A person struggles to cross a street in blowing and falling snow as the Gateway Arch appears in the distance, in St. Louis. The deep freeze that gripped much of the nation this week wasn't unprecedented, but with global warming we're getting far fewer bitter cold spells, and many of us have forgotten how frigid winter used to be.

WASHINGTON — We've become weather wimps.

As the world warms, the United States is getting fewer bitter cold spells like the one that gripped much of the nation this week. So when a deep freeze strikes, scientists say, it seems more unprecedented than it really is. An Associated Press analysis of the daily national winter temperature shows that cold extremes have happened about once every four years since 1900.

Until recently.

When computer models estimated that the national average daily temperature for the Lower 48 states dropped to 17.9 degrees on Monday, it was the first deep freeze of that magnitude in 17 years, according to Greg Carbin, warning meteorologist for the National Oceanic and Atmospheric Administration.

That is by far the longest the U.S. has gone without the national average plunging below 18 degrees, according to a database of daytime winter temperatures spanning from Jan. 13, 1997 to Monday.

In the past 115 years, there have been 58 days when the national average temperature dropped below 18. Carbin said those occurrences often happen in periods that last several days so it makes more sense to talk about cold outbreaks instead of cold days, and there have been 27 distinct cold snaps.

Between 1970 and 1989, a dozen such events occurred, but there were only two in the 1990s and then none until Monday.

“These types of events have actually become more infrequent than they were in the past,” said Carbin, who works at the Storm Prediction Center in Norman, Okla. “This is why there was such a big buzz because people have such short memories.”

Said Jeff Masters, meteorology director of the private firm Weather Underground: “It's become a lot harder to get these extreme (cold) outbreaks in a planet that's warming.”

And Monday's breathtaking chill? It was merely the 55th coldest day — averaged for the continental United States — since 1900.

The coldest day for the Lower 48 since 1900 — as calculated by the computer models — was 12 degrees on Christmas Eve 1983, nearly 6 degrees chillier than Monday.

The average daytime winter temperature is about 33 degrees, according to Carbin's database.

There have been far more unusually warm winter days in the U.S. than unusually cold ones.

Since Jan. 1, 2000, only two days have ranked in the top 100 coldest: Monday and Tuesday. But there have been 13 in the top 100 warmest winter days, including the warmest since 1900: Dec. 3, 2012. And that pattern is exactly what climate scientists have been saying for years, that the world will get more warm extremes and fewer cold ones.

Nine of 11 outside scientists who reviewed the said it showed that as the world warms from heat-trapping gas spewed by the burning of fossil fuels, winters are becoming milder. And the scientists dismiss those who say two cold days disprove climate change.