Watch How Arctic Air Blanketed the U.S. Last Week

As you might have noticed, it was unusually cold last week.

The animation above depicts the weather system’s bone-chilling journey across the country as the spinning expanse of Arctic air known as the polar vortex bulged south before righting itself and returning to its northern home. Along the way, it brought subzero temperatures, a Midwestern blizzard and a devastating snowstorm that buried the Buffalo area for days.

The animation visualizes data from the National Oceanic and Atmospheric Administration. It shows a detailed simulation of air temperatures that was created by blending high-resolution weather forecast models with recent observations from satellites and over 30,000 weather stations.

The frigid system began its descent on Dec. 21, when air from the polar vortex made its way into the United States, and largely ended Tuesday. The polar vortex spins in a counter-clockwise direction, creating the path of cold air that traveled south, then east, and finally north again.

In the Twin Cities, the edge of the system caused around eight inches of light, powdery snow, a not-unfamiliar sight for December in Minnesota. But what happened next was more disruptive. It became “pretty breezy” as the freezing air shifted east, said Caleb Grunzke, a meteorologist with the National Weather Service’s office in Chanhassen, Minn. The resulting winds reached 40 to 50 miles an hour, leading to blizzard conditions as the light snow became airborne. Wind chills fell far below zero degrees Fahrenheit.

The Buffalo area did not have the coldest temperatures in the country, but it had its own version of extreme weather. The interaction between the cold air moving east and warmer water in the Great Lakes contributed to more than four feet of snow, widespread power outages and at least 40 deaths.

By Christmas Eve, air from the vortex had traversed much of the country, reaching as far south as Florida, and most weather stations in the country recorded temperatures below freezing. More than a third recorded temperatures below zero degrees Fahrenheit. More than 500 weather stations broke daily low temperature records. In parts of Montana and Wyoming, temperatures were lower than negative 40 degrees. Below-freezing temperatures even reached warm Florida beach communities like Daytona Beach and St. Augustine.

Key Largo, just off the southern tip of the state, recorded the highest minimum temperature in the continental United States last Friday, 71 degrees Fahrenheit. But in the days that followed, temperatures there plunged to the 40s. For locals without home heating, ski coats or familiarity with winter weather, the evening lows were a shock. “To us, we were freezing,” said Shawna Norman, who books tickets for Sailors Choice, which offers party boats and fishing charters off the island. She said she was dismayed to hear about homes throughout the country that were freezing and without power. But a low temperature in the 40s still felt “terrible.” She said they were so used to warm weather that they “never have enough clothes to keep warm.”

But people in California were largely unaffected by the extreme weather. They can thank the Rocky Mountains, which effectively blocked the cold Arctic air from reaching them. You can see the effect easily in our animation, as the coldest, bluest air turns south and also east as it reaches the Rockies.

Judah Cohen, the director of seasonal forecasting at Atmospheric and Environmental Research, compared the dense, frigid air of the polar vortex to molasses. “It wants to flow through the lowest point,” he said. As the cold air traveled down the central part of the country, its path was deflected by the Rocky Mountains. The effect is a key reason that parts of the Pacific Northwest typically don’t get as cold as points equally north on the East Coast. “If you think of Portland, Maine, and Portland, Ore., they’re probably about the same latitude, but Portland, Ore., is much warmer.”

The vortex has since gone home. Forecasts predict warmer-than-average temperatures this week in most of the United States.