When Howling GaleHowling 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 StormEye Of The Storm when under attack.
Eye Of The StormEye 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 StormEye 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.
Janna is played primarily as a support, but can also be played as a viable AP carry and less viably as an AD champion.
You should build two or three gold per 10 before building legendary supportive items. As a support, you should not get last hits (unless no one else on your team is there to take them), so you need a different gold source - gold per 10 items.
You should always get a Philosopher's Stone as you need the mana regeneration and health regeneration.
Shurelya's Reverie builds out of Philosopher's Stone and offers a strong movement speed buff. That can help initiating fights, catching enemies or escaping. Because of the utility and the low price (including the gold gain from philosopher's stone) it's a great support item.
Although Janna is mainly played as a support, her damage output is still significant due to high AP ratios from Howling GaleHowling Gale and Zephyr. Do not neglect magic resistance.
Howling GaleHowling Gale damage output is tremendous, 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.
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 GaleHowling 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 will likely cause the enemy to win another head-on fight - it is best to cut your losses, and try to re-engage again later before Monsoon goes off its significant cooldown.