Monday, January 14, 2008

Measuring rainfall is difficult here

Several readers wondered why our totals for rainfall seemed so off base last week after the major story. I've included a couple of their queries here, followed by my response. -- Sandy Duerr

Q. For some time I have been comparing my own rain gauge with your weather page’s “past precipitation." The two have rarely come close. But now I am thoroughly puzzled. Your front page printed that San Luis Obispo had 2.45 inches of rain over (last) weekend. Yet your weather page stated that we have had only 0.57 inches during "month to date." Who's right, who's wrong, or better, why the discrepancy? I think The Tribune is an excellent newspaper, and I hope I don't sound critical.
-- David Sumi, San Luis Obispo

Q. Having run a small weather station for the city of Los Angeles years ago and having my own weather equipment, I check to see how my "totals" match up with your publication. I am very confused by the total for my city, Nipomo. Before this latest round of storms, the total was 2.36 inches. You stated, as did other publications, that the storm total was 2.28 inches. That would make the Nipomo total for the season 4.64 inches. You showed only 3.25 inches. …
-- Richard Tibben

A. We too are puzzled at the discrepancies and are working with Weather Central Inc., which provides our weather information, to remedy the ongoing problem. Weather Central told us that it published the same data for the city of San Luis Obispo during the most recent storm as the National Weather Service. The data it publishes for the cities of Morro Bay, Paso Robles and Pismo Beach are also from official National Weather Service stations. It prefers using the NWS data because “their sensors are large, expensive, calibrated on a regular basis and located only after significant scientific study,’’ according to Pat Weeden, customer service manager, newspaper services, for Weather Central.
The remaining cities whose rain totals we provide in "Yesterday Around SLO County" are from volunteer, private weather observers. Weeden says that his company does its best to contact these individuals at regularly scheduled times, to follow up when they are not home and generally make certain that we publish the best data possible. Since the observers aren’t always available to take readings, however, the company “often has missing readings, which produces ever increasingly bad data sets for the running totals,’’ Weeden says. “ I honestly do not have an acceptable solution for these cities, short of installing electronic weather stations with Internet access in each city, a dedicated PC with backup power, then assigning a data quality control person on my staff to check the readings every few hours and re-calibrate the sensors twice a year. This would cost tens of thousands of dollars a year, and is unrealistic.”
In addition, our county has many micro-climates – meaning that there could be different amounts of rainfall within just one community.
Looking ahead, we are considering asking Weather Central to install an official NWS station at The Tribune (the others are at Cal Poly and the San Luis Obispo County Regional Airport). We’ll keep you posted on that effort.
In the meantime, I hope this explanation helps you better understand the issues involved in providing you a complete and accurate weather report. Clearly, our system has inherent imperfections, for logical reasons. We’ll continue to do our best to resolve them, and we thank you for your understanding and patience as we do so.


DrGeneNelson said...

I have installed a digital weather station from Davis instruments at my residence at Chumash Village, about a mile away from the San Luis Obispo airport. My rainfall totals have lined up fairly well with the SBP data since I installed my equipment in July, 2006. Data logging would add about $200 to the $250 cost of my station. An old PC could connect the logger to my DSL connection. Thus, the cost estimates that you have provided seem on the high side.

Dr. Gene Nelson KE5HXX, San Luis Obispo

Dog Spot said...

Thank God the Tribune brass are tackling hard-hitting issues such as rain total accuracy.

Meanwhile, in Atascadero...

Nick K said...

The National Weather Service has published guidelines for sensor siting, you can find a brief description and a link to the full document here: