The Wind Herrscher's authority was the creation of ideal fluids.
So-called ideal fluids were incompressible fluids with negligible viscosity—zero viscosity, in other words.
Besides displaying Wendy's ability to manipulate storms and air, she could also create wind blades capable of cutting through anything. For a specific reference, think of how Golden Gilgamesh's Sword of Rupture from the neighboring Moon World set cut through space.
By exploiting the indivisible nature of ideal fluids, she could create an absolute shield to protect herself.
She could manipulate air and pressure differentials, control the global climate, and create all kinds of harsh environments. On the microscopic level, she could create vacuums, high-temperature zones, and more.
Of course, Wendy had gone to study at St. Freya Academy at such a young age. How could she possibly understand such profound fluid mechanics? This was the kind of thing that required understanding the Euler equations.
Clearly, Miss Wendy was not very Euler.
Of course, that did not mean the Wind Herrscher's abilities were invincible. In truth, when pushed to the extreme, every Herrscher's authority could be considered absurdly powerful. Destroying human civilization would be no more difficult than snapping one's fingers.
For example, the Reason Herrscher could directly recreate any relevant object as long as she understood its principles. If she knew how to make hydrogen bombs or antimatter weapons, nuking humanity into peace would not be difficult.
Likewise, the Thunder Herrscher could manipulate electromagnetic fields. If she directly reversed Earth's north and south magnetic poles, human civilization could immediately type out gg.
Of course, discussing abilities without discussing output was just hooliganism.
Not to mention whether Mei possessed that knowledge to make such manipulations, Herrschers themselves had limits as well. The first Herrscher of Reason, Welt Joyce, had died because he overused his Herrscher of Reason