Yesterday the temperature hit 61 degrees. Today it’s expected to do the same. That’s quite a change from the polar vortex that passed through last week, bringing near-zero readings.

For many people — particularly the folks who have to work outdoors — the warmth is a welcome reprieve. They don’t have to suit up in layers for work. For the rest of us, it’s so nice you could sunbathe on your fire escape or at a park.

So, why the big change?

“It was just a different air mass,” Carlie Buccola, a National Weather Service meteorologist, said of the polar vortex. “Now we have a different air mass that is not originating in the Arctic, so it’s warmer.”

This temperature whipsaw is “not uncommon,” she said. Same thing happened last winter.

On Jan. 7 last year, it was 5 degrees. Five days later, the mercury hit 61.