diff --git a/lib/microwaveprop/weather/weather_layers.ex b/lib/microwaveprop/weather/weather_layers.ex
new file mode 100644
index 00000000..6325114c
--- /dev/null
+++ b/lib/microwaveprop/weather/weather_layers.ex
@@ -0,0 +1,123 @@
+defmodule Microwaveprop.Weather.WeatherLayers do
+  @moduledoc "Derives scalar weather map layers from HRRR profile data."
+
+  alias Microwaveprop.Weather.SoundingParams
+
+  @doc """
+  Derives a map of scalar weather fields from a row containing HRRR profile data.
+
+  Expects a map with keys: `:surface_temp_c`, `:surface_dewpoint_c`,
+  `:surface_pressure_mb`, `:surface_refractivity`, `:profile`, `:duct_characteristics`.
+
+  Returns a map with derived fields: `:surface_rh`, `:surface_refractivity`,
+  `:temp_850mb`, `:dewpoint_850mb`, `:lapse_rate`, `:inversion_strength`,
+  `:inversion_base_m`, `:duct_base_m`, `:duct_strength`.
+  """
+  def derive(row) do
+    sorted = sort_profile(row.profile)
+
+    %{
+      surface_rh: compute_rh(row.surface_temp_c, row.surface_dewpoint_c),
+      surface_refractivity: row.surface_refractivity,
+      temp_850mb: level_value(sorted, 850, "tmpc"),
+      dewpoint_850mb: level_value(sorted, 850, "dwpc"),
+      lapse_rate: compute_lapse_rate(sorted),
+      inversion_strength: inversion_strength(sorted),
+      inversion_base_m: inversion_base_m(sorted),
+      duct_base_m: duct_field(row.duct_characteristics, "base"),
+      duct_strength: duct_field(row.duct_characteristics, "strength")
+    }
+  end
+
+  defp sort_profile(nil), do: []
+  defp sort_profile([]), do: []
+
+  defp sort_profile(profile) do
+    profile
+    |> Enum.filter(fn p -> p["pres"] != nil and p["tmpc"] != nil and p["hght"] != nil end)
+    |> Enum.sort_by(fn p -> p["pres"] end, :desc)
+  end
+
+  defp compute_rh(t_c, td_c) do
+    100.0 * SoundingParams.sat_vap_pres(td_c) / SoundingParams.sat_vap_pres(t_c)
+  end
+
+  defp level_value([], _target_pres, _key), do: nil
+
+  defp level_value(sorted, target_pres, key) do
+    nearest = Enum.min_by(sorted, fn p -> abs(p["pres"] - target_pres) end)
+
+    if abs(nearest["pres"] - target_pres) <= 25.0 do
+      nearest[key]
+    end
+  end
+
+  defp compute_lapse_rate([]), do: nil
+
+  defp compute_lapse_rate(sorted) when length(sorted) < 2, do: nil
+
+  defp compute_lapse_rate(sorted) do
+    surface = hd(sorted)
+    top = List.last(sorted)
+    dh_km = (top["hght"] - surface["hght"]) / 1000.0
+
+    if dh_km > 0 do
+      (surface["tmpc"] - top["tmpc"]) / dh_km
+    end
+  end
+
+  defp inversion_strength([]), do: 0.0
+
+  defp inversion_strength(sorted) do
+    sorted
+    |> Enum.chunk_every(2, 1, :discard)
+    |> Enum.reduce(0.0, fn [lower, upper], max_str ->
+      dt = upper["tmpc"] - lower["tmpc"]
+
+      if dt > max_str do
+        dt
+      else
+        max_str
+      end
+    end)
+  end
+
+  defp inversion_base_m([]), do: nil
+
+  defp inversion_base_m(sorted) do
+    sfc_hght = hd(sorted)["hght"]
+
+    result =
+      sorted
+      |> Enum.chunk_every(2, 1, :discard)
+      |> Enum.reduce({0.0, nil}, fn [lower, upper], {max_str, _base} = acc ->
