Why experiments of electrostatic induction are best done in dry weather? Basic experiments which demonstrate the presence of electric charges and forces..
For instance, when we rub a comb on hairs...it gets negatively charged..and can even suspend bits of paper due to this charge.
And many such experiments of rubbing glass rod, ebonite rod or rubber to silk, wool or paper which lead of their electrification.
They work best on dry days..why so?
I have read in a book that due to excessive moisture in environment the charges induced get leaked from the body to the earth.
How so?
 A: Electrostatic demonstrations work best when materials classed as insulators do not allow the flow (leakage) of charge.  However if even a very thin layer of water adheres to the surface of so called insulators then although the bulk of the material does not allow the passage of electric currents the thin layer of water does.
So instead of using glass and ebonite in electrostatic experiments plastics like acetate and polythene are used because they a less prone to have water adhering to them and heating them up in a stream of hot air from a fan heater usually removes any water present on the surface.
So a high humidity means that there is a greater chance of there being a very thin layer of water on the surface of an "insulator" and that layer in effect makes the material a conductor and charges can leak away to earth when you do not want them to.
In electrostatic experiments the human body is an excellent conductor as is a piece of wood where although the cellulose of which the wood is made is a good insulator water on the pores of the wood provide a conducting path.  Carefully heating a piece of wood does improve its insulating properties but if left in the air it soon becomes a conductor and more quickly in more humid conditions..
A: Suppose you have a negatively charged object. There are extra electrons on it. They repel each other, but not strongly enough to leave the object. If the object is an insulator, probably not even strongly enough to move around on the object.
The usual answer is that humidity makes air more conductive, allowing charges to be carried away. This is usually justified by saying that water is a conductor, which doesn't really sound that plausible. Never the less, this paper shows that indeed air does become more conductive at higher humidity.
As Farcher says, humidity provides another mechanism by making a thin layer of water on surfaces. This makes charges more mobile. They can flow along the surface, and onto whatever is supporting the charged object.
The surface layer can be hard to get rid of. Water molecules are sticky. In vacuum chambers, this can create problems. They stick well enough that water doesn't completely evaporate when the chamber is pumped down. The do come off enough to spoil a good vacuum. So vacuum chambers are sometimes heated to drive off the water and achieve a better vacuum. To get a really good vacuum, sometimes it must be heated to well over boiling temperature.
A: I think we can consider the air with humidity as a dielectric medium with permittivity slightly more than that of vacuum. Now in calculations we write the permittivity of free air which gives a deviation from from experimentally calculated values in moist setup.
