Mineral Deposits on Glass
When water evaporates from the surface of glass, it leaves behind everything other than hydrogen and oxygen. If this happens repeatedly, minerals in the water can bond with the surface of the glass and cause a water spotting condition whereby the glass always appears dirty. Simple cleaning won't remove this damage, only chemicals or glass polishing can.
Landscaping irrigation is a frequent cause of these types of deposits and requires adjustment or re-engineering of the irrigation system to prevent a recurrence. Rainwater streaming down the side of a building, picking up minerals from adjacent concrete or stone panels can also leave these deposits on glass.