Phono-semantic compound (形聲): semantic 口 (“mouth”) + phonetic 乎 – to exhale, call, or shout from the mouth 口. Note that 乎 is used as an exclamation particle, hence connotations of “an exclamation from the mouth, a shout”.
Note that the right side is 乎, not 平 – these differ in that 乎 has a slanted top and a hook at the bottom.
Han character 
呼 (radical 30 口+5, 8 strokes, cangjie input 口竹火木 (RHFD), four-corner 62049)
- KangXi: page 183, character 14
- Dai Kanwa Jiten: character 3471
- Dae Jaweon: page 402, character 14
- Hanyu Da Zidian: volume 1, page 605, character 2
- Unihan data for U+547C
呼 (Yale fu1)
呼 (hangeul 호, revised ho, McCune-Reischauer ho)