Agoraphobia, the medical notion that appeared in the 19th century to name the neurotic fear of public spaces, is likely to be of interest to the historical study of urban ambiances. An atmospheric approach shows that the description of the city-dwellers’ sensorial experience held a decisive role in the interpretative effort of the psychiatrists who constituted that clinical category. This article addresses the historical context of the “fear of squares” from the social, spatial and political perspectives required necessarily by atmospheric phenomena.