Optical signal/noise effects while driving

When I was driving I made an observation depending on two conditions

Condition 1: If my eyes are in sunlight but just in front of me there is shade of trees or anything else

Condition 2: There is dust on my windshield

When Condition 1 and 2 is there, I become near blind and cant see anything in front of me , rather dangerous scenario I observed,even though if i use sun glass, it makes no effect.

But if I remove dust with water and wipe it , I can see clearly and not get blinded.I can see clearly with dust in full sunlight and full shade scenario though.

What is reason of getting near blindness here, as only cause is dust on windscreen, can dust particle diffraction of sunlight or is it biological like eyes iris changing size due to different light condition, as my eyes are in sun light zone while my bonnet onward it is shade zone?