I realise most weather spells are typically Sight range, but how does that apply when you are in the middle of inclement weather that severely limits how far you can see?
And more importantly what happens when your mage tries to clear away inclement weather with a non-weather spell?
Essentially I want a flame proof Flambeau to burn all the driven snowflakes in the blizzard he is caught in. How would he do it?
Anything that restricts how far you can see restricts the range of Sight spells. It is a trade off made for allowing you to cast spells at a target a mile or more away if conditions allow it.
As for burning a mass of snowflakes, have you considered Touch range and lots of Size? A Base Individual for Ignem is a fire about 1 pace across. Size +2 would take that to 10 paces across and Size +4 would take that to 100 paces across. You don't need a whole lot of damage potential at all. Just using enough heat to warm things up hot enough to melt snow would work. Add a Duration longer than Momentary and you could have a mobile area that destroys snow.
As you say, Ignem does not make weather spells, so they cannot target the weather phenomenon as an individual. That means that he would be looking either at some kind of boundary effect or umpteen size magnitudes to get rid of every snowflake within sight. He might need a ritual.
However, you could make an analogue to the Lamp without Flame, heating up a sphere around you to melt, and keep melting the snow. With enough size magnitudes, he could make a pretty large oasis in the blizzard.
You could make a gigantic fireball and throw it ahead of you to clear a path ahead of you, but this would do nothing to the actual source of snow.
A snowstorm raging a county will make just one group, comprising every snowflake from just above the ground to the clouds, and there is unlikely to be any natural subgroups that you could target. Hence I am not convinced that 6-7 size magnitudes will suffice.
Concentration does not help you. The snowflakes not yet formed are not part of the target group. Hence, within a minute or two, you will have fresh snow upon you.
Admittedly, there are canon spells which violate the principles underlying 2 above, such as the forest walk in MoH.
Come to think of it, though, the target of an Ignem spell is the fire and not the snow, so sizes have to be calculated based on Ignem and not any snow element, and RAW only discuss sizes for actual flame, not heat, but I think the point remains. It is an awful lot of snow, so it takes an awful lot of magnitudes. Unless you argue by analogy from Lamp without Flame which does not seem to fit the guideline rules at all.
You overlooked that you do not actually have to directly target the snow.
If you are working with fire than you are creating a Base Individual 1 pace across without Size. That distance increases by a factor of x10 every +2 Size. Make a really big fire boom of low damage would do the trick.
If you are working with heat than things get a little more difficult. You have to pick what object you are heating up. There is no real discussion in the book on the size of the heated object so that is YSMV. Heating the air is weather magic so out since you want to avoid that. You could heat an area of the ground which would clear off the snow on it and thin out the snow in the air above.
No matter how you go about it though, it will be higher level and/or less effective than PeAu.
Troy's suggestion for a fireball looks the best if you insist on doing it with Creo Ignem, since the Creo Ignem guidelines don't really allow the creation of ambiant heat, rather than heating a specific target (remember that aristotelian physics precede thermodynamics), and I don't think you can target an auram phenomenon itself without actually dipping into the art of auram. Frankly, if I wanted a non-showy way of dealing with falling snowflakes, I'd use Muto Auram base 3, +1 touch, +1 concentration to switch the falling snowflake arround me into a light breeze. It's definitely the easiest option. Creo Auram base 1, +1 touch, +1 concentration, +1 ignem to create a wind hot enough to melts the snow works too. But then you end up with rain, which may not be what you want. Alternatively, actually igniting a snowstorm... that's MuAu base 5, +1 touch, +1 concentration with an ignem requisite.
You actually can create ambient heat, just not in a form usable for the melting of snow. Base 2 "Warm to the touch" and Range Room that makes everything in the room that warm.
Another alternative you could go for is to just use magic that allows you to ignore the visual species of snow. This would allow your vision to be totally unhampered by the snow without actually doing anything to it. There are already versions for seeing through earth and water in published material.
