With this build I have been using Janna as a ranged AP Carry with support qualities. Her damage is burst-oriented with plenty of slows for harassing and other cantrips to give her allies and herself some greater survivability and evasive maneuvering. For me, this build gives her a strength on the battlefield that gives truth to her title.

I picked up Janna some time ago and had an instant proficiency with her. With the build that I have been using for her, I have been winning consistantly with her, (roughly a 60-80% victory rate). As I play her, she becomes less of a support caster and more of an aggressor, capable of aiding allies when needed but focused widely on causing a lot of damage and manipulating the battlefield. If used properly, Janna can be a strong carry.

+Deals heavy damage with Howling Gale
+Farms well
+Easy to avoid combat--Howling Gale will strike after you are already gone
+Fast with abilities that improve survivability and aid in fleeing

-Fragile--If locked in melee with now escape available, Janna is an easy kill
-Somewhat Complicated--Predicting your enemies' moves is imperative to being effective
-Attracts Attention--Enemies will notice that, even if you aren't getting every kill, you are a major problem. Be aware of your enemies' positions because you will become a priority to kill.
-Needs a lot of mana

*Note: To keep in lane longer, never fall idle. Keep moving, this will make you harder to target and keep you on your toes for a timely escape.

To be typed shortly.

Overall it seems that these masteries are the best for characters that focus on AP and their mana, and I am inclined to agree, thusly this is what I shall recommend as well.

I tend to neglect the value of speed and being faster than your opponents, so I will often buy Boots of Speed immediately. If you would rather have a combative edge early on in place of speed, buy an Amplifying Tome instead. (Buy shoes soon after, though, as avoiding melee is key.)

If you haven't bought Boots of Speed by now, do so, Then buy yourself a pretty Blasting Wand. This will give you the AP you need to actually begin dealing damage and possibly getting kills with your Howling Gale. This will also allow you to cripple (though not neccessarily kill) crowds of minions. You should be able to start building you Catalyst the Protector now, too.

You may have noticed that you're a little fragile by now. If you find yourself getting too bruised up, work on Rylai's Crystal Scepter now with a Giant's Belt. Also you should finish Catalyst the Protector if you haven't been able to yet. If you have not been getting beaten up and have a fair streak of kills and assists, buy Meijai's Soulstealer instead. This will allow you to gain AP along with gold with each kill you make.

You should finish up Rylai's Crystal Scepter now. Now your spells will slow whomever they hit. A successful hit with a Howling Gale will leave your foes vulnerable to you, allowing to get in range to attack (and hopefully kill) with a Zephyr.

Get some extra speed and kill power with a pair of Sorcerer's Shoes if your enemies have been building up their Magic Resistance (as they should). If not, then let's finish up the Rod of Ages so it can start building up its power over time. If you can, buy another Blasting Wand for even more AP.

If you don't already have it, get your Meijai's Soulstealer. You should be getting plenty of kills and assists by now for this to power you up. If you aren't getting the kills and assists for it already, though, then just move on.

It's time for another Blasting Wand! This will become a Rabadon's Deathcap as soon as you can afford the upgrade. The AP that this item offers is ideal. (If you can afford it, buy the Needlessly Large Rod instead of the Blasting Wand, it gives you double the AP for a little less than twice the price.)

If the game drags on after you've built your cap, you can begin working on the Lich's Bane. This little wonder will turn some of Janna's physical attacks into magick attacks, which could help tip the balance of the game with how AP heavy you've become.

---

I don't recommend much variation, but I am aware that flexibility is key. If you find that Magic Resistance is becoming a problem, buy a Void Staff instead of a Meijai's Soulstealer.

Don't be afraid to buy items out of order if a situation calls for such. Most items require a Blasting Wand to be made, so this is a safe first piece to purchase in any scenario.

If you're up against a sneaky Evelynne or Twitch, or even a clever Teemo biding his time and soaking experience, buy and use an Oracle's Elixir after every death.

Start with one point in Howling Gale. This will allow you to check the bushes for hidden enemies when you are getting in lane and deals the bulk of Janna's damage. It will weaken minions while you and your buddy farm and it really hurts any champions that are hit, (usually 1/4 of their starting health). A couple hits with this will leave a champion extremely low on health, but not so much that they will recall, giving you and your ally an opportunity to get first blood.

Take either Zephyr or Eye of the Storm as your second ability. Likewise, so long as you have at least one point in each, you can max out either of the two. Both of these give you some resilience so it is all a matter of which you want to use more.

-Zephyr gives you extra speed to make use of when fleeing multiple enemies, or you can use it as an attack to deal a small bit of damage and slow your enemy while both you and your ally flee. (This can also be used on a fleeing enemy to slow them and give you and your ally an opportunity to kill).
-Eye of the Storm will give you or your ally a bubble of temporary health, which can act as a comfortable buffer between your soft skin and a kill strike. Take this first if you or your ally are at an initial disadvantage. (Also, if you are comfortable luring enemies at low health into your tower's range, you can use this ability to save you while you get a kill off a tower's shot).