+        dt = upper["tmpc"] - lower["tmpc"]
+
+        if dt > 0 and dt > max_str do
+          {dt, lower["hght"] - sfc_hght}
+        else
+          acc
+        end
+      end)
+
+    case result do
+      {str, base} when str > 0 -> base
+      _ -> nil
+    end
+  end
+
+  defp duct_field(nil, _key), do: nil
+  defp duct_field([], _key), do: nil
+
+  defp duct_field(ducts, "base") do
+    ducts
+    |> Enum.map(fn d -> d["base"] end)
+    |> Enum.min()
+  end
+
+  defp duct_field(ducts, "strength") do
+    ducts
+    |> Enum.map(fn d -> d["strength"] end)
+    |> Enum.max()
+  end
+end
diff --git a/test/microwaveprop/weather/weather_layers_test.exs b/test/microwaveprop/weather/weather_layers_test.exs
new file mode 100644
index 00000000..923c9f0b
--- /dev/null
+++ b/test/microwaveprop/weather/weather_layers_test.exs
@@ -0,0 +1,143 @@
+defmodule Microwaveprop.Weather.WeatherLayersTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.SoundingParams
+  alias Microwaveprop.Weather.WeatherLayers
+
+  @sample_profile [
+    %{"pres" => 1000.0, "tmpc" => 28.0, "dwpc" => 20.0, "hght" => 100.0},
+    %{"pres" => 975.0, "tmpc" => 26.0, "dwpc" => 18.0, "hght" => 340.0},
+    %{"pres" => 950.0, "tmpc" => 24.0, "dwpc" => 16.0, "hght" => 580.0},
+    %{"pres" => 925.0, "tmpc" => 22.0, "dwpc" => 14.0, "hght" => 830.0},
+    %{"pres" => 900.0, "tmpc" => 20.0, "dwpc" => 12.0, "hght" => 1090.0},
+    %{"pres" => 875.0, "tmpc" => 18.0, "dwpc" => 10.0, "hght" => 1360.0},
+    %{"pres" => 850.0, "tmpc" => 16.0, "dwpc" => 8.0, "hght" => 1640.0},
+    %{"pres" => 825.0, "tmpc" => 14.0, "dwpc" => 6.0, "hght" => 1930.0},
+    %{"pres" => 800.0, "tmpc" => 12.0, "dwpc" => 4.0, "hght" => 2230.0},
+    %{"pres" => 775.0, "tmpc" => 10.0, "dwpc" => 2.0, "hght" => 2540.0},
+    %{"pres" => 750.0, "tmpc" => 8.0, "dwpc" => 0.0, "hght" => 2860.0},
+    %{"pres" => 725.0, "tmpc" => 6.0, "dwpc" => -2.0, "hght" => 3200.0},
+    %{"pres" => 700.0, "tmpc" => 4.0, "dwpc" => -4.0, "hght" => 3550.0}
+  ]
+
+  @inversion_profile [
+    %{"pres" => 1000.0, "tmpc" => 28.0, "dwpc" => 20.0, "hght" => 100.0},
+    %{"pres" => 975.0, "tmpc" => 26.0, "dwpc" => 18.0, "hght" => 340.0},
+    %{"pres" => 950.0, "tmpc" => 22.0, "dwpc" => 14.0, "hght" => 580.0},
+    %{"pres" => 925.0, "tmpc" => 25.0, "dwpc" => 10.0, "hght" => 830.0},
+    %{"pres" => 900.0, "tmpc" => 20.0, "dwpc" => 8.0, "hght" => 1090.0}
+  ]
+
+  defp sample_row(overrides \\ %{}) do
+    Map.merge(
+      %{
+        surface_temp_c: 28.0,
+        surface_dewpoint_c: 20.0,
+        surface_pressure_mb: 1000.0,
+        surface_refractivity: 350.0,
+        profile: @sample_profile,
+        duct_characteristics: nil
+      },
+      overrides
+    )
+  end
+
+  describe "derive/1 surface relative humidity" do
+    test "computes surface RH from temp and dewpoint using Buck equation" do
+      result = WeatherLayers.derive(sample_row())
+
+      expected_rh = 100.0 * SoundingParams.sat_vap_pres(20.0) / SoundingParams.sat_vap_pres(28.0)
+      assert_in_delta result.surface_rh, expected_rh, 0.01
+    end
+  end
+
