You have to have seasons to have aliens, but as for the weather you can chose to have or not have certain ones, how long they last, temps and other stuff.
I play mine with rain and hail off, turned winter off but have it back on for I think five days, maybe less. Fall only three I think, summer the max number (don't remember the amount) and I think I have spring on seven days, not sure.
I keep the temps at 75 for spring and summer, not sure of fall or winter, I am still playing around with number of days, temps and such to find the one that I really like and plan to keep.
Hated the rain, it was nonstop and interfered with having fun and too much negative aspect, sims not liking to get soaked. Same with winter, not into letting my sims freeze to death so don't let the temps go below 50.
You can pick snow levels, if you have kids and want them to have the three days on the honor roll, just set it to low levels, then school won't be cancelled. On weekends you can set it to high levels so the kids can have fun building snowmen and igloos.
I play my sims on longer lifespan than the normal is, so you can set it for what works for you.
But whether you play on normal lifespans, extended, epic or whatever you can chose what seasons you want or don't want, the type of weather, temps, snow levels and more. So, don't let that keep you from buying the expansion pack.