Contact of windscreen with hands directly should be avoided. Human skin contains natural oils which stick to the windscreen. It happens very often when younger children are present in the car.
During normal driving during "dry" weather, we don't notice the presence of the human skin oil smudges on the screen ... however, during rain and bad/ cold weather, the presence of this oil coupled with the presence of mist, makes it very difficult to see across the windscreen.
As a preventive measure, the direct contact of hands with the windscreen and all other glass surfaces in the car should be avoided.
The oil and dust accumulates on the inside surface of the windscreen, nevertheless. In this case, it is recommended that a good glass cleaner like "Glint" be used once a week on both sides of the windscreen as well as the side windows.