I strongly suggest that you don't use this build in ranked play unless you have played it extensively. I first created this build both out of frustration from solo queuing as full support, and wanting to be a little more effective than the other AD builds I saw for Janna. This build is not recommended because it has what I consider serious problems in your overall teams performance. However, this build is extremely fun. So try it out, and if you feel like it give it a rating.

After playing as straight AD for several matches I found that my spells were virtually useless, and that my damage in the end wasn't stellar. It was good, but it wasn't that good to warrant how poor I felt my abilities are. I decided to try out hybriding and was impressed immediately. Not only do I have some punch like I was looking for, I still maintain a strong support role. I also find that most people, even after seeing me kill a couple people, are unsure what to do with an AD/AP Janna. Doing the blend allows me to focus on whichever defense they are neglecting the most, and makes my disruptive abilities more dangerous.

Pros:
Still effective at support
Somewhat unexpected damage output
Excellent chaser
Effective pusher
A tad bit more survivable

Cons:
Lower AP so less effective shields, and healing
Still squishy
Your team may be confused with what you are doing (solved by talking before the game starts)
Expensive builds
Fewer wards (potentially balanced depending on item path)

Recommended:
Ignite - This gives you a little extra kick and makes you especially effective at chasing, also helps people get a better idea of what you're gonna be doing at the start of the game.

Exhaust - This also helps with you chasing, and with you escaping. If you prefer flash to exhaust, feel free to take flash. Flash is a little more effective for escaping if there are multiple people. Where exhaust is more effective for escaping one person, and with the ability/attack speed reduction, also is really effective for fighting 1v1.

Good Alternatives:
Flash - Pretty much the same as exhaust, exhaust is simply my preference.

Teleport - Take instead of Flash. Good for moving to where the battle is since you are still the support, also a good spell to take if you want your opponents to be less privy to what you are doing.

Clairvoyance - All around good spell for a support even a hybrid support, helps keep your eyes up in place of wards.

Marks
Greater Mark of Desolation - armor pen is pretty standard for ad characters. If you really love AS (which I do) take Greater Mark of Attack Speed, but I find Desolation more effective overall.

Seals
Greater Seal of Attack Speed - attack speed is like 33% of this build, boosting it is good. If you find yourself using your spells more and having mana issues and you aren't getting a philo stone taking Greater Seal of Scaling Mana Regeneration is fine. I suggest you test it out and mix the two for the right balance.

Glyphs
Greater Glyph of Scaling Cooldown Reduction - I personally am a sucker for cooldown reduction. Since this build is hybrid around AD and AP I strongly suggest Celerity to make your spells slightly more effective. If you aren't a fan of CDR feel free to take whatever.

Quintessence
Greater Quintessence of Desolation - same as above. I am also a huge fan of movement speed, so Greater Quintessence of Movement Speed are also an option

I feel the masteries are pretty standard for damage builds. Some may wonder why I get Deadliness over Archmage's Savvy and the answer is relatively simple. Archmage's Savvy amounts to .6 x 18 = 10.8 ability power at level 18. It really isn't very much at all. I put a point in Cripple and in Burning Embers because I use exhaust and incinerate, make sure to move those talent if you aren't. Other than that the armor pen, spell pen, and bonus crit are all more worthwhile than 3 ad in my opinion.

The utility tree is also pretty standard. That last point can really go wherever. I pick up Greed sometimes, though it's not a huge boon, getting a Philosopher's Stone in a game is better. I also pick up Spatial Accuracy if i'm using teleport. If you feel you are using the jungle buffs a lot, take Utility Mastery instead.

I mostly build Zephyr and Eye of the Storm evenly. Zephyr keeps your speed up and gives you a slow as well. This is extremely useful for both ganking and escaping. Eye of the Storm is also good for staying alive and it DOES increase attack damage. This can help you farm a little bit early, or help keep your tower/ally alive and doing a little bit more damage. Get at least one rank of Howling Gale for it's disruptive ability. I usually get it 3rd, but getting it first can be useful. Pick up Monsoon at every point possible because it keeps everyone alive and helps you push better or split up the enemy team.

One thing I really want to emphasize here is that you are still a support character. Don't just spam your shield on yourself for more damage. Save your teammates when you can. You can also set up a gank by moving through the jungle and setting up a Gale. One thing I find in particular is that since I'm already in the thick of things fighting that I get into better Monsoon positioning. Use it to heal, use it to split a team apart. Use it to do both as much as possible.

