Adds a chill effect to your Blizzard spell. This effect lowers the target's movement speed by X%. Lasts 1.50 sec.
Improved Blizzard is a Mage talent that adds a chill effect to the Blizzard spell, which slows opponent movement speed for one and a half seconds. This talent is used primarily for frost AoE grinding, and ca be quite useful in PvP, or wherever one wishes to snare a large number of players all at once.
At max rank, Blizzard will be one of the mage's strongest snare spells, matching the 50% of Cone of Cold. These snare effects do not stack; if Cone of Cold is in effect (with more than 1.5 sec left), the snare from Blizzard will not overwrite it. 
Improved Blizzard makes the Blizzard spell an effective snare, but at a large mana cost. This chill can proc Frostbite, Brain Freeze and, for most important, Fingers of Frost. Since the snare is reapplied every time it hits (wich is often) Fingers of Frost will proc often, making frost mage AoE devastating when coupled with Shatter.
Patch 3.0.8 (2009-01-20): The snaring effect has been reduced to 20/40/50%.
Patch 1.3.0 (07-Mar-2005): Chill duration reduced to 1.5 seconds (4.5 seconds w/ Permafrost) and refreshes with each damage tick rather than at the beginning of the spell. In addition, a target that resists the damage will also resist the chill effect.