The first step alters the color of the triangles that are facing the sky. It affects all the triangles with a normal direction similar to _SnowDirection. As seen before in Chapter 3, Understanding Lighting Models, comparing unit vectors can be done using the dot product. When two vectors are orthogonal, their dot product is zero; it is one (or minus one) when they are parallel to each other. The _Snow property is used to decide how aligned they should be in order to be considered as facing the sky.
If you look closely at the surface function, you can see that we are not dotting the normal and snow direction directly. This is because they are usually defined in a different space. The snow direction is expressed in world ...
