Tornado counts have spiked dramatically around Philly and elsewhere. Here’s what is behind the surge.

The atmosphere is often described as an ocean of air, and in terms of the fear and terror they instill, tornadoes thus could be likened to sharks.

Most metaphors being imprecise, the big difference in this instance would be that tornadoes are far more dangerous, as the Philadelphia region just rediscovered.

The raw numbers might suggest that the atmosphere is becoming frighteningly more tornado-infested around here and nationally in recent years, further reinforced by the several tornadoes that touched down Thursday, including the EF3 that devastated part of Bensalem Township with winds up to 140 mph.

Could this be yet another frightening by-product of a warming world?

» READ MORE: Tornado that hit Bucks was an EF3 with winds of 140 mph: ‘I’d never seen anything like it, except on television’

“We’ve seen some evidence that the variability of tornado incidence has increased,” said Harold Brooks, senior scientist at the National Severe Storms Laboratory in Norman, Okla. But, he added, “We don’t necessarily know the cause.”

Brooks and other experts cautioned against drawing hard conclusions about the trends based on recent tornadic activity.

And Thursday’s outbreak that left ribbons of damage from the Lehigh Valley to the Jersey Shore and parts of Delaware might even offer a window unto the reasoning for the caveats, said Matthew Elliott, warning coordination meteorologist for the government’s Storm Prediction Center.

When the storms started popping Thursday afternoon, a whirlwind of reports poured into the National Weather Service Office in Mount Holly. Meanwhile Doppler radar was detecting evidence of tornadoes, and the meteorologists on duty were issuing warning after warning.

If something was spinning out there, Elliott said, someone was bound to know it.

» READ MORE: A tornado outbreak killed 64 people in Pennsylvania 35 years ago. We still know terrifyingly little about the storms.

Some tornadoes do swirl in the empty spaces — Brooks told of a 1991 tornado in Alabama with winds perhaps 300 mph, that traveled 66 miles and encountered exactly two buildings — but more places than ever are inhabited, and people are sharing what they witness.

“Urban expansion, just in general more cellphones, you see the reports much more often,” said Elliott, and those trends have accelerated in the last decade.

The core of Thursday’s outbreak occurred along a populated, 130-mile corridor from near Allentown to Long Beach Island. How different the warnings and the tornado counts would have been in the 20th century is impossible to say. But it is safe to conclude that on Thursday nothing was likely to be missed.

These days, storms are being added to the tornado counts that in years past might never have appeared on the public radar. By far the biggest increases in sightings have occurred among the weaker tornadoes, those with winds of 85 mph or less.

In 1950, they constituted less than one in 10 of all tornadoes. By 2010, the figure was about six in 10. One of the twisters verified Thursday was an EF0, with winds 60 to 80 mph, in Northeast Philly.

In Pennsylvania, a grand total of 11 smaller tornadoes were reported between 1951 and 1970, according to the government’s storm database. In the last decade, 74 were.

Meanwhile, no discernible trend has been evident in the numbers of stronger tornadoes, those with winds greater than 85 mph across the country since 1991, generally the beginning of the period of reliable records.

Unfortunately, Brooks said, the earlier data are suspect. He said some of it hadn’t been rigorously vetted and was assembled based on secondhand reports.

Thus it isn’t comparable with what’s available today, which is frustrating for trend-tracking.

» READ MORE: Storm Prediction Center is the eye in a year of killer storms

“it’s very difficult to go back in time,” said Elliott, “It’s just really difficult to draw conclusions.”

Further confounding teasing out the effects of worldwide warming is the very nature of the tornado itself.

In a complex process, twisters are spawned by thunderstorms, which form from warm air rising from the surface, “like a hot-air balloon,” said William Gallus a severe-storm specialist at Iowa State University.

The air begins to spin when it encounters winds blowing in different directions. For a tornado to form, the air also needs to be spinning near the surface.

It is unclear precisely how the rising global temperatures would affect this matrix, said Gallus and other tornado specialists.

Brooks said the one major difference researchers have seen in recent decades isn’t an increase in the number of tornadoes but a rise in the number of days with intense outbreaks. Conversely, that has meant more tornado-free days.

What happened in Bensalem notwithstanding, it is unlikely the region will be seeing increasing visits by those EF3s, Brooks said.

The one in Trevose was the first in the Philadelphia region since July 1994. Before that, according to the available records, the most recent one had touched down in the West Chester area in March 1955.

One of similar strength affected Bucks County in May 1896 and was blamed for killing four people.

A deadly outbreak with even stronger tornadoes struck northern and Western Pennsylvania in 1985.

But storms of that intensity around here are decided anomalies, Brooks said.

For one thing, said Brooks, “Your season isn’t well-defined.” Philadelphia is not in the same atmospheric battleground as areas out his way and in the nation’s Tornado Alley, which can count on tornadoes almost every spring with their ready access to Gulf moisture.

“Farther from the Gulf the atmosphere has to work harder,” he said.

But on occasion, evidently it is willing to do so.