What is "wetterau"

The Wetterau is a fertile undulating tract, watered by the Wetter, a tributary of the Nidda River, in the western German state of Hesse, between the hilly province Oberhessen and the north-western Taunus mountains.

Bettina von Arnim writes of Wetterau in her text Diary of a Child in the chapter "Journey to the Wetterau".