訓 あたた(かい),あたた(まる) ・ 音 ダン
Sino-Vietnamese: noãn
The sun 日 gives the meaning: this is the warmth of sunshine and air, while 温 with its water radical is warmth you touch in liquid. See danbou, indoor heating.
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今日は暖かい日です。 (きょうはあたたかいひです)
Today is a warm day.