The Collaborative International Dictionary of English v.0.48:

Hibernal \Hi*ber"nal\, a. [L. hibernalis, from the root of hiems
   winter; akin to Gr. ? snow, Skr. hima cold, winter, snow: cf.
   F. hibernal.]
   Belonging or relating to winter; wintry; winterish. --Sir T.
   Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hibernal
    adj 1: characteristic of or relating to winter; "bears in brumal
           sleep" [syn: brumal, hibernal, hiemal]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "hibernal":
   Siberian, aestival, algid, arctic, autumn, autumnal, below zero,
   biting, bitter, bitterly cold, bleak, boreal, brisk, brumal,
   canicular, cold, cold as charity, cold as death, cold as ice,
   cold as marble, crisp, cutting, equinoctial, freezing,
   freezing cold, frigid, gelid, glacial, hiemal, hyperborean,
   ice-cold, ice-encrusted, icelike, icy, inclement, keen, midsummer,
   midwinter, nipping, nippy, numbing, out of season, penetrating,
   piercing, pinching, raw, rigorous, seasonal, severe, sharp, sleety,
   slushy, snappy, solstitial, spring, springlike, stone-cold,
   subzero, summer, summerlike, summerly, summery, supercooled,
   vernal, winter, winterbound, winterlike, wintery, wintry