But such a temperature provision does seem rare, if not unique. City firefighters were excused from performing inspections during “inclement weather,” which included when the wind-chill factor dropped to 20 degrees and below. But the city won elimination of that restriction in 1988.

Because of the structure of the city’s library systems and the vagaries of labor negotiations, there are varying definitions of an extreme condition.

The city’s public libraries are run by three nonprofit agencies, funded by the city. Each negotiates working condition contracts with a local chapter of District Council 37, the city’s largest municipal union.

All three contracts require some action when the temperature drops below certain thresholds for two hours. Librarians may be reassigned to a warmer library. In every borough except Brooklyn, library workers can elect to continue working in the extreme conditions in exchange for paid leave, or compensatory time, to be taken later. In Brooklyn, workers are either sent home with pay or reassigned.

Image Librarian trainee, Anne Barreca, 26, left, sorts books at the Seward Branch of the New York Public Library Monday, January 11, 2009. Credit... Ruth Fremson/The New York Times

But there is no agreement on what constitutes extreme cold. The level is below 63 degrees in Queens, below 65 in Brooklyn, and below 68 for branches in Manhattan, Staten Island and the Bronx, which are all controlled by the New York Public Library.

In the last round of contract negotiations, the clause for branches controlled by the New York Public Library was modified so that workers could collect comp time and keep working, meaning the branch could stay open. The temperature threshold was also reworded from “below 68” degrees to “67 degrees or below,” which, of course, means basically the same thing.