Beware Daily Weather Records!!


During big rain events, a particular rainfall amount may be touted as record rainfall for the date, just as during hot weather the temperature may be proclaimed a daily maximum record. Ideally, meteorological data should be based on where we are in our orbit around the sun and not a particular calendar day. This is because our calendar is slightly flawed when it comes to specific days as it takes 365.25 days to circle the sun giving us a leap day every four years.

Consequently, the dates in a leap year after February 29th, are all displaced by one day. That is, April 6th in 2017, 2018 and 2019 is the 96th day of the year, while April 6th in a leap year (i.e., 2016, 2020) is the 97th day. So, if a record event for a particular date happened in a leap year versus a non-leap year we are actually comparing different days based upon of position relative to the "solar year".

For example, the largest rainfall amount for San Francisco for April 21st is 1.39” in 1880, a leap year so it was the 112th day of that year. But the record for April 22nd, is 0.43” in 2007, not a leap year so it is also the 112th day of the year!
Which is correct?  The bottom line is that neither is really right all of the time!

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500