial_link_boost(75, %{degradation_db: 12.0, n_links: 3})
+      assert boosted > 85
+      assert boosted <= 100
+    end
+
+    test "single-link result still boosts (n_links >= 1)" do
+      boosted = Scorer.commercial_link_boost(70, %{degradation_db: 6.0, n_links: 1})
+      assert boosted > 70
+    end
+
+    test "empty-links (n_links == 0) is a no-op" do
+      assert Scorer.commercial_link_boost(75, %{degradation_db: 10.0, n_links: 0}) == 75
+    end
+  end
+
   # ── score_sky/1 ──────────────────────────────────────────────────
 
   describe "score_sky/1" do
diff --git a/test/microwaveprop/propagation_test.exs b/test/microwaveprop/propagation_test.exs
index 130ea54d..ba3c886f 100644
--- a/test/microwaveprop/propagation_test.exs
+++ b/test/microwaveprop/propagation_test.exs
@@ -39,6 +39,72 @@ defmodule Microwaveprop.PropagationTest do
         assert is_integer(result.band_mhz)
       end)
     end
+
+    test "NEXRAD reflectivity drops the 24 GHz rain score when HRRR precip_mm is 0" do
+      # Same profile, once without NEXRAD and once with a heavy-rain dBZ. The
+      # 24 GHz rain-score factor must drop because NEXRAD can report rain that
+      # HRRR's hourly precip accumulation hasn't caught yet.
+      base_profile = %{
+        surface_temp_c: 25.0,
+        surface_dewpoint_c: 18.0,
+        surface_pressure_mb: 1013.0,
+        hpbl_m: 500.0,
+        wind_u: 3.0,
+        wind_v: 2.0,
+        cloud_cover_pct: 15.0,
+        precip_mm: 0.0,
+        profile: [
+          %{"pres" => 1000.0, "tmpc" => 25.0, "dwpc" => 18.0, "hght" => 100.0},
+          %{"pres" => 950.0, "tmpc" => 19.0, "dwpc" => 10.0, "hght" => 600.0}
+        ]
+      }
+
+      valid_time = ~U[2026-07-15 13:00:00Z]
+
+      without = Propagation.score_grid_point(base_profile, valid_time, 33.0, -97.0)
+
+      with_nexrad =
+        base_profile
+        |> Map.put(:nexrad_max_reflectivity_dbz, 45.0)
+        |> Propagation.score_grid_point(valid_time, 33.0, -97.0)
+
+      rain_dry = Enum.find(without, &(&1.band_mhz == 24_000)).factors.rain
+      rain_wet = Enum.find(with_nexrad, &(&1.band_mhz == 24_000)).factors.rain
+
+      assert rain_wet < rain_dry
+    end
+
+    test "NEXRAD is ignored when HRRR precip_mm already reports heavier rain" do
+      base_profile = %{
+        surface_temp_c: 25.0,
+        surface_dewpoint_c: 18.0,
+        surface_pressure_mb: 1013.0,
+        hpbl_m: 500.0,
+        wind_u: 3.0,
+        wind_v: 2.0,
+        cloud_cover_pct: 15.0,
+        # 30 mm/hr → far above the ~3 mm/hr NEXRAD gives for 30 dBZ, so HRRR wins.
+        precip_mm: 30.0,
+        profile: [
+          %{"pres" => 1000.0, "tmpc" => 25.0, "dwpc" => 18.0, "hght" => 100.0},
+          %{"pres" => 950.0, "tmpc" => 19.0, "dwpc" => 10.0, "hght" => 600.0}
+        ]
+      }
+
+      valid_time = ~U[2026-07-15 13:00:00Z]
+
+      with_light_nexrad =
+        base_profile
+        |> Map.put(:nexrad_max_reflectivity_dbz, 30.0)
+        |> Propagation.score_grid_point(valid_time, 33.0, -97.0)
+
+      without = Propagation.score_grid_point(base_profile, valid_time, 33.0, -97.0)
+
+      rain_no_nx = Enum.find(without, &(&1.band_mhz == 24_000)).factors.rain
+      rain_with_nx = Enum.find(with_light_nexrad, &(&1.band_mhz == 24_000)).factors.rain
+
+      assert rain_with_nx == rain_no_nx
+    end
   end
 
   describe "upsert_scores/1" do
diff --git a/test/microwaveprop/weather/era5_batch_client_test.exs b/test/microwaveprop/weather/era5_batch_client_test.exs
index 9197c796..47f0a75c 100644
--- a/test/microwaveprop/weather/era5_batch_client_test.exs
+++ b/test/microwaveprop/weather/era5_batch_client_test.exs
@@ -29,6 +29,21 @@ defmodule Microwaveprop.Weather.Era5BatchClientTest do
   end
 
   describe "fetch_month_into_db/1 idempotency" do
+    # This describe block exercises the uncached path which would otherwise
+    # hit the real CDS API if the developer has ERA5_CDS_API_KEY set via
+    # direnv. Clear the env var for these tests so the expected "API key not
+    # configured" short-circuit fires and the tests stay hermetic.
+    setup do
+      previous = System.get_env("ERA5_CDS_API_KEY")
+      System.delete_env("ERA5_CDS_API_KEY")
+
+      on_exit(fn ->
+        if previous, do: System.put_env("ERA5_CDS_API_KEY", previous)
+      end)
+
+      :ok
+    end
+
     test "returns {:ok, 0} when the month-tile already has any profile" do
       # Seed one profile inside the 2014-03, tile (32, -98) window.
       %Era5Profile{}