Mostly literary; you'll meet 霄 inside compounds rather than alone - 云霄 (the high heavens), 九霄 (the highest heavens), 霄汉 (the sky and the Milky Way), 霄壤之别 (a difference like that between sky and earth).
memory
Top radical 雨 (rain) signals weather/sky; 肖 below gives the sound. Original meaning was 'sleet,' which broadened to 'sky / high clouds.'
Top rain radical - a pictograph of clouds with droplets falling beneath. Indexes 霄 in the weather and sky family alongside 雪 snow, 雷 thunder, 云 cloud. It marks the high atmosphere where rain forms, the cloud-veiled upper sky, by extension the heavens themselves.
Bottom 肖 supplies the sound directly - xiāo to xiāo, identical. The resemble meaning doesn't transfer; borrowed purely as a sound marker. The same phonetic anchors 消 to vanish, 销 to melt down, 削 to pare - a tight xiāo family.
