Several parts combine into one character.

暖 is a phonetic-semantic compound: 日 (sun) + 爰 (yuán, "to draw / pull / two clasping hands" — providing the sound). The composite reads as "sunlight reaching out like extended hands to touch you" = warm. The metaphor is intimate and tactile — warmth not as a measured temperature but as the sun's gentle reach. 暖 forms the canonical pair with 寒 (cold), and the two characters together cover the human-experiential temperature spectrum.

Korean reading "nan." 暖房 (nanbang, heating — used for heated indoor spaces, contrasting with naengbang for air conditioning), 溫暖 (onnan, warm climate / mild — used in ongnanhwa for "warming," as in jigu ongnanhwa "global warming"), 暖流 (nallyu, warm ocean current — geography vocabulary), 暖帶 (nandae, temperate zone), 暖爐 (nallo, heater / stove), 寒暖計 (handan-gye, "cold-warm meter" = thermometer).

Mandarin nuǎn, 3rd tone. 暖 (nuǎn, warm), 暖和 (nuǎnhuo, warm and pleasant — extremely common spoken adjective), 温暖 (wēnnuǎn, warm — used both physically and emotionally), 暖气 (nuǎnqì, "warm air" = central heating — northern Chinese cities have municipal 暖气 systems), 暖宝宝 (nuǎn bǎobao, "warm baby" = chemical hand-warmer / heat patch — winter convenience-store essential in northern China).

Japanese on-reading ダン (dan) — 暖房 (danbō, heating — paired with 冷房 reibō, air conditioning), 温暖 (ondan, mild / warm), 暖流 (danryū, warm current), 暖冬 (dantō, mild winter — used in weather forecasts). Kun-reading あたたかい (atatakai, "warm") — 暖かい. Here Japanese makes another famous tactile-vs-ambient distinction with the same kun-reading: 暖かい vs 温かい (both pronounced atatakai). 暖かい is used for ambient warmth — climate, weather, atmosphere: 暖かい春 (atatakai haru, warm spring). 温かい is used for tactile / object warmth — warm food, warm tea, warm-hearted feelings: 温かいスープ (atatakai sūpu, warm soup), 温かい言葉 (atatakai kotoba, warm words). Mixing them sounds odd: 温かい春 would suggest spring is something you can touch and feel warm. The 暖 / 温 split parallels the Japanese 暑い / 熱い distinction for "hot."

Memory aid: sun (日) reaching out with clasping hands (爰) — sunlight's embrace is warmth.