As I'm sure I tried to explain erlier, though oddworld in general is an imaginary world, oddworld AO and AE (the subjects that this thread is rotating around) are games. There is no rain because oddworld inhabitants decided against the idea, the cacti could be floating on the clouds for all I care.
The only part where there is rain is, as you say, the circle. But that is there for the same reason as meeps are in MO... oddworld inhabitants wanted something new for abe to do.
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