That’s cold enough to get frostbite on exposed skin, so cover everything up. North and west of the city it is colder still: 9 below zero in Sussex, N.J., with a wind chill of -20.

In the city, the mercury will creep up to only 16 degrees by afternoon. Overnight, more of the same, down to 10, with below-zero wind chills.

By tomorrow afternoon, the worst should be over. Temperatures are expected to crawl up to the low 20s, then the 30s on Saturday, the balmy 40s on Sunday and into the 50s early next week.

Brace yourselves: City officials are warning people to stay indoors as much as possible today, and use mass transit instead of driving. But city public schools and government offices will remain open.

In the suburbs, dozens of school districts are opening late, and several are closed, including Bloomfield and Belleville, N.J.