I'm at work and don't have my books on me and was hopeful someone has the info...
Are auras full area items or do/can they form in "bubbles".
An example would be a tower where the first floor is level 5 aura, the second floor is half 5 and half 6, the 3rd floor is level 5.
Had an idea for an adventure or maybe a covenant but can't recall much.
Thanks
However you want. There can certainly be bubbles, like lacunae.
"Bubbles" would be the easiest way to handle parts of the same Aura with higher strength.
If you want rules for it, then look in RoP: M. While tethers normally effect the whole Aura, if a smaller portion of the Aura had a tether that only effected it than that portion could easily be 1 point higher. In fact, in the Aura Variation Rules one of the possibilities is that the Aura in a limited area increases by 1.
No one has reported an Aura with a score of 2.5 but it's not clear if thats because there's something mechanical in the universe that defines auras at these states or if it's a fault in Hermetic theory. Much as the vis pawn was treated as an individble quantum in theory, while vis sources clearly contained fractional amounts (which we explored with the Soqotran "spark").
I suppose the people who craft auras (Architects and the Criamon clutch on Arran, for example) might have spotted the Aura strengthening rules in world, and be able to tell an Aura about to change.
How do we know the noone has reported an aura of strength +2.5?
I always assumed that the measure of aura strength on a scale from +1 to +10 is purely a game term.
Ars Magica generally does not have many pages dedicated to explorations of how characters inside of its world sees the world of Ars Magica. I dont think that people in the ArM universe measure auras on a scale from +1 to +10, probably they mostly dont measure them at all, except by some deeply personal measure caused by the application of InVi magic, such as e.g. the intensity of shivering induced or how spicy the taste of the aura is.
As you suggest aura strengthing is a good example. Imagine for simplicity's sake a magic aura of strength +2 that increases in strength at the solstices to +3. In game terms we would say that the change is a sudden jump of +1. But again this could be a reflection of the reality of the characters or it could be a game mechanical simplification. Looked at from the perspective of the characters it is possible that around the time of the solstices this aura is slowly building in intensity until it climaxes at the solstice and then slowly comes down to its normal level again.
The same goes for vis.
There is nothing to say that vis is not naturally divisible beyond the pawn, only that hermetic magic has no good way to consistently interact with and measure such. how would a magus know that the pawn they just used was actually 1.12 pawn instead of 1.0? It is hinted in RoP:M that auras contain/are made of "fluid vis" which would be divisible on much smaller scales that "pawned" vis. Likewise it is hinted that "pawned" vis occurs because the fluid vis concentrates, but does it concentrate instantaneously or a little at a time? There are canonical vis sources implying both are true.
There are good reasons why the rules are as they are, since it would be a nightmare to do the maths based on equations for magical phenomena. But IMO the underlying metaphysics make more sense that way and to a sufficient degree that it makes for better stories too.
Well, magnitudes of spells exist as things discussed by player characters. . That a magnitude of X does not work, but X+1 does gives you a tool to build the layers of the Realm Interaction table.
I've always thought that they were like boundaries, with each change in aura usually having some change in a conceivable boundary when it comes from natural features.
Often but not always - as is a common answer to questions regarding Aurae and Regiones
The shape of magic auras is a bit crop circlish in my game, if there are Criamon about.
In my games we usually play the aura as a bubble. One side of a line it's +2, the other side it is +1. The line can move around during the day/season, but there is a definite line. Like walking out of a shadow into sunlight.
I agree that there is a good argument for fractional auras - probably why we have so many botch dice 
There is an example of just such an aura, but seperated into regiones in Legends of hermes:the garden of Herrison.