A bit lengthy for a spell description, but I think it's okay for a higher level spell. My only concern is whether this is actually a 5th level of power...
Stormstrider (Nature 5)
Casting Time: 4 Turns
Duration: 10 minutes (+5 per success)
Resistance: Speed or Minimum Damage
Reagents: Focus: A small talisman created from four tail feathers of an elemental Storm Eagle.
Powerful winds gather around you, and the air crackles with the elemental power at your command.
You can tread on air as if walking on solid ground, but at one and a half times your normal movement rate. Moving upward is similar to walking up a hill. The maximum upward or downward angle possible is 45 degrees. In addition you can jump up to your movement rate vertically, or twice your movement rate horizontally with powerful gusts of wind.
Strong winds have no effect on your movement.
In addition to movement, you can spend one attack action and send a burst of lightning at any target within 200 feet. Targets must pass a Speed test 18 or take +9 electrical damage. You must wait 1d6 turns between lightning attacks while the energy builds.
Should you cast this spell in the midst of an actual thunderstorm, you can send the storm's own lightning as a powerful attack against your enemies. Your strike zone can be any ten foot square within sight and open to the sky. Any target within this area must make a Speed test 20 or take +12 electrical damage. Anyone within 15 feet of the strike zone must make a Speed test 18 or take +9 electrical damage. Even those who make their speed test take minimum light wound damage and all are stunned for 1d6 turns.
This lightning attack is subject to the storm's own whim. Each turn the storm and spell last, roll 1d6. A result of 5-6 signals a lightning strike that takes your action for the turn, should you use it. Don't start rolling for another strike for 1d6 turns. You are unable to move or perform any attack actions in the turn after the blast.
Should the spell duration end while the subject is still aloft, the magic fails slowly. The subject floats downward 60 feet per round for 1d6 turns. If he reaches the ground in that amount of time, he lands safely. If not, he falls the rest of the distance. Since dispelling a spell effectively ends it, the subject also descends in this way if it is dispelled, but not if it is negated by an antimagic field. No lightning strikes are possible once the spell duration ends.