Mana density zones
By default, a planet would have an even distribution of atmospheric mana. Mana behaves somewhat like a gas in that it disperses naturally. However, once a certain threshold of local mana density is reached, mana compacts and drags in its surrounding mana, until a maximum density is reached, and the suction stops. Different materials can store different amounts of mana per volume.
Differences in atmospheric mana density are chiefly caused by mana crystals.