Onset of pandemic period at county capitals and temperature prior to onset. The map shows time for onset of pandemic period at all Swedish county capitals. Diagrams depicting temperature fall many weeks prior to the onset are attached for 8 major cities representing 24.4% of the Swedish population in 1957. Weather stations are located at these cities. Norrköping and Sundsvall are not county capitals, but the major cities in their respective counties. Regression statistics are available in Table 2.