  The distribution of visual interstellar extinction $A_V$ has been mapped in
selected areas over the Northern sky, using available LAMOST DR5 and Gaia
DR2/EDR3 data. $A_V$ was modelled as a barometric function of galactic latitude
and distance. The function parameters were then approximated by spherical
harmonics. The resulting analytical tridimensional model of the interstellar
extinction can be used to predict $A_V$ values for stars with known parallaxes,
as well as the total Galactic extinction in a given location in the sky.
