“warm / temperature” in Japanese is 温, read on, nuru, atata (おん, ぬる, あたた). It is a Grade 3 Kyōiku kanji, taught in year 3 of Japanese elementary school, and appears in words such as 温度 (temperature).
What does 温 mean?
きょうは温かい日だ。
Same sound, different character — the classic trap. Compare them side by side: