Source: Warhammer Fantasy: 4th Edition

Wizards (Flying & Aerial Combat)
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Wizards often ride large ferocious winged monsters such as griffons and hippogriffs. When they are on the tabletop wizards may use their powers as normal, but when they fly high above the battlefield wizards can no longer cast spells on to the ground as they are too far away. By the same token spells cannot be cast against them by wizards on the ground. High flying models can cast spells against each other. All high flying models are considered to be within range of spells as they weave and soar around each other.