Question

electrostatic experiments cannot be conducted successfully on humid days .explain

Answer 1

for charges to be static they should stay in an insulating environment, otherwise they cannot stay still "static". any conducting medium that can allow the charges to move will mess up your electrostatic experiment.
Now humidity means that you have water molecules hanging around everywhere, well water is a poor conductor but it does ionize to a certain extent and thus its capable of transferring charge and messing up your experiment.