+  describe "derive/1 850mb extraction" do
+    test "extracts temperature at 850mb from profile" do
+      result = WeatherLayers.derive(sample_row())
+      assert result.temp_850mb == 16.0
+    end
+
+    test "extracts dewpoint at 850mb from profile" do
+      result = WeatherLayers.derive(sample_row())
+      assert result.dewpoint_850mb == 8.0
+    end
+  end
+
+  describe "derive/1 lapse rate" do
+    test "computes lapse rate in C/km from surface to top of profile" do
+      result = WeatherLayers.derive(sample_row())
+
+      # (28.0 - 4.0) / ((3550.0 - 100.0) / 1000.0) = 24.0 / 3.45 = ~6.957
+      expected = (28.0 - 4.0) / ((3550.0 - 100.0) / 1000.0)
+      assert_in_delta result.lapse_rate, expected, 0.01
+    end
+  end
+
+  describe "derive/1 inversion detection" do
+    test "detects no inversion in standard lapse profile" do
+      result = WeatherLayers.derive(sample_row())
+      assert result.inversion_strength == 0.0
+    end
+
+    test "detects inversion strength and base height" do
+      result = WeatherLayers.derive(sample_row(%{profile: @inversion_profile}))
+
+      # Inversion: 950mb (22C, 580m) -> 925mb (25C, 830m), strength = 3.0
+      assert_in_delta result.inversion_strength, 3.0, 0.01
+      # Base height AGL: 580m - 100m (surface) = 480m
+      assert_in_delta result.inversion_base_m, 480.0, 0.01
+    end
+  end
+
+  describe "derive/1 duct characteristics" do
+    test "extracts duct base and strength from duct_characteristics" do
+      ducts = [
+        %{"base" => 500.0, "top" => 800.0, "strength" => 12.5},
+        %{"base" => 200.0, "top" => 400.0, "strength" => 8.0}
+      ]
+
+      result = WeatherLayers.derive(sample_row(%{duct_characteristics: ducts}))
+
+      # min base
+      assert result.duct_base_m == 200.0
+      # max strength
+      assert result.duct_strength == 12.5
+    end
+
+    test "handles nil duct_characteristics" do
+      result = WeatherLayers.derive(sample_row(%{duct_characteristics: nil}))
+
+      assert result.duct_base_m == nil
+      assert result.duct_strength == nil
+    end
+  end
+
+  describe "derive/1 nil/empty profile" do
+    test "handles nil profile gracefully" do
+      result = WeatherLayers.derive(sample_row(%{profile: nil}))
+
+      assert result.temp_850mb == nil
+      assert result.dewpoint_850mb == nil
+      assert result.lapse_rate == nil
+      assert result.inversion_strength == 0.0
+      assert result.inversion_base_m == nil
+    end
+
+    test "handles empty profile gracefully" do
+      result = WeatherLayers.derive(sample_row(%{profile: []}))
+
+      assert result.temp_850mb == nil
+      assert result.dewpoint_850mb == nil
+      assert result.lapse_rate == nil
+      assert result.inversion_strength == 0.0
+      assert result.inversion_base_m == nil
+    end
+  end
+
+  describe "derive/1 surface refractivity passthrough" do
+    test "passes through surface_refractivity unchanged" do
+      result = WeatherLayers.derive(sample_row(%{surface_refractivity: 342.7}))
+      assert result.surface_refractivity == 342.7
+    end
+  end
+end