A tell-tale sign of the seasons changing is the passage of robust cold fronts with legitimate air mass changes as what happened on Wednesday. For Estherville (NW Iowa), the high temperature for Wednesday was set during the early morning hours well before sunrise. This situation is somewhat rare for September with only a handful of other dates since 2010 having a similar occurrence. The featured chart looks into the hourly frequency by week of the year for the high temperature to occur at based on NWS CLI-mate reporting sites (read: mostly automated airport weather stations). The chart uses a logarithmic scale as the frequency counts during the mid-afternoon hours dwarf any other hour of the day. During the cold season, the influence of air masses can be seen with increased overnight frequencies than during the warm season. Additionally, you can see enhanced frequencies near midnight during the cold season as frontal passages will lead to multi-day warming or cooling events. The lowest frequencies are found during the pre-dawn hours during the warm season as solar heating is the strongest and cold-air advection during the daytime is not sufficient to counter.

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