Well tornados can form by themselves, I think. Okay let's try to find another example I know it rains where you live, it comes and goes. It's natural weather. But to RainbowDash she can make it rain, or move the clouds around or make it thunder when ever she wants to. You're not getting this are you.
Oh. My. Gosh! Incredible! Look at all the little Detailwork, and what a nice perception of depth.
Though something bugs my Eye. I would have suggested drawing the Wings a little "fluffier", they look like Paper-thin and not really suitable for some heavy Pony-flanks. But, thats more a matter of taste, technically perfectly fine!