*Disclaimer*
This guide is not intended for Ranked play, as Janna is a better support champion than AP nuke. This build is more for fun than anything else.

Overview
Janna is one of the best - if not very best - champions that is positively affected by AP. All 4 of her abilities are scaled with AP and the ratios are incredibly high.

Her Howling Gale (the reason your creeps will constantly be beating on your opponent's mid turret) has a .75 AP ratio - not to mention that it does more damage the longer it channels, which can wipe out an entire line of minions in one fell swoop.

Her Zephyr (the reason she doesn't need early boots) scales at .80 AP which will be her main constant harass nuke.

Her Eye of the Storm has a .90 ratio which is why you only take one rank of this in the beginning. Her shield comes mostly from AP, which you have plenty of. So the base number from ranking the shield up won't be that much of a difference.

Lastly, Monsoon - which has infinite uses - has a .35 AP scale for each of the 4 seconds that you can channel it. At a full channel, you can heal for 140% of your AP + 4x the base.

This is simply an overview, but the skills will be covered more in-depth later in the guide.

Pros

Can harass often without giving up CS
Her shield can prevent her from taking any damage while harassing
Her Howling Gale can wipe an entire wave of minions
Her passive AND Zephyr give her speed buffs along with being able to pass through minions
Can sustain in lane for long periods of time with clarity and her ultimate
Still works beautifully to support in team fights
Hitting Q twice quickly can interrupt certain ultimates and other channel abilities
Your mid turret WON'T go down

Cons

Requires blue buff to be fully effective (mana hungry)
Squishy
CC can pick her apart quickly
Cannot mid against anyone with a silence
Only has 2 damaging abilities

When you start off the game, you're gonna want to grab a Doran's Ring. It lets you get a little tankier, helps out with some of your MP5 problems, and gives a little boots in AP. On your first trip back, pick up a Blasting Wand or a Needlessly Large Rod if you have the gold. The first item you'll want to complete is your Rabadon's, to maximise your AP ratios.

After this, grab tier 1 boots and another Blasting Wand or a Sheen if you have enough. You're gonna want a Lich Bane because you only have 2 damaging spells. If you don't spam all of your abilities at once, you can take advantage or the Lich Bane autoattack proc twice while you're waiting on the CD for your Zephyr. Also, it gives a 7% speed boost which stacks with your Zephyr and passive Tailwind.

To maximise the utility of Lich Bane, throw your Zephyr on the enemy and hit them with an autoattack while slowed. Immediately after, knock them up with a quick double tap of your Q. Because Lich Bane has a 2 second CD, your Q won't trigger the effect. You'll have to wait about a second and a half to use your shield which will make the Lich Bane proc and add to your autoattack damage as well. Hopefully by then you'll only have a second or 2 left on your Zephyr.

By now, it should be mid to late game. AD carries will probably have a Phantom Dancer and that Singed might have a Force of Nature. It's finally time to complete your Sorcerer's shoes. Also, while you're back at the base invest in another Blasting Wand until you have enough cash for Rylai's Crystal Scepter. This item will help you be more tanky and will help you chase or escape more easily. This slow from Ryalai's will proc on your Howling Gale, proc on top of Zephyr's slow, and will even slow them if you knock them back with your ultimate. This is definitely a great item on Janna.

By now, it's definitely late game, and you'll need more damage along with more max mana so you can stay on the offensive longer. An Archangel's Staff will be perfect for this situation as it will give you around 150 more AP combined with Rabadon's passive and the 3% mana conversion on the staff.

For your last item, you can either get another Archangel's if the enemy team doesn't have much MR or you can grab a Void Staff to counter a team with high Magic Resist.

Howling Gale

- This is Janna's best skill. You can put a Q down in the direction of your opponents minions and go do something else while you get the gold and exp for killing them. This ability is why Janna can harass without giving up farm.

When you're trying to escape or need to quickly knock someone up or put some damage on them, hit q once and quickly hit q again. This will do around 250 damage less than a full channeled gale, but it's very unlikely that any of your opponents will sit around and let you channel your gale to take the damage.

Zephyr

- This is your main harass nuke and your speed buff/debuff. After you set your Howling Gale to dominate a wave of minions, pop your shield and float in toward the enemy champion. Throw a Zephyr at them and float back to the safety of your minions.

This combination is the most annoying thing that Janna can do. When you pop your Q, you will still be getting farm. Popping your E will make sure that you don't take damage back from the enemy champion or minions when you go to harass them with Zephyr. And Zephyr's speed debuff will prevent them from catching up to you to land any high damage combos to break the shield.

Eye of the Storm

- This skill definitely helps with Janna's (and of course whoever you want to put the shield on - including turrets) survivability. If used correctly, and in time to stop damage from hitting your actual health, you can stay in lane indefinitely.

One thing that people often forget about for this skill is the attack damage buff that it gives. If you pop a shield on someone like Nocturne right before he uses his ultimate to dive into an enemy, he will not only have a few hundred false Health Points, but he will also have a large increase in his damage. This can also be helpful if you notice that one of your turrets are going down. Pop a shield on the turret and it will also output higher damage for the duration.

Monsoon

- This is my personal favorite one of Janna's abilities. While the heal is definitely nice, I use it mainly for its utility which is why I only get 1 point in it until levels 17 and 18. If you use Monsoon tactically, and land a great knock back with it, you feel so accomplished. :3

One of my favorite combos with Monsoon goes like this: If you're being chased toward your turret, or if somehow your opponent managed to push the creep wave close to your turret, you can pull off one of the most awesome combos ever.

Right before you attempt this combo, throw a shield on the turret so it will do more damage when it targets your enemy. If you can manage to flash to the other side of your opponent, you can use the knockback from Monsoon to send them flying into your turret. If you quickly move toward the enemy and throw a quick doubled-tapped Q, it will knock them up for a brief period, causing them to take even more damage from the tower. And finally, you can use your Zephyr to ensure that they take as long as possible to get out of the danger of your turret. If SOMEHOW your opponent survives, give them a round of applause in /all chat, because that's impressive. :)

In no means is this guide meant to meant to be taken as a hard guide on Janna. Again, Janna is a support champ, and in serious ranked games, she should be played as such.

However! This is a super fun way to play Janna, and I thoroughly recommend Janna players and non Janna players alike to try this build. It actually works. :3