diff --git a/src/pocketmine/level/generator/normal/Normal.php b/src/pocketmine/level/generator/normal/Normal.php
index c13b80c7d..8a5cdddd7 100644
--- a/src/pocketmine/level/generator/normal/Normal.php
+++ b/src/pocketmine/level/generator/normal/Normal.php
@@ -145,6 +145,9 @@ class Normal extends Generator{
 					return Biome::BIRCH_FOREST;
 				}
 			}else{
+				//FIXME: This will always cause River to be used since the rainfall is always greater than 0.8 if we
+				//reached this branch. However I don't think that substituting temperature for rainfall is correct given
+				//that mountain biomes are supposed to be pretty cold.
 				if($rainfall < 0.25){
 					return Biome::MOUNTAINS;
 				}elseif($rainfall < 0.70){