WordNet (r) 3.0 (2006):

hiemal
    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 "hiemal":
   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, hibernal, 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