Long days and short nights are characteristic of summer.
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On its own 昼 is literary. In speech and modern writing it almost only appears in compounds, especially 昼夜 (zhòuyè, day and night), 白昼 (báizhòu, broad daylight), and 昼伏夜出 (zhòu fú yè chū, 'hide by day and emerge by night' - said of nocturnal animals or covert activity). Everyday 'daytime' is 白天 (báitiān).
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The simplified 昼 is the bottom-half abbreviation of traditional 晝, which combines 日 (sun) with the upper part of 畫 (a boundary/division) - etymologically 'the sun-side of the day', i.e. the part of the day-night divide where the sun is up.
Top 尺 is a graphic simplification of traditional 晝's upper portion (originally a hand-and-brush measuring out the day). In the modern shape it wraps 旦, distinguishing 昼 from 屑, 尾. Indexed under Kangxi #44 尸 (shī, corpse) by tradition via the bent-body silhouette of 尺 at the top.
Bottom 旦 pictures the sun 日 rising over the horizon line - a natural fit for a daytime word. The 旦-on-bottom does the heavy semantic lifting: 昼 is the stretch of time after dawn 旦 has happened, when the sun is up and visible.
Filed under radical 尸 (shī, #44) by convention. 尸 is not a separate component in 昼, so no strokes are highlighted.
