Wind has absolutely no bearing on structural integrity and so long as you have a roof on your build pieces, the rain won't affect it either - though rain only affects durability, not structural integrity.

As you go up each square in height, the colour indicator of stability will change, the ratio of levels to change in integrity is different depending on the materials being used to build. There's no fluctuation - the game has VERY defined rules in regards to this and with experience you learn how far you can push it and where/what things need to be placed.

It's also important to note that horizontal beams do not add to the structural integrity as much as vertical beams do.

It does take a few seconds for the game to check that the right supports are in place so if you are quick, sometimes you can use those few seconds to connect pieces that would otherwise fall down if placed on their own and left. Again, it all just comes down to practice and experience.

If you move parts of a structure, it will recalculate the integrity whilst it seems like it might fluctuate, it does settle. There's a bunch of advanced building tutorials on Youtube, I highly recommend Vetlive, Unknown Factions, and Gaming It Out has a great Advanced demo too.

Good luck! Hope you're enjoying it.