There WAS rain in the world. Science "prooves" this (depending if you agree.... or care)... but in the area, wherever this place was, there was NO rain. This is not totally unthought of. The Egyptians very seldomly had rain, but their streams and floods feeded their plants.

This would explain why the inhabitants of the earth laughed at the idea of such a large flood when Noah told them of his reasons for building the Ark.

The hebrew word used in 2:6, "Ed" could stand for a "stream", "flood", or "mist". If it was a mist, it would create the "Matar" ("Rain" in 2:5 which can be translated to any form of liquit precipitation).