Assertion : The lightening conductor at the top of high building has sharp pointed ends.
Reason : The surface density of charge at sharp points is very high resulting in setting up of electric wind. 

A charged cloud induces opposite charge on pointed conductors. At sharp points of the conductor surface density of charge is very high and charge begins to leak from the pointed ends by setting up oppositely charged electric wind. This wind, when comes in contact with the charged cloud, neutralizes some of its charge lowering the potential difference between the cloud and the building. This reduces the chances of lightening striking the building [if the lightening strikes the building, the charge is conducted to the earth and the building remains safe].