Several parts combine into one character.

暑 is a phonetic-semantic compound: 日 (sun) on top + 者 (zhě, "one who / person" — but originally a pictograph of stacked firewood being burned). Beyond the sound, 者's original "burning firewood" image contributes a hint of meaning: the heat is conceived as "sunlight burning like firewood" — the sun blazing down with kindled intensity. The 日 radical anchors a family of light-and-time characters: 暑 (hot weather), 暖 (warm), 晴 (clear sky), 暗 (dark), 晚 (evening), 早 (early).

Korean reading "seo." 避暑 (piseo, escaping the heat — Korean families taking summer vacations to mountains and beaches), 暴暑 (pokseo, severe heat / heatwave), 暑氣 (seogi, hot air / summer heat). And three of the 24 traditional solar terms incorporate 暑: 小暑 (soseo, "Small Heat" — early July), 大暑 (daeseo, "Great Heat" — late July, traditionally the hottest day), 處暑 (cheoseo, "Place of Heat ending" — late August, marking the end of summer). The lunar calendar gives Korean weather conversation a poetic structure missing from purely numerical Western calendars.

Mandarin shǔ, 3rd tone. 暑 (shǔ, summer heat), 暑假 (shǔjià, "heat vacation" = summer vacation — etymologically transparent, paralleling 寒假 winter vacation), 中暑 (zhòngshǔ, "stricken by heat" = heatstroke — the standard Mandarin medical term), 暑期 (shǔqī, summer period). 中暑 directly corresponds to Korean 일사병 / 열사병 in meaning.

Japanese on-reading ショ (sho) — 避暑 (hisho, escaping heat / summering), 残暑 (zansho, "lingering heat" = the late-summer heat that stretches into September; appears in Japanese seasonal greeting cards 残暑見舞い "late-summer greeting"), 大暑 (taisho, the Great Heat solar term). Kun-reading あつい (atsui, "hot") — 暑い (atsui). And here is one of the most notorious Japanese kanji-disambiguation challenges: the kun-reading あつい takes three different kanji depending on what is hot. 暑い = hot (weather, ambient air), 熱い = hot (an object, water, food, fire — tactile), 厚い = thick (book, paperwork, friendship — completely different concept that just shares the sound). Japanese students drill this trio relentlessly: 今日は暑い (today is hot weather), お茶が熱い (the tea is hot), 本が厚い (the book is thick).

Memory aid: sun (日) plus burning firewood (者) — the sun blazing like kindled wood. Note Japanese 暑い (weather hot) vs 熱い (object hot).