2) Then I checked guide and found ( https://tomenet.eu/guide.php?chapter=6.8 ) that it's totally dull. Almost all spells are useless and bonuses also not good.
3) I went through http://angband.oook.cz and... no one using Air magic. If no one using it - maybe it means that it has to be buffed?

Analysis:
Noxious Cloud
Fires a poison cloud at a designated target.
Lv31: Unbreath cloud, harder to resist since it prevents
living creatures from actually breathing.
Required skill level for first spell level: 3.
// The only good spell there. Good at beginning and VS living creatures later on. But a lot of stuff immune to it.
Lightning Bolt
Fires a lightning bolt at a target.
Required skill level for first spell level: 6.
// Manathrust better
Elemental Shield
Gives temporary resistance to heat and cold, and at
higher tier also to lightning and acid. (auto-project)
Required skill level for first spell level: 15.
Shared spell (see above): Fire, Water.
For the complete version: Fire, Water, Earth, Air.
// It was fun idea to make shared spell, so more people would take Air too. But it's much easier (and useful) to use res.potions
Thunderstorm
Throws thunder bolts at random monsters nearby.
(33% lightning, 33% sound, 33% light)
Required skill level for first spell level: 15.
Shared spell (see above): Air, Nature.
// Worst spell ever. Looks cool, but it could mess everything very easy. Stealth would be ruined with this.
Wings of Wind
Grants feather falling.
Lv16: Lets you levitate.
Required skill level for first spell level: 16.
Shared spell (see above): Air, Conveyance.
// Useless.
Invisibility
Turns you invisible.
Required skill level for first spell level: 30.
// Not bad.
BONI: Feather falling at 30, // useless
poison resistance at 40, // something, but not really. one of the most easier resists.
levitation at 50. // useless (at 50 so much items gives you levitation)
===============
Suggestions:
1) Make boni like this:
BONI:
Feather falling at 20
poison resistance at 30
levitation at 40
poison imm. at 50.
2) Rework some spells:
- Move "Noxious Cloud" to Nature school (also very unpopular/quite useless school; cloud would make it much better). Instead cloud put 'Lightning Bolt' as 1st spell with level: 3 req.
- Second spell --> 'Chain Lightning'; AoE spell which hits several creatures one by one
- make 'Thunderstorm' attack only 1 enemy which you are already attacking. So it wouldn't work till you start fighting with particular mob.
I hope it would help to make Air school more popular among istari

p.s.
Also 'mind' school is pretty dull.. It makes not sense to put there skillpoints (_temporary_ ESP is not enough, maybe make it constant at lvl 50?).. And add at least one dmging spell.