Thanks for looking at this build, its one I have had a lot of fun with. AP Janna built right is far more effective than just support, your spells for support are much more effect in damage health and protection which other teams then have to pay attention to cause now you can kill as well.
Please take a look and let me know if you like it or not, whether you do or not I'm gonna keep playing it, I'd just like to let people know my take on Janna, a good APer with some staying power who really needs to be feared whether in a group or alone.

With this build I play back and use the speed I gain from Zephyr to keep from buying boots. This allows me to buy Doran's Ring for more health and Damage and save money for items that will give me more potency. In emergencies you can hit your Ghost to get out this early too. If your not used to playing cautiously you'll have difficulty. With that being said sit back and just keep the enemy off your tower, use the Howling Gale to kill minions without having to get dirty and if enemy Champions come after you use the Zephyr to 1, slow them down to get away and 2, hurt them and make them think twice about charging, you also have Eye of the Storm if you need a quick Defense or to boost your ally if he's countering to help you. If you can play Janna well once you get 2 points in Zephyr you can do some good damage and hunt down your opponents who are low on health, again using Ghost if things get sticky. Build up till you get 1600 to buy the Needlessly Large Rod, which will build into the Cap to make people not want to get close to you. With your Ghost you can do some serious damage now and chase people down.

Your Howling Gale by mid game should do enough damage to keep enemy minions far from your tower and your AP should hurt enemy champions enough to make them stay back, you can solo bot now and allow your partner to go mid and help take that out, or if you can push bot and take the tower out, at which point you or your partner can go push mid. Once you get the cap your next focus should be to make yourself at least a little more survivable. Buy Giant's Belt for the life, it really makes a huge difference and builds into Rylai's Crystal Scepter which will give you a little more life and the AP that will make your deadly to anyone with half life or less. That and your moves being great CC will allow your small group fights here to become very lopsided as you do a ton of damage and slow them or throw them in the air allowing your teammates to pounce and gain an easy kill. Make sure to space your two damage moves out unless your combo can take the kill. After the scepter you will need boots, allways with Janna you want Ionian Boots of Lucidity, the cooldown reduction is awesome and with the Speed passives Janna has you will move faster than almost everyone if not everyone in the game, include your Ghost and you should feel comfortable to get out of bad spots.

Now that you have the AP and the speed the next two items are based on the situation, or personal preference. You can buy Zhonya's Hourglass to get some armor, a lot more AP, and the ability to be invulnerable for 2 seconds. Remember in team fights people will look to take you out first since your still pretty squishy. With this let them come, hit the ability and let your team kit them while they do nothing to you. If it wears off and their are still all over you it your shield or Ultimate to help yourself and go on the offense, several of them should be weak enough at that point for you to get one hit kills and turn the tide.
To go with some support and help yourself still get Abyssal Scepter, this weakens them all form your and your allies AP attacks so you guys can get rid of them faster, plus it gives you more AP and MR which is Awesome.
You could go for Will of the Ancients but more than likely you'll not help as much as the other two can, but if your team is AP heavy and they have little MR, which should never happen, but if it does, buy this.
Lastly is one that could really help you out a lot too. Athene's Unholy Grail will give you a good amount of AP, some MR and a pasive is more cooldown reduction which means more tornados to go around.
Based on the situation pick between these 4 for the last two items, and if the game still isn't over sell the dorans and get yourself something nice.
Now enough of the buying late game is all about team fighting. Don't forget your filling two spots in this battle, your an AP beast and your support capabilities can also give you a big edge. So do what you can in damage but don't leave your partners hanging, you should be ready to use Eye of the Storm on your buddies and Monsoon too to keep them alive and in the fight. next if you see a squiter about to get away stop them with Ghost and Zephyr or Howling Gale to either kill them yourself or get you team to finish him, or even THEM.
In the retreat you should be between your team and the enemy, save them since you have much better get away speed and your Kitting is awesome which has turned to tide of a team fight many times for me. You have enough abilities to keep yourself alive, or alive long enough to save 4 others.

- You are not a support so don't sit back and let others take all the minions, but be generous so you don't screw your buddy, your howling gale and items together can kill a whole line of minions in one hit so don't get selfish.
- At towers always kill the enemy minions, with Howling Gale, about to get to you, this will allow your minions and you to focus on the tower.
- People wont expect you to hit so hard, if you think you can take someone one on one do it, you have a lot more weapons and getaway ability than them.
- Remeber that you hit HARD, so when you try to slow people for your team you may just kill them. At which point I've been called a Kill Stealer a lot. Just trying to slow people with that damage may have that effect, if you have a weakling who gets hurt about you trying to help and taking a kill don't let it destroy the team morale but don't worry about it, let them knwo what you were doin.
- Your an awesome Ganker with the Zephyr and Ghost, do it when you can.
- Some champions are just gonna kick your but, so be careful around them. You should know this though.
- Can't think of anything else, let me know what you think.