Early
I start with a Vampiric Scepter because I like the life steal and it builds well into your later items. I feel that the life steal is just as effective, if not better than, taking anything else and a potion. Since you are trying to farm in the end, you might as well just keep shooting and getting any lost life back. I follow up with Berserker's Greaves because I love my movement speed, and it gives some initial attack speed. I don't get Mercury Treads because I do still tend to stay back and if you react well not having the tenacity is not too bad. Later in the game when you get Monsoon if you DO get caught you can just push everyone off you. I build a Tear of the Goddess next because it's got some mana regen on it and it starts building your mana pool with it's passive. I then get a Hextech Revolver for some AP, but mostly for the spell vamp. Having both spell vamp and life steal makes you really sustainable, even if you get into a big fight and get hurt you usually don't have to head back since you can just vamp all your life back. You also have Eye of the Storm to protect your health pool while your vamping up.

Mid
As I move into the mid game I start builing my bigger items. The first one I get is Manamune. This item gives you a little AD as well as 2% of your mana pool as AD. This may not sound like much, but at level 18 you should have around 3k mana. 2% of 3k =~ 60 AP which is nice. You also get a larger mana pool which helps with your support capabilities and your AP damage. I follow up with finishing my Hextech Gunblade. Spell vamp and Life steal plus a good amount of AP and AD. This is your first true split build item and is probably the most useful item you can get. You even have a cute activate ability which can really help when chasing or trying to escape or starting a nuke with.

Late Mid/Late
Now I'm working on my specialty items. I'm already doing a pretty good amount of damage. But I'm lacking in some of the AS that really makes this build fun. I build a Zeal next in preparation of my Trinity Force. The TF is one of the strongest items in the build. It has basically a little bit of everything you want. More AD, more AP, a little bit of crit (which your masteried into), more health, more mana, and movement speed. And don't forget the unique passive that comes with the Sheen I get next. Even if the item didn't come with all those nice stats the Sheen ability is amazing in this build. Toss out a shield and do 200% damage. Throw out your Zephyr to slow and do 200% damage. Use your gale to disrupt someone and do 200% damage before they even hit the ground. The 200% with the shield is particularly nice if you happen to be shielding yourself, since the shield also buffs your AD. After this I pick up a Guinsoo's Rageblade. It's the only other item that has both AP and AD on it that you haven't built yet, and it gives you the stackeable AS which is just devastating at this point. The last item in the build is situational. I pick Madred's Bloodrazor because... well you can. And it's funny. It also kills health stacking opponents, which is just hilarious. Janna owning tanks... think about it. Madred's isn't necessarily the best item. I have taken Thornmail before, because nobody kills auto-attacking opponents better than auto-attacking opponents. Banshee's Veil can also be really useful if you've got crazy AP opponents that can just nuke you. Chances are you are gonna be a little bit of a focus target so consider these options carefully.


Optional:
Wiggle's Lantern is a good early game option. It helps you farm minions a little easier and gives you a free ward since I tend not to buy wards as much in this build.
Philosopher's Stone, Heart of Gold and Kage's Lucky Pick are good items if you want to farm gold faster while giving you a little bit of early bonus stats, just remember to sell them back when its time to move into bigger items.
Ionian Boots of Lucidity can be useful if you find you are using your spells more often. I find that I almost never need them, but I have run a few games with them and did just as well as my normal games.

During the laning phase you should really just poke as much as you can. Be careful because you tend to have fewer hit points than others. So use your speed to poke in and get a shot off and then dart away. If you're laning partner is good, once you get them low enough charge in and use your Zephyr to slow them so you're teammate can capitalize. After you have Eye of the Storm, remember that it does increase AD and provides a shield. If you are pushing for a kill use it on AD champions to let them dive the tower a bit more, and give them that little extra punch to get the kill. Be extremely careful when using Zephyr because it takes away your natural movement speed, if you don't have boots yet and you are trying to chase, chances are you won't catch them. Also it hurts your chances of getting away from their laning partner if they try and counter you. Once you have Howling Gale it gets a little easier to gank and/or escape. You can drop the initial spell without stopping and you can hit Q again to cast it immediately. If they are close enough this will disrupt them giving you and your teammates a chance to catch up and/or flee. Monsoon is a good spell to use to heal up, and also throw someone back. If you can push an enemy into your own tower you can almost ensure a kill while staying safe.

As always, be careful of their middle and/or jungler. You don't want to push in too deep to their tower, because it doesn't take much to kill you. Even with all your disruption a few good hits or an ult and you're toast. Hang back and kill minions and poke when you can.

If you've made it this far, then you should have some interest in this style of play. This is my first guide and I may have assumed you already understood certain aspects of Janna's playstyle. If you happen to be in the dark, most of what I learned about how to play Janna I got from Panglot's guide Janna - cause your girlfriend cant blow good enough. Of particular note is his description of aiming with Howling Gale. His guide is also almost identical to how I play my Janna as a full support which is why I didn't discuss that at all in this guide.

I hope this has been an interesting read for you and that you do try the build. Once again I'm not suggesting these builds for ranked play, but this game is about fun, and this build is fun. Happy ganking.