rain: Water condensed from atmospheric vapor and falling in drops.
Fortunately humans have recently developed a technology called "shelter". They have even assembled semi-permanent constructions ("houses", "buildings" and the like) which can effect this "shelter". Perhaps the aliens could make use of them. No of course not - what am I thinking.
You really want to have a "scientific" excuse to dislike that movie, don't you? You don't have enough confidence in your gut/aesthetic reaction to just say "I didn't like it"? You MUST build a case for why the Alien story was scientifically implausible!
Oh brother.
Keep it coming - these are really fascinating (and IMPORTANT) objections to M. Night Shyamalan's film (about faith).