Hi, Grishka - "Long Is Our Winter" appears on page 153 of Sol Weber's Rounds Galore! book, attributed to Mozart (?) [question mark his]. Sol's notes:

Less familiar than the "Exultate" (by Mozart) but also exceptional, here's a moving Mozart round uncovered by Dotti Siftar, whose Philadelphia-based round singing family has been "spreading the word" for many years now.

-Joe-