diff --git a/vendor/clouds3d.js b/vendor/clouds3d.js
index a57db79..230a384 100644
--- a/vendor/clouds3d.js
+++ b/vendor/clouds3d.js
@@ -188,8 +188,9 @@ var FRAG_SRC = [
         // is not the same as 'is it night').
         // Only darkens the sky, NOT the clouds — we want them to stay
         // moon-lit (their colour came from uLight) while the sky around
-        // them goes dark. The cloud mask 'cloud' tells us how much of
-        // this pixel is cloud, so fully cloudy pixels keep their colour.
+        // them goes dark. transmittance tells us how much light made it
+        // through the cloud layer (1 = clear, 0 = fully cloudy).
+        'float cloud = 1.0 - trans;',
         'float skyDarkness = nightFactor * 0.85;',
         'col = mix(col, col * (1.0 - skyDarkness), 1.0 - cloud);',
         'gl_FragColor = vec4(col, 1.0);',