The Collaborative International Dictionary of English v.0.48:

nippy \nippy\ adj.
   Pleasantly cold and invigorating; -- of weather conditions.

   Syn: crisp, frosty, nipping, snappy.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nippy
    adj 1: a sharp biting taste; "a nippy cheese"
    2: pleasantly cold and invigorating; "crisp clear nights and
       frosty mornings"; "a nipping wind"; "a nippy fall day";
       "snappy weather" [syn: crisp, frosty, nipping, nippy,
       snappy]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "nippy":
   Siberian, algid, arctic, below zero, biting, bitter, bitterly cold,
   bleak, boreal, brisk, brumal, burning, chill, chilly, cold,
   cold as charity, cold as death, cold as ice, cold as marble, cool,
   crisp, curried, cutting, freezing, freezing cold, frigid, frosty,
   gelid, glacial, hibernal, hiemal, high-seasoned, hot,
   hot as pepper, hyperborean, ice-cold, ice-encrusted, icelike, icy,
   inclement, keen, lively, nipping, numbing, penetrating, peppery,
   piercing, pinching, racy, raw, rigorous, seasoned, severe, sharp,
   shivery, sleety, slushy, snappy, spiced, spicy, stone-cold,
   subzero, supercooled, tangy, winterbound, winterlike, wintery,
   wintry, with a kick, zestful, zesty, zippy