Eh, didn't know where to put this or if it was mentioned, but if you are inside and you, say, look north while a sandstorm goes on, it mentions nothing of a sandstorm. Shouldn't it be so that you can't see anything?
try reading help weather.
Basically, you're able to "weather n" to identify whether or not a sandstorm is out, amongst other things. Unfortunately the code is mixing it up a little in terms of lighting and weather. Basically, sometimes you're able to look into zero-visibility rooms if you are in a room with good visibility. And at the same time, unable to look into a well lighted clear room, if the room you're in happens to be too dark or obstructed by sand. There are frequent exceptions though ... I think.
Try 'weather north'.