Howling Gale can be used for farming after leveling the skill some and building AP.
Despite being a telegraphed move, Howling Gale boasts a deceptively long range that can catch enemies off guard.
When Howling Gale is charging in the brush, it is visible to the enemy and produces a sound cue, but once the attack is released, 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 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. Bear in mind using Zephyr's active disables Janna's ability to ignore unit collision.
Eye of the Storm can be used on allied turrets, thereby making it an excellent defense. A max-level Eye of the Storm base shield (240) boost a non-nexus turret's health on Summoner's Rift by slightly less than 10%. Nexus turrets have less HP than non-nexus turrets, so the max-level base value for Eye of the Storm boosts their HP by 12.5% HP. Evidently, building AP will improve the level of protection offered for turrets relative to their HP.
Eye of the Storm grants increased attack damage, so use it on an ally (or yourself) when attacking 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.
Eye of the Storm also adds its AD bonus to shielded turrets. The bonus AD ramps up with successive hits due to turret hit mechanics. This can be very useful during an enemy tower dive. Even if Janna doesn't land an attack on an enemy champion, if a shielded turret kills a champion, she will be granted the kill/assist.
Baiting an enemy champion to attack an apparently vulnerable Janna under a turret that's about to die is a great way to create opportunities for kills, specially in early stages of the game. Try shielding an enemy's last hit as he switches target to Janna so he attracts aggro from the barely alive turret to make him over-commit under fire or back out.
Bear in mind the AD bonus from Eye of the Storm only remains as long as the shield is active. If the shield if broken before reaching its full duration, the bonus attack damage is lost.
You must judge when it is appropriate to use Monsoon, especially in team fights. There are multiple functions for this ultimate:
To peel enemies away from allied champions, specially carries.
To allow space for your team to regroup after being engaged on in a disadvantageous position.
To deny an enemy engage or to disengage a losing fight.
To disrupt enemy champions' channels.
To restore lost health to multiple wounded allies during the course of team fight from a safe position.
To heal the team after a fight for an extended push.
To push enemy champions towards your team's champions or turrets.
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 there 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 her some utility.
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 StormEye Of The Storm scales extremely well with AP and quickly turns into a huge health barrier, and -ironically- getting even larger than the summoner spell barrier, as well as on a 6 second cooldown. This ability is not very useful late game without some AP.
Zephyr damage scales moderately but the slow gets very harsh with some AP.
Howling Gale, while it is Champion main damaging 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.
Having the second 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 it takes time.
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 building magic resistance.
Howling Gale's damage output is moderate, but it needs a few seconds before it can deal its 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 StormEye Of The Storm becomes a huge bulwark of health. Be careful of getting baited in to kill one of her low health allies. She may suddenly give them a very strong shield.
Do not underestimate Champion 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 a 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 Janna to use her 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.