The warmest day of any week tends to be the first or last day of that week

Abstract

I analyze daily maximum temperature data collected at the San Francisco International Airport from 1949 to 1994. For each of the 52 weeks in each year, I compute the day on which the weekly maximum temperature falls. I find that the warmest day of the week is more likely to occur on the first (or last) day of the week than on a day in the middle of the week. This conclusion does not depend on which day is designated as the first day of week. The same phenomenon is observed when the data is detrended. I argue that this surprising phenomenon results due to positive temporal correlation in the daily maximum temperature data. I show that the same phenomenon occurs in various stochastic time series models.