Mage · Frost · tier 3 · from level 20

Improved Blizzard

Rank 3/3

Adds a Chill effect to your Blizzard spell. This effect lowers the target's movement speed by 40% for 1.5 sec.

Doesn't change the damage or healing of any Mage spell.

Averages, counting crits and misses, for a level 60 Mage with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.