Where the shape comes from
Three streaming lines of vapour drifting sideways, the middle one shortened and the lowest given a hook so that the graph could never be read as 三. The full character 氣 added 米, the steam that comes off cooking grain; simplified writing went back to the bare vapour. It is the 气 of 天气, 空气 and 生气, and it heads the names of the gases: 氧, 氢.
汉字 filed under 气
No published character on HSK Guide is filed under this radical yet. 汉字库.