Howling Gale can be used for farming after building AP. It is also a great harassing ability against champions that stand behind their minions.
Janna can finish off wounded enemies with Howling Gale with its deceptively long range.
When Howling Gale is charging in the brush, it is visible to the enemy. But once the gale proceeds it will become invisible until it comes out from the brush.
Because you lose your movement speed boost after firing Zephyr, consider the consequences of doing so.
When you're escaping from an enemy gank, keeping Zephyr's passive may be more useful than activating Zephyr to slow down your enemy and losing the movement speed bonus.
Because Zephyr allows Janna to move through units, she can easily harass enemy champions while using their minions as an obstacle. Doing so will result in drawing minion aggro, so it is beneficial to self-cast Eye Of The Storm when under attack.
Eye Of The Storm can be used on allied turrets, thereby making it an excellent defense.
Eye Of The Storm grants increased attack damage, so use it on an ally (or yourself) when attacking an enemy turrets or in team fights. To maximize the potential of the shield's attack damage bonus, Eye of the Storm is best used on a friendly DPS champion during a team fight. Take into consideration the current attack speed and attack damage scaling of your potential allied targets.
Monsoon is one of the best counters to a team with multiple AoE ultimates/abilities as it will knock enemies back, preventing initiation, canceling channeled spells, and healing your team.
Janna's abilities allow her to have a natural movement speed bonus that should be used to her advantage.
A quick combination of Howling Gale and Monsoon is a very good way of saving your team in a losing fight.
Discharge a tornado ahead of you when fleeing an enemy, make sure they follow you in its' path and release in a timely manner to stall them.
Janna is built to be a highly utilitarian support, but her high scaling of ability power allows her to function as a CC-oriented mage also. Her prime statistics are ability power and cooldown reduction.
As an AP support Janna enjoys gigantic ability power ratios, notably on Monsoon's heal.
Unlike other AP supports, like Lulu or NamiJanna's attack range is very short, making the AP support core item Spellthief's Edge not very useful against ranged opponents. As a normally passive and defensive support Ancient Coin is more fitting for her
From that on feel free to grab AP or pure support items. AP will transform into powerful utility and damage while support items will plain out give the utility to her.
At about 350 AP Monsoon's heal is so powerful that it can make Janna's team almost invulnerable to non-burst damage for its duration
Eye Of The Storm scales extremely hard with AP and quickly turns into a huge health barrier, and -ironically- getting even larger that the summoner spell barrier, as well on a 6 second cooldown. This ability is not very useful late game without AP
Zephyr damage scales only moderately but the slow gets very harsh with AP
Howling Gale while is Janna's main damage ability is also the one that scales the worst with AP. The ratio is mediocre if fired right away and decent at best if fully charged
In a more supportive build items like Mikael's Crucible, Zeke's Herald and Locket of the Iron Solari are useful. You should bear in mind that Janna's scaling with AP already boosts her utility a lot -not to mention her damage- so building this items is a questionable decision.
Lich Bane is a core item on AP Janna, as it allows for more sustained damage with only two damage spells. To maximize AP Janna's efficiency, spread out her skills so that each one procs Lich Bane.
Building Deathfire Grasp on AP Janna is also required as her burst is not that strong, and the secondary effect improves the rest of her otherwise mediocre damage.
Having THE largest mana pool in the game, building Archangel's Staff is a good idea. However bear in mind that Janna has a hard time charging it up. Only pick it up if you are sure the game will last for long. It bears great results but takes time
The Howling Abyss
The Crystal Scar
The Twisted Treeline
Although Janna is mainly played as a support, her damage output is still significant due to high AP ratios from Howling Gale and Zephyr. Do not neglect magic resistance.
Howling Gale damage output is moderate, but it needs a few seconds before it can deal full damage. Quickly avoid the spell's path to avoid the damage.
Have your team stagger to avoid the knock-up, which can be used easily almost instantly.
By late game Eye Of The Storm becomes a huge bulwark of health. Be careful of getting baited in to kill one of her low health allies. She my suddenly give them 500 bonus health.
Do not underestimate Janna's mobility, as her Zephyr increases her movement speed passively and allows her to move through minions.
Using Zephyr as a slow removes her movement speed bonus while it is on cooldown. Baiting her to use the slow and then attacking her with a gap-closer can render her more vulnerable. However, this will only really work if Howling Gale and Monsoon are already on cooldown since she may use those in response and as a melee champion, if your team can slow her as well since the duration of Zephyr's slow is significant enough to likely stop as few autoattacks for melee champions trying to chase her without Janna being slowed herself.
When you see Janna channeling her Monsoon, quickly disable her to prevent her from healing her teammates.
If your team cannot easily stop it, retreat. The healing from Monsoon is so gigantic that gives her team near invulnerability while it lasts- it is best to cut your losses, and try to re-engage again later before Monsoon goes off its significant cooldown.
During early game, try to force her to shield early(through damage, movement feints, etc.). Then wait for it to wear off, so you can use your higher damage spells on her. This can severely cripple her presence in lane or force her to go back, potentially leaving her ADC defenseless or turning her into an easy kill.