Frequency-time (f-t) spectra of quasi-periodic (QP) ELF-VLF emissions in the period range 5--200 s are examined by using a few hundred QP events observed at high latitudes. These f-t spectra are classified into five types: nondispersive, rising tone, falling tone, mixed, and burst. Statistical characteristics of QP emissions in each type are examined by using f-t spectra of ELF-VLF emissions recorded continuously for 50 days in Iceland. A phenomenological model to explain the f-t spectra is proposed; that is, the rising tone type of QP emission is generated by compressional mode Pc 3 magnetic pulsations which propagate in a radial direction toward the earth, while the nondispersive type is generated by standing oscillations of the magnetic fields which have effective compressional components.