Rain can be triggered by a 25 flux event. However, to be fair, we do mostly get it with thunder, which is a 50 flux event.
There are a lot of ways flux is created, which aren't always obvious. A silverwood sapling generating a node causes flux (and it's aura is pulled from the local area, not generated from nothing) when it's first grown. Anything that drains vis and causes the aura to have to recharge causes (a very small amount of) flux. Nodes merging because they're close together causes flux. Using an infernal furnace causes flux.
Because of the way node recharging works, flux will spread much
farther than you might think when your local node recharges. When a node is below max vis, it will draw vis from any nearby node that is larger than itself. Then, the node it drew from will try to recharge in the same manner. Because the node data is kept in a separate file, nodes can draw from each other even if they're in unloaded chunks. Eventually, the node recharging will get to a node that is bigger than all its neighbors and it won't have anything to draw from. This node will charge by turning the infused ore nearby into dull ore (the kind that produces grey shards) at the rate of 10 vis/block. Eventually, it will run out of ore and be unable to recharge. Once you hit this point, you've entered a downward spiral. Vis auras are not
an infinite resource. There are a couple of Thaumcraft items that can generate aura, but they do so very slowly, and if your node gets to the max charge, they will supercharge it a bit, which will cause flux. (Do we see a pattern here yet?