What constitutes ‘normal’ weather in Philly and elsewhere is surprisingly complicated, and always changing

With the new year, C.J. Hoffmeir was confronted with a mystery: Philadelphia’s hefty precipitation surplus, a robust 20% above the annual “normal” value, had evaporated.

Through Curious Philly, the forum in which Inquirer staffers answer readers’ queries about the area, Hoffmeir asked what had happened to that hard-earned surplus accumulated in an eventful year that featured two tropical-storm deluges, multiple floods, and barrages of house-darkening thunderstorms.

» READ MORE: Ask us a question through Curious Philly.

Hoffmeir diligently checks the daily weather almanac that is published in The Inquirer, and was perplexed by what he saw in the precipitation report on Jan. 2. For him and his wife, both retirees in Cinnaminson, pondering the enigmas of the atmosphere isn’t merely recreational. It “keeps us from yelling at the TV with all the chaos going on,” he says.

We would like to inform the Hoffmeirs that yelling at one’s TV constitutes farmore normal behavior than what the atmosphere usually exhibits. In weather, “normal” is quite abnormal, something that can be verified statistically. That appears to be especially true these days as the nation and the world await new guidance on just what to expect from the atmosphere in the next 10 years.

The simple answer to Hoffmeir’s question is related to the human impulse to impose order on an uncaring universe: Climate records very much obey a calendar that nature doesn’t.

For the government, the new year brings a clean logbook, as pure as the untrodden snow. The National Weather Service produces a “Daily Climate Report” that includes a summary of precipitation that fell that day along with a year-to-date total. What Hoffmeir saw was the result of a century-old protocol.

“Cumulative yearly precipitation totals are kept through the entire calendar year, then reset to zero at the beginning of the new year,” explains Alex Staarman, a meteorologist at the weather service office in Mount Holly. As of Saturday, the region was running a deficit of about 0.4 better inches.

Hoffmeir’s query also speaks to the overarching question of just what is “normal” in matters of the atmosphere. That’s a climatic can of worms.

» READ MORE: Isaias leaves destruction and thousands in Philly region without power: ‘The worst ... is yet to come’

Who said Philadelphia should have expected 41.53 inches of precipitation in 2020, rather than the 49.76 measured officially at Philadelphia International Airport?

An abnormally paltry amount of that, about 0.06%, fell as snow, and the official 0.3 inches of snow in Philadelphia was 22.1 inches below normal.

So far, no snow has been measured in Philadelphia in January, meaning that the monthly snowfall deficit is about 3 inches and growing. But what is the source of those computations?

Although they often are used interchangeably, “normal” is not to be confused with “average.” Average is derived from simple arithmetic. Normal requires complicated data analysis.

Take January. The normal daily high is 40.3. But the average daily highs based on 146 Januaries differ by almost 3 degrees, from 38.8 on the 17th and 29th, to 41.6 on the 23rd.

On average, 0.4 inches of snow has fallen on Jan. 7, yet the normal daily total is 0.1.

» READ MORE: 30.7 inches of snow fell upon Philly on Jan. 7, 1996

“The raw day-to-day average values will vary widely,” explains Michael Palecki, physical scientist at NOAA’s National Centers for Environmental Information and the government’s go-to person on matters of normality.

The current normal values were derived from data for the 30-year period from Jan. 1, 1981, to Dec. 31, 2010. For the normals, rather than using straight averages, the data were “smoothed to reduce spurious variations in day-to-day increases,” he said. Describing that process in detail, he suggested, could drive readers to seek news about the Eagles’ coaching search or some other diversion as quickly as possible.

Those normals went into effect on Jan. 1, 2011, and in the spring will be replaced by a new set based on the 30-year period that began on Jan. 1, 1991, which then will reign as the normals for the next 10 years.

What is so special about 30 years? Nothing in particular, the climate center says.

In the worldwide weather community, 30 years has been viewed as an adequate sample period. Roughly 90 years ago, what is now the World Meteorological Organization decreed that weather agencies compute normals for stations based on 30-year data sets, Palecki said.

The updating is important given that climate is always in motion, and worldwide warming has been gaining steam since about 1975.

In Philadelphia, the normal annual precipitation now is about 4% higher than it was in the 1970s. The January average temperatures in Philadelphia are up 0.7 degrees. It is quite likely that both those numbers will increase with the new data set.

The updating holds value for energy companies, farmers, people who are considering relocating, and the like. Watching the career of normals also holds value for monitoring climate.

The axiom that climate is what you expect, weather is what you get, does hold water. In the last 10 years, the annual average snowfall, 23 inches, was quite close to the normal, 22.4. Yet seasonal amounts varied from 68 to that 0.3, and not one winter finished anywhere close to normal.

» READ MORE: When winter doesn’t come: Here are the winners and losers of Philly’s nearly snowless season

None of the winters came within 1.5 degrees of the normal temperature of 35.2, with eight of them above and two below.

No one is quite sure where climate is heading or what chaos will rain out of the skies in the next 10 years, but the probability of continued day-to-day abnormality borders on 100%.