Improved Scorch

From WoWWiki

Improved Scorch
Increases your chance to critically hit with Scorch, Fireball and Frostfire Bolt by and additional X% and your damaging Scorch spells have a Y% chance to cause your target to be vunerable to spell damage, increasing spell critical strike chance against that target by 1%, stacks 5 times.
Usable by
Class	Mage
Properties
Location	Fire, Tier 4
Affects	Scorch, Fireball, Frostfire Bolt
Ranks	3
Requirements
Points required	15

Improved Scorch is a mage talent that increases the critical strike chance of your Scorch, Fireball, and Frostfire Bolt spells by up to 3% and gives your Scorch spell the ability to debuff your target to become vulnerable to spell damage, increasing spell critical strike chance against it by 1% from all attackers, stacking 5 times for a total maximum of 5%.

The self-specific critical chance increase is passive, meaning it does not rely on the Improved Scorch debuff to be up on the target. This means Fire raiders will likely want to take it even if they do not apply the debuff in their raid groups.

Rank table

Rank	Chance to debuff
1	33%
2	66%
3	100%

Improved Scorch versus Winter's Chill

This effect does NOT stack with Winter's Chill which provides the exact same bonus. As such, a single charge of either will not allow the other to be applied to the mob, making mistaken Scorch or Winter's Chill applications costly in terms of raid DPS and time.

A mage must decide which to use in his spell rotation. While this would naturally follow one's chosen spec (either Fire or Frost), careful consideration of either Winter's Chill or Improved Scorch may prove very beneficial to one's build and strategy.

Currently, Winter's Chill is preferred over Improved Scorch. Only Scorch applies Improved Scorch, while Winter's Chill is applied by any frost damage spell, including (as of Patch 3.0.2) Blizzard and Frostfire Bolt. This makes it easy for Winter's Chill to overwrite Improved Scorch, but not vice-versa.

When it comes to speed of application, it takes at least 7.5 seconds to apply 5 stacked Improved Scorch debuffs because of Scorch's 1.5 second casting time and repeated casts, whereas Blizzard (which stacks Winter's Chill per damage tick, making it the fastest way to apply the debuff) needs only a single cast and at least 5 seconds to apply a full stack of Winter's Chill. The Glyph of Improved Scorch can speed Scorch up, as it causes Scorch to generate 3 applications per hit.

In terms of debuff duration, Improved Scorch has the advantage. It lasts 30 seconds compared to Winter's Chill's 15 seconds. This means a Fire mage does not have to Scorch often to maintain the debuff or lower his DPS. However, Winter's Chill is refreshed by any Frost spell from a mage with the Winter's Chill talent, making it the easier to maintain of the two debuffs. Because even high-powered, damaging Frost spells can be used to maintain Winter's Chill, a Frost mage will not lose as much DPS as a Fire mage who has to switch to Scorch to refresh Improved Scorch on a target.

That is not to say Scorch and Improved Scorch are completely without advantages. Scorch has excellent DPM and is good for non-solo PvE as it attracts little threat, at the cost of high damage. Between Scorch applications, a Fire mage's already high damage spells will be doing even more damage thanks to Improved Scorch.

Ultimately, it is recommended that either be looked into by a mage player for use in his build where applicable.

For parties and raids, in the event another mage is present who uses the talent opposing one's own, communication must be made so that either applications of Scorch or Winter's Chill do not cancel each other out or that the stronger debuff is used.

Patch changes

Hotfix (2009-04-21): "Shadow Mastery and Improved Scorch will no longer stack with each other. ^[1]"
Patch 3.0.2 (2008-10-14): Increased spell critical strike chance against the affected target (old - increased fire damage done to the target by 3% per stack.)