When Hazō says 'we got that snow appearing in mid-air', what he means without realizing it is 'you pushed the temperature below -78C and the carbon dioxide in the air started condensing out and turning into dry ice.'
The effects of the Elemental Mastery nuke are extremely hard to predict. We-the-MfD-community have spent literally half a decade trying to figure this out, talking to different experts in various flavors of physics, and our best models are still very uncertain guesses. Based on all that, here's what the QMs have decided to go with:
Elemental Mastery changes the temperature of the air in a given zone by up to 5C per level, meaning that Akane's level 40 allows her to change things by +/- 200C. This change comes on slowly, the air cooling/heating over the course of a few seconds. Air that leaves the zone initially retains its magically-assigned energy but thereafter answers to normal thermodynamics and will shift to match the ambient temperature.
The jutsu lasts for as long as the caster concentrates and then another 10 minutes per level thereafter, meaning 400 minutes (6.66 hours) in Akane's case. The duration cannot be altered. What happens when Akane cools an area as much as she can, especially when she does so on a cold day and there happens to be a thunderstorm rolling in from not far away?
Carbon dioxide (CO2) condenses at a measly -78C. Oxygen is a bit more resolute, sustaining its gaseous nature down to -186C. Nitrogen is the most obdurate of the group, making it all the way to a beefy -196C.
The exact size of a zone is flexible, but in open terrain we're calling it a circle 50m [EDIT: this later got revised to 30] meters in diameter. When the temperature in this area drops to about -80C you're going to see dry ice precipitating out as snow. Once it drops to -200C you're going to see the air itself converting into a liquid and raining down. It will hit the ground, instantly boil back into gas, and spread out to the sides. The air liquifying will reduce the pressure in the zone, thereby pulling air down from above and around the zone. The result will be a sphere of inward-drawn air blasting a torrent of icy hell at the earth.
Should there happen to be any interesting weather formations in the area that aren't too high up, they will be sucked down into the effect and lend their own sparkle to the destruction.
The aforementioned destruction will come in a variety of forms. In the immediate area of the zone you get Mach 1 winds, meaning around 400 mph given the cold air. (For reference, the strongest hurricane ever recorded had winds of 185 mph and the strongest category-5 tornados are up to 318 mph.) Then you've got the cryogenic liquids freezing everything. If the 'anything' has water inside it (or sap, or blood, or...) then it will freeze and expand, destroying whatever it was inside. The liquid nitrogen (LN2) and liquid oxygen flow outward as a flood being driven by those hyperhurricane winds. They freeze everything they run into and also batter it with tons of kinetic energy.
Everything within a 0.75 mile radius is completely obliterated and a crater is dug into the ground, ranging from 'massive' in sand or loam to 'modest' on stone.
Everything within a 1.5 mile radius is destroyed. Concrete buildings are leveled, trees are demolished, etc.
Every living thing within a 12 [EDIT: the revision in zone size reduces this to 9] mile radius is killed through a combination of wind, cryogenic flood, breathing cold air causing the water in your lungs to freeze into lots of tiny sharp ice crystals that will shred your alveoli like grapes on a grater, etc.
If the caster starts inside the inner or middle regions then they're dead, period. (Well, unless they manage to escape during the ~1 minute that it takes for the effect to go from 0 to hellstorm, perhaps by reverse summoning or tunneling really fast and deep, then sealing the tunnel behind them. And conceivably there's some bullshit S-ranker out there with sufficient bullshit to survive being in the heart of the storm because S-rankers are made of bullshit. Hazō has never heard of anyone with such an ability and can't imagine what it would be, but he can't completely rule it out because S-rankers are bullshit.)
If the caster starts in the outer zone then they have a chance of escaping if they run immediately and very quickly, and they have skywalkers or some other way to get off the ground so as to escape the LN2 flood, and they have a way to breathe air that's killing them via grapes-on-a-grater cold.
Given the above, a brief summary of the EM nuke scene looks like this: over the course of ~7 hours Akane turned ~17 cubic miles (yes, miles) of air into cryogenic liquid and threw it into the middle of a katabatic Mach-1 hyperhurricane that blasted it outwards across a circle with area of ~500 (EDIT: after the zone size reduction, 250) square miles, demolishing the landscape and killing every living thing in that area except herself and Hazō. Despite Hazō's best efforts to be far from habitation, the affected area was vastly larger than he could have anticipated and Akane almost certainly killed a bunch of Fire's civilian population. Best guess, somewhere in the 200-600 range, although the Gōketsu likely will never know exactly.
For the record, the players did very well in selecting security precautions. If you were less cautious and had done this anywhere near Leaf then Akane and Hazō would have died, Leaf's civilians would almost certainly have all died, and a good share of Leaf's ninja would have died.
