Maps of the number of HI clouds along the line of sight at high galactic
  latitude
  Characterizing the structure of the Galactic interstellar medium (ISM) in
three dimensions is of high importance for accurate modeling of dust emission
as a foreground to the Cosmic Microwave Background (CMB). At high Galactic
latitude, where the total dust content is low, accurate maps of the 3D
structure of the ISM are lacking. We develop a method to quantify the
complexity of the distribution of dust along the line of sight with the use of
HI line emission. The method relies on a Gaussian decomposition of the HI
spectra to disentangle the emission from overlapping components in velocity. We
use this information to create maps of the number of clouds along the line of
sight. We apply the method to: (a) the high-galactic latitude sky and (b) the
region targeted by the BICEP/Keck experiment. In the North Galactic Cap we find
on average three clouds per 0.2 square degree pixel while in the South the
number falls to 2.5. The statistics of the number of clouds are affected by
Intermediate-Velocity Clouds (IVCs), primarily in the North. The presence of
IVCs results in detectable features in the dust emission measured by
\textit{Planck}. We investigate the complexity of HI spectra in the BICEP/Keck
region and find evidence for the existence of multiple components along the
line of sight. The data (https://doi.org/10.7910/DVN/8DA5LH) and software are
made publicly available, and can be used to inform CMB foreground modeling and
3D dust mapping.
