  We summarize the results of long-term spectral monitoring of yellow
hypergiants (YHGs) of northern hemisphere with a R$\ge$60000 resolution. The
spectra of these F-G stars of extremely high luminosity, compactly located at
the top of the Hertzsprung-Russell diagram revealed a variety of spectral
features: various types of H$\alpha$ profile, the presence (or absence) of
forbidden and permitted emissions, as well as circumstellar components.
Variability of spectral details of various nature is studied. Absolute
luminosity, circumstellar envelope expansion rate and amplitude of pulsations
are determined. The reliability of the YHG status for V1427 Aql is reliably
confirmed; manifestations of a significant dynamic instability of the upper
layers of the atmosphere of $\rho$ Cas after the 2017 ejection and
stratification of its gas envelope are registered; the lack of companion in the
system of the V509 Cas hypergiant is proven; a conclusion is made that the
V1302 Aql hypergiant is approaching to the low-temperature boundary of the
Yellow Void.
