This is a well known problem on the Jazz, you need to enlist the aid of the airconditioning to 'dry' the air blowing into the car and up the windscreen, also make sure heater lever is to the right (fresh air) position - with lever to the left the same old humid, stale air is re-circulated around the cabin.
Put fan on position 3, airconditioning on, heater control at least 1/4 way on and the heater blows warm, dry air into the car - hey presto the screen will clear very quickly, no matter how hard its raining.