Are Thunder Strikes (EVO 4) affected by the caster's proficiency with air magic, and to what extent? In other words, is there any difference in destructive power between a thunder strike cast by a lv3 and a lv8 Air mage?
I don't really think it's well explained, but a + in a spell description means a spell improves with levels.
Thunder strike reads 26++ so that means damage increases by two per extra air level. More common is just one + where it would be 1 damage per extra level.
Ranges are often written with a simple + too but in that case it means range increases by 5 per additional level.