This will act as your primary skill throughout the game, and thus will be used in the most ways.

1) Checking Bushes
-Simply stand at one end of a bush and launch this down its length. Most people tend to flee the bush at the first shot fearing a second hit. If nobody runs out, the bush is usually safe for you, yourself, to hide. On occasion, however, you will run into an enemy still waiting for you to walk into their waiting claws, but this is uncommon and not much of a concern when you expect it-- just let a hardier character check the bush after your gust.

Weakening and Killing Enemies
Simply point and let charge, the whirlwind will do its work. It deals a lot of damage and greatly puts you and the ally you are in lane with to claim a kill.
-Howling Gale has a long range and, if you put your enemies at the very end of its area, they might not even see it coming.
-If your enemies start getting wise and begin dodging the gust, analyze how they avoid it. Once you identify a pattern, point the Gale in their path of escape and let them run into it instead of away.
-If an enemy is at low health and you want to ensure a kill, let them get you locked in melee. Your enemies will know that you are fragile and that your physical damage is weak, but they will not expect you to hit them with a Howling Gale that isn't charged.

Harassing
-This ability will go through walls and terrain, so don't be afraid to hide just outside of your land and throw Gales at your enemies while they are quarreling in lane. A lot of times they will even ignore you knowing that you don't even need to be near the origin of the Gale when they're struck.
-Feel free to move around and attack after you've placed a Gale, most enemies will focus on you instead of it, so you'll be able to strike enemies from angles they aren't even expecting.

Aiding Escapes
If you are being chased by an enemy, place a Gale while you are running IN THE SAME DIRECTION you are running. If your foe continues pursuit then he will stay in the Gale's path for as long as he chases you and will get hit by it after it fully charges and launches. If this doesn't kill the aggressor, it has still knocked him airborne and put more space between you and him.
(Note: this is an effective strategy for luring enemies into a futile tower dive, too. Just place the Gale as you get near the tower.)

Farming
After you've upgraded this ability a couple of times and have about 50 AP, this ability will one-shot basic minions and earn you great quantities of gold.

Fighting near Enemy Turrets
-Simply direct this at an enemy turret-hugger and back away. Howling Gale will deal its damage and you can advance against the tower without attracting any of its aggro.
-This ability will allow you and an ally focus on destroying a tower while also clearing away enemy minions, making your puch as effective as it can possibly be.

Striking the Unsuspecting Assassin
If you know where an assassin is waiting for you to encounter a moment of weakness, pretend you don't see her. Instead, act like you're farming minions, but point the Gale at the invisible assassin. After the Gale hits, focus your attention on that enemy.

Zephyr is a much simpler ability to use. For the most part you will just use it for its passive speed boost. Besides that it has two other applications:

Slowing an Enemy
If you or an ally needs an escape, a hit from Zephyr can slow an enemy just enough to allow for fleeing.

The Killshot
In higher levels Zephyr will become your tool to getting those final hits on a weakened enemy. Even if Zephyr doesn't kill, it will allow you and your allies to take advantage of a slowed and crippled enemy.

This is, perhaps, the most straight-forward ability. I use it primarily at low health, allowing myself or an ally to take a hit or two with no panalty when fleeing. A timely use can also nullify the strike of a turret. Do not abuse this ability because mana is precious; use when it seems necessary.

Aside from being a healing Area of Effect, the ability to push enemies away adds some tactical finesse.

Fleeing
While you or an ally is running and in great danger of death, using this will push an enemy far away to provide an escape while gaining health.

Trapping
If an enemy has pushed past the creep line and needs to get away, you can use this ability to push the enemy deeper into your team's territory--a dangerous place for weak and solitary enemies.

Pushing
Though it is a short distance, using this ability can allow your minions to advance on a turret if need be.

Destroying Turrets/Nexus
Things can get intense when downing structures. Monsoon can push away all of those pesky enemies while you and your team focus on the structure being destroyed, all while getting healed.

Protecting Turrets
When your own turret is in danger of falling, using this can push enemies just out of range and let your minions hold the line for you.

Mass Heal
The most obvious way to using this, you can heal your whole team, minions included, while fighting a team battle.

Lv. 1-9: During this time you are not going to be dealing enough damage to get a kill alone. Whether you are soloing mid or side-laning with a partner, you are just doing enough damage to force your enemies to retreat, allowing you (and your partner, if you have one) an advantage in both gold and level.
You shouldn't be farming minions for yourself too much at this time if you're with a partner, it is important to let your ally get enough gold to play effectively as well.

Lv. 10-15: During this time is when you really shine as an aggressor. Here you will have the speed, damage potential, and survivability to get your few kills and many assists. Be careful, though, because your enemies should see you as an issue of priority by now. Be very aware of your minimap.

