October 15 - A Very Dry Day in North America

Precipitation map for October 15, 2015

I am always struck by the number of grey dots (zero reports) on the CoCoRaHS map when precipitation is generally absent from the map. October 15 was one of those days.  Out of 8791 reports submitted in the U.S. and Canada (as of this post), 7867, (89.5 percent) were zero reports. That's pretty remarkable in a couple of aspects. A dry day such as this is pretty rare across the continent. There have been only 14 days where there have been 80 percent or higher zero reports this year. As you can see from the chart below the vast majority of days have 60 percent or less zero reports, and it's rare to have 40 percent or less, i.e. on any day the number of zero reports is usually between 40 and 60 percent of the total reports submitted.

The day that has the highest percentage of zero reports so far this year is March 7. On that day 9348 (93.5 percent) of the 10006 reports submitted were zero reports. In terms of raw reports, May 3rd leads the list with 9588 zero reports, 86 percent of the 11215 reports submitted. What's really interesting is how similar the surface and upper air weather maps are for March 7th and October 15th.

Surface map for March 7, 2015 (L) and for October 15, 2015 (R)

500 millibar maps for March 7, 2015 (L) and October 15, 2015 (R)

We constantly remind observers that a zero report is an observation. When you submit that zero it says "I observed no precipitation". If you submit only when it rains or snows, then we don't know if days with no measurement are zero, or it's missing for some other reason. "No report" is ambiguous. Zero reports are important to drought monitoring. They are necessary for climatological calculations (daily and monthly averages, for example). On top of it all, zeroes are easy to report. When you log in to CoCoRaHS, the default precip amount is zero. Hit SUBMIT on the page and you're done.  If there is a stretch of dry weather and you haven't been able to enter your observation each day, you can use the Monthly Zeros Report to submit these. Just click the box on the dates you had zero, then submit he report. A daily report will be generated for each of those days. It can't get much easier.

How to complete a Monthly Zeros report

A couple of years ago I came up with "Be a hero, report your zero!" while encouraging an observer to report zeroes. That may be overstating it a bit, but it rhymes and gets the point across. Observers who report every day tend to have complete records, and the more complete records we have the better. Long-term records are gold to climatologists.