[RightWhale]

The tail of the comet is all gas, so the water vapor would mix as vapor with the atmosphere. Eventually it would precipitate out to the ground.

[Pyro7480]

I remember reading that in 1910, the Earth actually passed through the tail of Halley's Comet. The ice particles, since they are moving so fast (tens of thousands of miles per hour) that they would instantly vaporize in the Earth's atmosphere.