To me the only way to understand a spell inside out is to write it yourself, from scratch, relating every element back to the goal you are trying to achieve.
The spell you have found here sounds very interesting but if the author of the spell hasn't even managed to make clear what general area of magic his/her working falls into then it probably isn't a very good spell to be honest. Specifics are not "nice to haves" or useful in a spell they are the intrinsic nature of the magic itself. If a spell doesn't even manage to be specific enough as to name the desired outcome then it is fair to say it is going to be a flop.
Picking up the idea of weather magic - this is a difficult area of magic for even the most experienced of magical practitioners to perform. Any witch with the ability to pull off this level of magic will almost certainly not be a witch who looks for spells in books.
I don't like to leave answers on a downer so to look for a positive way forward for you - the only thing I can suggest is that you study the spell and you own it. Decide in your own mind which kind of storm you wish to use it for and then ensure you understand all the wording and how it relates to the purpose you have selected for the spell. It doesn't really matter if your intent differs from that of the person who wrote it, the important thing is that there is a clear link in your mind between the steps of the spell and the outcome that you personally want to achieve.
But my primary advice is to write your own spells. The work of others will only get any witch so far and no further.