Air Magic
is about the secondary skill. For air magic spells, see School of Air Magic.
Air magic is a secondary skill, that increases the effectiveness and reduces costs of spells from the School of Air Magic.
Discussion
The main reasons for choosing air magic are fly, dimension door and haste spells.
The first two are most powerful adventure map spells together with town portal (Earth Magic). However, fly and dimension door are often banned from custom maps. Nevertheless, expert air magic causes haste to be cast at mass spell, which makes it worth the secondary skill slot.