IEM Daily Feature
Monday, 11 June 2018

Precipitation Days Bias

Posted: 11 Jun 2018 05:33 AM

A climate metric you will find on this website and others is a simple summation of the number of days per month / season / year with precipitation. While this metric may seem straight forward enough, the concept of 'days' can be tricky. It turns out that there are very few platforms out there that report true calendar day precipitation that also accounts of daylight saving time. Almost all of the present day NWS COOP data reports its 'daily' total during the early morning hours while most of the automated equipment reside in standard time all year round and thus reporting summertime 'daily' totals at 1 AM. Does the reporting time of day make a difference to the number of days reported per year with precipitation? The featured chart looks into this using a long period of hourly precipitation reports from Des Moines. The blue line reports the bias against average based on a daily reporting at the given hour. The largest values shown are in the 1-2 day per year range, which amounts to about a 1% error in the yearly totals.

Good = 5
Bad = 1