diff --git a/assets/js/compute.js b/assets/js/compute.js
index 521fbe5..25ba062 100644
--- a/assets/js/compute.js
+++ b/assets/js/compute.js
@@ -55,11 +55,12 @@ export function buildHourlyRows({ forecast, airQuality, location, vehicleType, v
     const soilT0 = h.soil_temperature_0cm    ? h.soil_temperature_0cm[i]    : null;
     const soilT6 = h.soil_temperature_6cm    ? h.soil_temperature_6cm[i]    : null;
     const soilM  = h.soil_moisture_0_to_1cm  ? h.soil_moisture_0_to_1cm[i]  : null;
-    // Scale radiation by cloud cover before passing to concrete calc.
-    // shortwave_radiation and cloud_cover are separate forecast fields that
-    // don't always agree — this prevents hot concrete values on cloudy days.
-    // 100% cloud -> 85% reduction, 0% cloud -> full radiation.
-    const effectiveRad = glob * (1 - (cc / 100) * 0.85);
+    // Use direct_radiation (beam sunlight) + a fraction of diffuse for concrete.
+    // direct_radiation is zero on fully overcast days — far more accurate than
+    // shortwave_radiation which can be unreliably high even at 100% cloud cover.
+    // Diffuse (scattered light through cloud) contributes ~20% as much heat to
+    // a surface as direct beam, so we weight it accordingly.
+    const effectiveRad = dir + dif * 0.2;
     const concreteT = calcConcreteTemp(Ta, effectiveRad, va);
     // iso is a local wall-clock string e.g. "2026-05-13T14:00" (no Z).
     // For display we slice the string directly — no Date object needed.