Philadelphia recorded its first official freezing temperature for the 2019-20 season on Nov. 8.

That was almost three weeks after a 32-degree reading was first logged in Millville, in the belly of South Jersey. And Millville’s was two weeks ahead of the first freezing readings in both Allentown and Reading, even farther to the north.

“It can be considerably colder than some of the other areas,” said Valerie Meola, a meteorologist at the National Weather Service office in Mount Holly. And, conversely, sometimes warmer during the day.

What is going on down there, and why does Millville seem to be such a thermal outlier? That was a question a reader posed through Curious Philly, the forum through which Inquirer reporters answer queries about the area.

This chilling saga winds through the mysteries of where, how, and why we measure temperature; what we know and don’t know about the state of the atmosphere; and whether machines are better than humans in observing the weather.

The temperature differences have something to do with microclimate effects; its location in Cumberland County, which calls itself the “Garden Spot” of the Garden State; and, just maybe, a slightly tipsy automated temperature sensor.

So, yes, Millville is different.

Where is the Millville temperature measured?

Actually, it’s outside Millville, at Millville Executive Airport. According to its website, when the airport was built in 1941, the War Department declared it “America’s First Defense Airport,” and it is the state’s second-largest airport after Newark.

However, it is not to be confused with Chicago O’Hare or Philadelphia International: It is just to the west of the legendary Pine Barrens in a county about five times the size of Philadelphia but with one-tenth of its population. Thus, it is insulated from significant urbanization effects that could bump up readings.

The measuring site is elevated, about 53 feet above sea level, according to the weather service, and that would make a small difference. Temperature decreases slowly and subtly with height.

Microclimate effects are clearly evident in the temperature readings. On cloud-free nights with light winds, daytime heat radiates into space way more efficiently in open areas like Millville than it does in Philadelphia, with all its heat-holding buildings and paved surfaces.

That’s a big reason why overnight lows often are lower in Millville, where it got down to 24 degrees under clear skies and calm winds early last Friday, and just 32 at PHL, where it was overcast and breeze from the south was blowing at 7 mph.

Wind and clouds, however, can be equalizers — and then some. On Wednesday morning with overcast and northwest winds at both locations, the low in Millville was 33, and 34 at PHL. Winds had a dramatically different effect on Nov. 24: The temperature shot up to 56 in Millville on a south wind that didn’t make it to Philadelphia, where the daily high was nine degrees lower.

But the overall day-night differential pattern between the two stations has been remarkably consistent during the last 70 years. For example, Millville’s daily high for all the Januaries was 41, compared with 39 in Philadelphia, but the low, 23, was 2 degrees cooler than Philly’s.

Still, it is at least possible that the sensor at Millville, which is part of an Automated Surface Observing System, or ASOS, is slightly out of calibration, although Meola said that the weather service has no reason to suspect that it is malfunctioning.

What is ASOS?

It’s a robotic system that measures several weather variables, including dew point, visibilities, and sky cover. It relays this real-time data frequently, which in turn gets publicly distributed and ingested into computer models.

An ASOS unit.
National Weather Service
An ASOS unit.

The temperatures are measured electronically by thermistors rather than mercury thermometers.

The government has been using ASOS systems since the early 1990s, replacing human observers. The thermistors generally are considered more accurate than their predecessors.

Historically, station moves have bedeviled the nation’s observation network. Philadelphia, for example, has changed its measuring station multiple times. One advantage of an ASOS is that it can stay in one place theoretically forever.

The systems do have drawbacks. They can read only patches of the sky, unlike the humans who could scan it 360 degrees. And they have no mechanisms for measuring snow.

The systems require periodic maintenance. Thermometers can fall out of “tolerance,” which officially is plus or minus 2 degrees Fahrenheit.

No one lives at the Millville airport, so who cares about the temperature?

Weather observations are necessities at all airports, particularly for visibility and winds, and, yes, temperature does matter.

“On a practical level, it is critical to have accurate temperature observations at airports,” said David Robinson, the Rutgers University professor who is the New Jersey state climatologist. “Probably worse to have an instrument read too high than low, especially when it comes with issues hovering around the freezing point.”

Millville isn’t one of Mount Holly’s primary or “first order” stations, whose climate reports are vetted and submitted to the National Centers for Environmental Information.

“Every site is important” for forecasting purposes and monitoring how a given forecast is performing, Meola said. “We’re looking at that data every single day.”