diff --git a/internal/normalizers/nws/day_night.go b/internal/normalizers/nws/day_night.go
new file mode 100644
index 0000000..6a3efe6
--- /dev/null
+++ b/internal/normalizers/nws/day_night.go
@@ -0,0 +1,74 @@
+// FILE: internal/normalizers/nws/day_night.go
+package nws
+
+import (
+	"math"
+	"time"
+)
+
+const (
+	degToRad = math.Pi / 180.0
+	radToDeg = 180.0 / math.Pi
+)
+
+func observationLatLon(coords []float64) (lat *float64, lon *float64) {
+	if len(coords) < 2 {
+		return nil, nil
+	}
+	latVal := coords[1]
+	lonVal := coords[0]
+	return &latVal, &lonVal
+}
+
+// isDayFromLatLonTime estimates day/night from solar elevation.
+// Uses a refraction-adjusted cutoff (-0.833 degrees).
+func isDayFromLatLonTime(lat, lon float64, ts time.Time) *bool {
+	if ts.IsZero() || math.IsNaN(lat) || math.IsNaN(lon) || math.IsInf(lat, 0) || math.IsInf(lon, 0) {
+		return nil
+	}
+	if lat < -90.0 || lat > 90.0 || lon < -180.0 || lon > 180.0 {
+		return nil
+	}
+
+	t := ts.UTC()
+	day := float64(t.YearDay())
+	hour := float64(t.Hour())
+	min := float64(t.Minute())
+	sec := float64(t.Second()) + float64(t.Nanosecond())/1e9
+	utcHours := hour + min/60.0 + sec/3600.0
+
+	gamma := 2.0 * math.Pi / 365.0 * (day - 1.0 + (utcHours-12.0)/24.0)
+
+	eqtime := 229.18 * (0.000075 + 0.001868*math.Cos(gamma) - 0.032077*math.Sin(gamma) -
+		0.014615*math.Cos(2.0*gamma) - 0.040849*math.Sin(2.0*gamma))
+
+	decl := 0.006918 - 0.399912*math.Cos(gamma) + 0.070257*math.Sin(gamma) -
+		0.006758*math.Cos(2.0*gamma) + 0.000907*math.Sin(2.0*gamma) -
+		0.002697*math.Cos(3.0*gamma) + 0.00148*math.Sin(3.0*gamma)
+
+	timeOffset := eqtime + 4.0*lon
+	trueSolarMinutes := hour*60.0 + min + sec/60.0 + timeOffset
+	for trueSolarMinutes < 0 {
+		trueSolarMinutes += 1440.0
+	}
+	for trueSolarMinutes >= 1440.0 {
+		trueSolarMinutes -= 1440.0
+	}
+
+	hourAngleDeg := trueSolarMinutes/4.0 - 180.0
+	ha := hourAngleDeg * degToRad
+	latRad := lat * degToRad
+
+	cosZenith := math.Sin(latRad)*math.Sin(decl) + math.Cos(latRad)*math.Cos(decl)*math.Cos(ha)
+	if cosZenith > 1.0 {
+		cosZenith = 1.0
+	} else if cosZenith < -1.0 {
+		cosZenith = -1.0
+	}
+
+	zenith := math.Acos(cosZenith)
+	elevation := 90.0 - zenith*radToDeg
+
+	isDay := elevation > -0.833
+	return &isDay
+}
diff --git a/internal/normalizers/nws/observation.go b/internal/normalizers/nws/observation.go
index cb004a2..27988b0 100644
--- a/internal/normalizers/nws/observation.go
+++ b/internal/normalizers/nws/observation.go
@@ -76,9 +76,13 @@ func buildObservation(parsed nwsObservationResponse) (model.WeatherObservation,
 	// Determine canonical WMO condition code.
 	wmo := mapNWSToWMO(providerDesc, cloudLayers, phenomena)
 
+	var isDay *bool
+	if lat, lon := observationLatLon(parsed.Geometry.Coordinates); lat != nil && lon != nil {
+		isDay = isDayFromLatLonTime(*lat, *lon, ts)
+	}
+
 	// Canonical condition text comes from our WMO table.
-	// NWS observation responses typically do not include a day/night flag -> nil.
-	canonicalText := standards.WMOText(wmo, nil)
+	canonicalText := standards.WMOText(wmo, isDay)
 
 	// Apparent temperature: prefer wind chill when both are supplied.
 	var apparentC *float64
@@ -95,7 +99,7 @@ func buildObservation(parsed nwsObservationResponse) (model.WeatherObservation,
 
 		ConditionCode: wmo,
 		ConditionText: canonicalText,
-		IsDay:         nil,
+		IsDay:         isDay,
 
 		ProviderRawDescription: providerDesc,
 
diff --git a/internal/normalizers/nws/types.go b/internal/normalizers/nws/types.go
index 3024ab0..dc5327f 100644
--- a/internal/normalizers/nws/types.go
+++ b/internal/normalizers/nws/types.go
@@ -8,7 +8,11 @@ import (
 // nwsObservationResponse is a minimal-but-sufficient representation of the NWS
 // station observation GeoJSON payload needed for mapping into model.WeatherObservation.
 type nwsObservationResponse struct {
-	ID         string `json:"id"`
+	ID       string `json:"id"`
+	Geometry struct {
+		Type        string    `json:"type"`
+		Coordinates []float64 `json:"coordinates"` // GeoJSON point: [lon, lat]
+	} `json:"geometry"`
 	Properties struct {
 		StationID       string `json:"stationId"`
 		StationName     string `json:"stationName"`