上空
shàngkōng · HSK 7
in the sky
🌌🛩️ 上 = above + 空 = sky/air, so the air above = overhead
上 means above or up; 空 means sky, air, or empty. Together they mark the airspace overhead. Note shàng keeps its 4th tone here, not the neutral tone it takes in words like 上头.
上 — horizontal line above a baseline
上 is an indicative character (指事): a short stroke placed above a longer reference line shows the concept of 'above' or 'up'. The earliest oracle-bone forms used two horizontal lines for this; the modern shape added a vertical stroke for clarity. The core meaning of 'upper, above, to ascend' has stayed stable for millennia.
空 — cave radical 穴 + 工 (phonetic)
Originally a phono-semantic compound: 穴 (cave, hole) gives the meaning of emptiness or hollow space, while 工 supplies the sound. The earliest sense was 'hole' or 'cavity,' which extended to 'empty,' 'sky,' and 'in vain.' The reading kòng (4th tone) specifically means 'free time' or 'vacancy.'