  The distribution of atomic hydrogen in the Galactic plane is usually mapped
using the Doppler shift of 21cm emission line. We calculate the emission
spectrum in velocity slices of data (channel maps) and derive its dependence on
the statistics of HI velocity and density fields. We find that (a) if the
density spectrum is steep, i.e. n<-3, the short-wave asymptotics of the
emissivity spectrum is dominated by velocity fluctuations; (b) the velocity
fluctuations make the emission spectra shallower, provided that the data slices
are sufficiently thin. In other words, turbulent velocity creates small scale
structure that can erroneously be identified as clouds. The contribution of
fluctuations in warm HI is suppressed relative to cold component when velocity
channels used are narrower than warm HI thermal velocity and small angular
scale fluctuations are measured. We calculate how emission spectra vary with
the change of velocity slice thickness and show that the observational 21cm
data is consistent with the explanation that intensity fluctuations within
individual channel maps are generated by a turbulent velocity field. As the
thickness of velocity slices increases density fluctuations get to dominate the
emissivity. This allows to disentangle velocity and density statistics.
Application of our technique to the Galactic and SMC data reveals spectra of
density and velocity with the power law index close to -11/3. This is a
Kolmogorov index, but the explanation of the spectrum appealing to the
Kolmogorov-type cascade faces substantial difficulties. We generalize our
treatment for the case of a statistical study of turbulence inside individual
clouds. The mathematical machinery developed is applicable to other emission
lines.
