Synonyms for Mildness (same or very similar meaning)
WordNet sense 1 (good weather with comfortable temperatures):
clemencyWordNet sense 2 (acting in a manner that is gentle and mild and even-tempered):
gentleness, softnessWordNet sense 3 (mercifulness as a consequence of being lenient or tolerant):
lenience, leniency, lenityFrom the ODE community, based on WordNetadd/edit