It’s been my observation that ants seem to build up there pile before a heavy rain comes. So do they know rain is coming, and if so… how?
I’m not sure if ‘know’ is the right word; as far as we know, no individual ant is smart enough to actually ‘know’ stuff at all (in an abstract sense); the colony responds to stimuli including atmospheric temperature, pressure and humidity.
Ants are always building up their piles. They don’t have television, so they have a lot of free time.
haha funny 
No seriously, the pile will get higher before a flood.
I live in fire ant territory and have not noticed any real changes in ant hill geometry before the rain. What I have noticed is that once the weather changes – like the cooler weather of fall – the ant hills dramatically change shape. The hills that were spread out on the ground are now errect. This serves some ventilation purpose but prob just minimizes surface area.