fix: derive feels-like when NWS observation omits it (#721)

src/accessiweather/thermal_comfort.py

@@ -11,6 +11,8 @@
 APPARENT_TEMPERATURE_SOLAR_ALLOWANCE_PER_DEGREE_F = 0.15
 HEAT_INDEX_MIN_TEMP_F = 80.0
 HEAT_INDEX_MIN_HUMIDITY = 40
+WIND_CHILL_MAX_TEMP_F = 50.0
+WIND_CHILL_MIN_WIND_MPH = 3.0
 
 
 @dataclass(frozen=True)
@@ -115,6 +117,20 @@ def calculate_heat_index_f(temperature_f: float, humidity: int | float) -> float
     return heat_index
 
 
+def calculate_wind_chill_f(temperature_f: float, wind_speed_mph: float) -> float | None:
+    """Calculate the standard NWS wind chill in Fahrenheit when applicable."""
+    if temperature_f > WIND_CHILL_MAX_TEMP_F or wind_speed_mph <= WIND_CHILL_MIN_WIND_MPH:
+        return None
+
+    wind_factor = wind_speed_mph**0.16
+    return (
+        35.74
+        + (0.6215 * temperature_f)
+        - (35.75 * wind_factor)
+        + (0.4275 * temperature_f * wind_factor)
+    )
+
+
 def warm_apparent_temperature_is_coherent(
     temperature_f: float | None,
     humidity: int | float | None,

src/accessiweather/weather_client_nws_parsers.py

@@ -8,7 +8,11 @@
     normalize_pressure_pair,
     normalize_temperature_pair,
 )
-from .thermal_comfort import sanitize_thermal_comfort_readings
+from .thermal_comfort import (
+    calculate_heat_index_f,
+    calculate_wind_chill_f,
+    sanitize_thermal_comfort_readings,
+)
 from .weather_client_nws_common import *  # noqa: F403
 from .weather_client_parsers import normalize_pressure
 
@@ -81,13 +85,26 @@ def parse_nws_current_conditions(
         props.get("heatIndex", {}).get("unitCode") or "wmoUnit:degC",
     )
 
+    derived_wind_chill_f = None
+    if wind_chill.fahrenheit is None and temperature.fahrenheit is not None and wind_speed_mph is not None:
+        derived_wind_chill_f = calculate_wind_chill_f(temperature.fahrenheit, wind_speed_mph)
+
+    derived_heat_index_f = None
+    if heat_index.fahrenheit is None and temperature.fahrenheit is not None and humidity is not None:
+        derived_heat_index_f = calculate_heat_index_f(temperature.fahrenheit, humidity)
+
+    derived_feels_like_f = (
+        derived_wind_chill_f if derived_wind_chill_f is not None else derived_heat_index_f
+    )
+
     comfort = sanitize_thermal_comfort_readings(
         temperature_f=temperature.fahrenheit,
         temperature_c=temperature.celsius,
         humidity=humidity,
-        wind_chill_f=wind_chill.fahrenheit,
+        feels_like_f=derived_feels_like_f,
+        wind_chill_f=wind_chill.fahrenheit if wind_chill.fahrenheit is not None else derived_wind_chill_f,
         wind_chill_c=wind_chill.celsius,
-        heat_index_f=heat_index.fahrenheit,
+        heat_index_f=heat_index.fahrenheit if heat_index.fahrenheit is not None else derived_heat_index_f,
         heat_index_c=heat_index.celsius,
     )
 

tests/test_nws_current_parser.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from accessiweather.thermal_comfort import calculate_heat_index_f, calculate_wind_chill_f
 from accessiweather.weather_client_nws import parse_nws_current_conditions
 
 
@@ -42,3 +43,45 @@ def test_parse_nws_current_conditions_normalizes_measurements_to_current_model()
     assert current.heat_index_f is None
     assert current.condition == "Mostly Cloudy"
     assert current.uv_index == 2.5
+
+
+def test_parse_nws_current_conditions_derives_wind_chill_when_field_missing():
+    payload = {
+        "properties": {
+            "temperature": {"value": 0.0, "unitCode": "wmoUnit:degC"},
+            "relativeHumidity": {"value": 72.4, "unitCode": "wmoUnit:percent"},
+            "windSpeed": {"value": 16.09344, "unitCode": "wmoUnit:km_h-1"},
+            "windChill": {"value": None, "unitCode": "wmoUnit:degC"},
+            "heatIndex": {"value": None, "unitCode": "wmoUnit:degC"},
+            "textDescription": "Mostly Cloudy",
+        }
+    }
+
+    current = parse_nws_current_conditions(payload)
+
+    expected = calculate_wind_chill_f(32.0, 10.0)
+    assert expected is not None
+    assert current.feels_like_f == pytest.approx(expected, abs=0.1)
+    assert current.wind_chill_f == pytest.approx(expected, abs=0.1)
+    assert current.heat_index_f is None
+
+
+def test_parse_nws_current_conditions_derives_heat_index_when_field_missing():
+    payload = {
+        "properties": {
+            "temperature": {"value": 32.2222222, "unitCode": "wmoUnit:degC"},
+            "relativeHumidity": {"value": 70.0, "unitCode": "wmoUnit:percent"},
+            "windSpeed": {"value": 7.0, "unitCode": "wmoUnit:mi_h-1"},
+            "windChill": {"value": None, "unitCode": "wmoUnit:degC"},
+            "heatIndex": {"value": None, "unitCode": "wmoUnit:degC"},
+            "textDescription": "Hot",
+        }
+    }
+
+    current = parse_nws_current_conditions(payload)
+
+    expected = calculate_heat_index_f(90.0, 70)
+    assert expected is not None
+    assert current.feels_like_f == pytest.approx(expected, abs=0.1)
+    assert current.heat_index_f == pytest.approx(expected, abs=0.1)
+    assert current.wind_chill_f is None