The air is just too cold and too dry for it to do so. 又冷又干的空气使它们无法解冻

16. particularly now, in winter,
    尤其是现在——冬天
17. when the little there is is locked up as ice.
    那里仅有的一点水也结成了冰
18. Surprisingly, snow here never melts.
    奇怪的是 这里的雪永远都不会融化
19. The air is just too cold and too dry for it to do so.
    又冷又干的空气使它们无法解冻
20. The sun's rays turn it straight into vapour.
    阳光把它们直接变成了水蒸气
21. It evaporates.
    它们升华了
22. But it is the only source of water,
    可是这是唯一的水源

来自《行星地球 第一季 第五集 Planet Earth S01E05 沙漠》第 19 行