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](lore/Mana ore and crystals.html).