If you can, then it could be cool if a spell only can be cast in a certant weather type. Ie lightning bolt requires cloudy weather etc.
This is the principle behind the call lightning spell for clerics. The idea is that mages can
create a lightning bolt so do not need to depend on weather, whereas clerics have to manipulate the existing weather because they do not share any such power.
Though I don't fully understand how it can access cast_arg2, I thought you had to send it to the function specifically - this will make me rethink a LOT of my previous code 
The fact that cast_arg2 is a
global variable is the reason that it can be accessed in any file by any function, so long as the file knows it exists.
Note that while these seems to make things simpler, and might make it seem tempting to make more variable global, it is in general a bad habit to get into, and should only be done in certain circumstances.
Historical Note: Jamdog added the global variable cast_arg2 (there seemed no other reasonable thing to do without adding very ugly code) in 2008 in order to improve upon the what was at the time a very flawed locate_object spell which originally did a poor job of responding to what the caster was actually searching for. It turns out to be a valuable tool in all cases where the argument passed to a spell cannot be converted to an item in the same room as the caster.
