Who was the bishop in (339-397) who successfully challenged and limited imperial authority over the church?

St. Ambrose of Milan was decreed as a bishop while serving as catechumen. He was Governor of Milan from 339-397. He ejected Emperor Theodosius from the church and compelled him to do penance over a massacre. He was a supporter of monasticism and instrumental in developing the formal acts of the Church.  

the causes of the dust bowls were severe droughts,   lands became dry and hard, temperature was higher than usual, hot and dry conditions killed crops, high winds. because of those factors, the winds kicked up dry soil and created clouds of dust.

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