Strange Lighting on normal mapped Terrain

Hi,

I need your Unity wisdom here :)

What you see in the attached screenshot is a normaled mapped terrain. There is a snow texture with a normal map and a snowy path with a normal map. Both normal maps are created with Crazybump if that matters:

The odd thing is that the snow texture by the side of the path is actually considerably brighter than the path texture when viewed in an image viewer. In Unity, however, there are certain lighting situations in which the snow texture appears much darker then the path texture (like in the screenshot). There is an abrupt transition between the brightness of the snow and that of the path.

With "certain lighting situations" I mean that I am using a scripted weather system which simulates a whole day from morning to night lighting. When the sun is at its zenith and shines brightest all the lighting seems correct. Same goes for night lighting when there is only blueish ambient light in the scene. But for most sun directions and intensities in between the terrain is reflecting light as you can see in the screenshot.

What do you think is causing this problem? The normal maps? Or is this maybe a bug?

Thanks a lot! :)