Divisors on Hurwitz spaces: an appendix to ‘The cycle classes of divisorial Maroni loci’

The Maroni stratification on the Hurwitz space of degree $d$ covers of genus $g$ has a stratum that is a divisor only if $d-1$ divides $g$. Here we construct a stratification on the Hurwitz space that is analogous to the Maroni stratification, but has a divisor for all pairs $(d,g)$ with $d\leq g$ with a few exceptions and we calculate the divisor class of an extension of these divisors to the compactified Hurwitz space.