So I am thinking of a ritual that provides ideal weather for farming for a small area (A village or so). Thus the spring rains won't be too heavy or too light, the summer will be long and pleasant, winter will be mild etc etc. I am not quite sure what the level of the effect ought to be, or what requisites are required.
So the basic effect will be Creo Auram, does it need a muto, rego, or herbam requisites?
This is probably quite a stupid question, in my defence I am new to ArM5 and it looks like I am playing a weather mage.
It's very tricky to do so with a single Ritual - weather is a collection of many different phenomena over a long span of time, and their "ideal" proportion varies a lot depending on what you are trying to (rye vs. dates for example). You might have a spell to protect from frost, one to protect from excessive rain etc.
In any case, I think the right Arts are Auram and Rego, not Creo -- unless you are trying to create unnatural weather for the region/season. The Base level is 3 to protect against a mild weather phenomenon, e.g. hot and humid weather that makes farming unconfortable. It's level 4 to protect against a normal type of weather phenomenon, e.g. winterfrost. It's level 5 to protect against a severe type of weather phenomenon, e.g. thunderstorms. It's level 10 to protect against the most severe and destructive types of weather, like a hurricane.
Also, keep in mind that Rituals are going to be expensive -- it might be better to use normal Formulaic spells. One really effective way to do so is to ask the farmers to build and maintain low circular walls around their fields. Then you can cast your magic with Range: Touch, Target: Circle and Duration: Ring, for a total of +3 magnitudes over the Base effect, and have it last until the circle is broken. This means level 10/15/20/25 spells to ward against any type of mild/normal/severe/very severe weather.
A trick to snip off two magnitudes off (bringing the level down to 4/5/10/15) is to cast them with Duration: Momentary, and use a Rego Vim spell that is at least one magnitude higher to extend that Duration to Ring (so a level 20 Rego Vim will allow you to sustain spells that ward against the most severe weather) - see "Maintain the Demanding Spell" in the core book. This means that, if you learn the Rego Vim spell as a Formulaic, you can probably pull off all the weather wards you want as spontaneous spells.
A flavourful alternative is to summon a weather spirit, and ask that spirit to take care of the local weather. Such a spirit probably has all the necessary powers. This is a Rego Vim or Rego Auram effect with a Base Level 15, though for the summoning you'll probably need to boost it up to Arcane Connection Range and to a suitable duration. You also probably need an Arcane Connection to the spirit. You can exchange 5 xp in Magic Lore with 1xp in Magic Lore plus knowledge of a Spirit's True Name, which acts as a permanent arcane connection to the spirit. Of course, once the spirit it's there, you'll have to convince it -- or use your magic to control it (Base Level 5, though enslaving spirits is always risky...).
Eh, you DO realise how hard that will be? Those circles are going to be hundreds of paces long.
And a magi is limited to tracing the circle at 10 paces per round.
AND requires a Conecentration roll at ease 6 every round, its going to be extremely high risk to botch the Concentration roll eventually, and if tracing the circle fails, its an automatic spell botch. Big badness.
By RAW, mostly Rego Auram. Sometimes Creo.
Im not sure its possible to squeeze in something so complex into a single ritual... There´s simply too many effects needed, and needed at the right times or during the right conditions.
You´re probably better off having one or a few magi learning some standard weather spells and then cast them as and when needed.
If the goal is to have the crops be healthier, it's probably more direct to just use a Creo Herbam spell on them, weather be damned.
But if you're already a weather mage, then go for it....
Let's see, if you want to do things right, you need ReAu Base 5 (control a very severe weather phenomenon), +3 Sight, +4 Year, +4 Boundary, and say +2 complexity so you can have the right mix of good weather (without the complexity, you would have unchanging weather). That's a level 70 ritual and 14 pawns of vis, but that will make your peasants happy.
Personally, I think you would be much better off developing a couple of formulaic spells : one to turn bad weather away (or destroy it), using ReAu or PeAu, and a CrAu to bring a nice rain. Then you just cast the first whenever the weather turns too bad and the second if you need rain. Of course you have to look at the weather every day, but you don't need a ritual. Or do as one of my Verditius did, put them in magic items and let a grog worry about it. And really, once you make sure there is enough rain, and no rainstorm will destroy the crop, you are already ensuring a good year.
You can specialise in ReAu and use Enchanted Items. If a Verditus, it can be one big one, using the Rego and Auram Elder Runes.
You can't use Cr Au if you want to help crops. But ReAu can attract or repel clouds, or force them to start or stop raining.
Rain sticks, anyone?
Overall, the most common combination you'll find is CrHe for improving crops, ReAu or PeAu to prevent disastrous storms or even frosts, and keep a CrAu or CrAq ritual in your back pocket in case of a severe drought.
You might also consider where your closest water source is. If you have an irrigation system, it might be easier to make a CrAq ritual.
As for the giant circles, there's an Ex Miscellania tradition, the Columbae, which specialise in giant pseudo-rings and could handle the giant boundary circles, no problem, especially if its a kind of ward. That's their specialty. Check them out in Houses of Hermes: Societas.
I don't think you need to do target boundary though. Standard target size for a Auram spell is a a weather phenomena that fits within a standard boundary. If bounder y is not required and the duration is below a year then it doesn't HAVE to be a ritual and you could put it in a standard magic item. If you keep the duration to one day or even concentration I could see the added complexity being only a plus 1.