Windows must have a roof overhang to keep water off of them. In this case the window is only inches from the ground. Rain dripping off the dome roof above is either traveling over the surface of this window and/or the lack of an overhang is causing the water to hit the ground and “splash-back” onto the window. Snow may also pile up on this window, melting from the interior heat loss. The wood window was a very bad choice to start with – It should be vinyl or clad wood. However, windows this low need an 18″ to 24″ overhang.