Now you got me thinking though. You could do it with Target Circle as well, which would really be the only way to work it outdoors without a ritual. Still don't think it would be effective though since it would be damn hard to draw a circle in the middle of a blizzard.
I think it is worth mentioning that if you create a gigantic fireball (with low damage) big enough to catch all of the snowstorm within its range, you are more than likely to be singing a significant number of mundanes, houses, fields, forests, mundane animals, and also reasonably likely to be hitting a few magical creatures and a magus or two.
Chances are a few someones are going to take offense at being singed in a gigantic sea of low intensity fire. Also you might cause a few actual fires.
MuAq (if snow is considered liquid water) or MuAu (if precipitation) to turn the storm to oil or alcohol. Apply flame. Ignites the storm. Ignites the trees, other plants, local wildlife, villages, peasants and other mundanes, grogs, companions, and probably the magus, too.
Is it possible to invent such a spell without an Hermetic breakthrough? It seems to contradict RAW, which is not uncommon for MoH.
I find a new target to be a far greater stretch than extending light to heat by analogy, to make ambient heat analogous to lamp without flame.
If you err on the conservative side, it could be possible to use Rego to have your CrIg fireball move quickly around you to melt and evaporate the snow. I would not worry about hot steam; water evaporates at room temperature (even at freezing temperatures to some extent). If it does not, rain does not obscure sight nearly as much as snow. Obviously, if the snow is really dense, there may be enough water to annoy you one way or another, but it would still be a problem.
I was thinking along the lines of a Pilum of Fire makes an individual flame strike a target.
Can you not invent a Mass Pilum of Fire that creates multiple flames that each strike a member of a roup of targets? such as everyone wearing the Duke's colours in that squad over there.
Or am I getting Target: Group confused in CrIg?
So my magus can't target a flame on each snowflake in a flurry that touches my magus?
I have had (read lost) that dispute in my troupe. The majority concluded that a group pilum of fire will create a group of pilums which could directed at one or multiple victims, with one or multiple pilums per victim.
If you take their interpretation, you could obviously make a million pilums and direct them at the one million closests snowflakes, and since the snowflake will evaporate at a lot less than +5 damage, you can fit thousands of the pilums you need within a single group of standard individuals. The group limit is by volume/mass and not by number.
If you take my interpretation, where a group pilum would hit a group of victims with one fireball each, however, you could only use it to target the entire snowstorm, with every snowflake in the county, because the snowflakes in your vicinity do not form a discernible group. If you do not have enough size magnitudes, the spell fails for lack of eligible targets.
The authors' interpretation remains unknown to myself. Which one did you have in mind?
Do we have a numeric size for a base individual for Ignem? I can't locate anything besides "a large campfire". And I think you are calculating based on area, not volume. If you go with volume it should be an increase of 10x by each +3 size.
Another option is to make a coat of flames using baseline 5 (+5 damage with unnatural shape) to create flames at R: Touch around you and add a Rego requisite to make the flames expand around you in the shape of a dome, isolating you from the snowstorm. Based on Circle of Encompassing Flames you should be able to manage up to 50m² with an individual, or an hemisphere with a 3m radius. Here, keeping the thickness of the dome constant and increasing only the surface area, each +2 magnitudes for size should give you x10 times the radius.
I find this difficult to express in terms of hermetic magic. Maybe something non-standard (for example, something like Treading the Ashen Path). Also, note that since the target is the created flame you can't really target only the snow unless you add requisites (but if you stick to the "heat to be hot/warm" guidelines this should prevent unwanted damage to most things.
So, going with that as inspiration: Trailblazing the Snowstorm (Base 3 (heat to be hot on touch), +1 Touch, +2 Group, +3 size, +1 fancy effect (keeps being cast while the caster walks seven miles)): while you walk sparks dart from you, heating everything in a 10 paces radius. Under a snowstorm this means you will melt the snow falling around you. This spell can be used in other situations, and can ignite very flammable objects (a Flambeau once burned down a village after accidently igniting a nearby wheat field). You can walk for 7 miles with each casting of this spell.