The author's central idea about the weather conditions in 1871 Chicago revolves around the notion of extreme and devastating weather events during that period. This is evident from several pieces of evidence in the text. Firstly, the reference to the Great Chicago Fire of 1871 suggests a catastrophic event, which was exacerbated by the dry and windy conditions at the time. Secondly, the mention of the heat and drought conditions in the summer of 1871 highlights the harsh weather that plagued the city.
Finally, the author's description of the "inferno" that swept through the city emphasizes the severity of the weather's impact. Together, these pieces of evidence underscore the author's central idea that the weather conditions in 1871 Chicago were extreme and had dire consequences for the city and its inhabitants.