All that a spell really is is a way to use energy to influence the world. You use your mind to control or influence the natural flows of energy, suiting it to your purpose. Energy has been described as the fourth state of matter, past even gasses. So, if energy is matter, it stands to reason that it would follow the laws of physics.
The most commonly known of Newton's Laws of Physics is that for every action there is an equal and opposite reaction. This means that you need at least 101lb of force to lift a 100lb object. So, to fly, if you weighed 100lbs, you would have to be able to put out 101lb of force of pure energy, straight down. Since the human mind cannot focus enough, nor has enough power, to put out that much energy, unless you are really small and/or able to concentrate to a super human level and haveenough energy somewhere, the easiest way to fly is to buy a ticket for an airline flight.
Newton's other two laws state that an object in motion will stay in motion unless an outside force acts on it and that an object at rest will stay at rest unless an outside force acts on it. Since there are so many outside forces acting on clouds, the odds of your relatively tiny amount of energy having an influence that makes a difference in the weather is very small. Another law, the law of conservation states that, at a macro level, matter cannot be created or destroyed, only changed. Of course you cannot create clouds out of nowhere. So weather spells, unless done by a large group, will either have very little influence, or not work at all.
I'm not going to say werewolf, vampire, dragon or other similar transformation spells don't work because these creatures do not exist, as that is unprovable. I will say, however, that to cause a mass mutation in only days, to change who or what you are, is impossible. Your cells don't replicate fast enough to produce a noticeable change in anything less than years. And ome cells in your body do not replicate at all, such as your red blood cells. And, while humans do have a gene that makes their skin pale, their teeth longer and sharper, and probably a physical endurance gene, we do not have a gene that makes us hunger for blood, turn to a wolf at the full moon, grow large scales and wings to become a reptilian beast, or anything such as that. There is no genetic way to become one of these creatures.