Lv. 16-18: We're at the endgame now. You should be at your enemies' base at this time, and now your jobs have changed. You are not going to be acting as a carry, but rather more as a support with an edge. While your allies and minions are downing towers and the nexus, you should be focusing on killing all the enemies' minions with Howling Gale and harassing enemy champions, keeping their team at a disadvantage.
If, by chance, your team gets pushed out of the enemies' base, charge mid and blow a Howling Gale and a Monsoon to create an opening.

I strongly suggest Clarity and Heal

Clarity-Mana is Janna's most precious resource, and she will be using a lot of it. Clarity will allow her more time doing what she needs to do, and the short cooldown time will allow you to stay in lane for a long time, even when you do run short.

Heal-Though you shouldn't get hit much, you are still in danger of dying. A single hit can be crippling, so keeping Heal ready can save you from the inevitable death. This will also be a strong tool in keeping your allies alive as well, which is always a boon.

Janna works best with a Tank or an Initiator. Though you will be using Janna as an offensive character, your partner is still going to be the one soaking the damage, so you need to be ready to give him a buff when the situation calls. You might have to use an uncharged Howling Gale to save your fighter so that he can survive and protect you.
Also, don't be angry when all you get are assists. You might be dealing the bulk of the damage, but you do not have the attack speed to kill without giving your enemy an opportunity to run.

When your partner recalls or dies and you are fighting 1v2, switch your playstyle to defensive. Focus on minion kills to prevent your enemies from pushing, players will heed the creep line. If you need to, tower hug, using Howling Wind to thin out enemy minions and hope that your enemy champions will be caught next to your tower without minion support.

Janna can play mid effectively if not poised against a ranger. She can torment and harass short-ranged characters, even kill them, with no problem alone. Because of her low health, however, once she is put up against a character with a similar range she is in trouble and should not lane alone. Miss Fortune, Caitlyn, and Vayne are all bad match-ups for Janna, so be wary.

Now that I have reached level 20, I have decided to keep a running record of my Janna matches. Games recorded in Green are victories, those in Red are losses. Games can be assumed to be 5v5 games on Summoner's Rift unless stated otherwise.
Format
-kills/deaths/assists- -turrets/barracks destroyed- -minions killed- -gold earned-

Level 20
2/7/12 2/1 188 22,902gp
0/14/21 4/2 172 14,611gp
0/5/0 2/1 135 7,645gp (1v1 against LeBlanc on Twisted Treeline)
0/2/8 2/0 86 6,711gp
0/0/5 1/0 124 6,869gp
1/8/4 0/0 65 5,444gp
1/5/11 1/0 99 7,910gp
4/9/10 1/0 101 8,490gp
1/5/12 2/0 102 7,998gp
7/5/7 1/0 237 14,248gp
2/5/8 0/0 128 10,469

Level 20 Averages:
Kills-1.64
Deaths-5.91
Assists-8.91
Structures-1.81
Minions-130.64
Gold Earned-10,299.91gp

Win Rate- 63.64%

-kills/deaths/assists- -turrets/barracks destroyed- -minions killed- -gold earned-

Level 21
4/8/8 0/0 135 10,818gp
2/6/9 1/1 143 18,261gp
2/7/15 3/3 157 12,414gp
1/4/9 1/0 149 8,600gp
7/8/19 2/0 138 13,736gp
2/7/15 0/0 156 12,748gp
1/7/9 0/0 160 10,770gp

Level 21 Averages:
Kills-2.71
Deaths-6.21
Assists-12
Structures-1.57
Minions-148.29
Gold Earned-12,478.14gp

Win Rate- 57.14%

-kills/deaths/assists- -turrets/barracks destroyed- -minions killed- -gold earned-

Level 23
3/6/11 0/0 95 7,783gp
3/12/17 3/0 118 12,462gp
5/12/6 2/0 190 12,933gp
3/7/13 0/0 111 10,256gp*
2/4/11 1/1 121 10,136gp
0/2/4 1/0 128 8,918gp
9/10/14 1/2 145 15,225gp

Level 23 Averages:
Kills-3.67
Deaths-7.67
Assists-10.5
Structures-3.67
Minions-132.83
Gold Earned-11,342.83gp

Win Rate- 66.67%

*I don't know whether this game was a victory or a loss, I lost my internet connection at level 18. Because of this, I am omitting it from any of my level averages. By the way, I dislike Cox's rampant unreliability...

-kills/deaths/assists- -turrets/inhibitors destroyed- -minions killed- -gold earned-

Level 30
1/4/23 0/0 119 12,930gp

I wholly recommend that you all offer any critiques that you can.
Thank you all for reading, I shall ammend as needed and make the guide smoother over time.
~Sullindir

P.S.- Before voting, I would like to encourage that this build be tried before hand. Thank you for your courtesy.

7/14/2011
-Added "Edits" section
-Added "Phases of the Game" section

7/30/2011
-Added "Match History" section

11/28/2011
-Updated Mastery Trees"
-Added Runes