薄 here reads bó (not báo) — the literary/compound reading meaning 'thin / slight'. 薄雾 = 'thin mist', lighter than 雾 (fog) and 浓雾 (thick fog). Common in scenic descriptions and poetry.
mistakes
Don't pronounce as báo-wù. 薄 has three readings: bó (literary, in 薄雾, 薄弱), báo (colloquial, 这本书很薄 'this book is thin'), and bò (in 薄荷 'mint'). In 薄雾 it's bó.
