  The development of broadband microresonator frequency combs at visible
wavelengths is pivotal for the advancement of compact and fieldable optical
atomic clocks and spectroscopy systems. Yet, their realization necessitates
resonators with anomalous dispersion, an arduous task due to the prevailing
normal dispersion regime of materials within the visible spectrum. In this
work, we evince that silicon nitride microring resonators with air cladding on
top and sides -- a deviation from the frequently employed silica-embedded
resonators -- allows for the direct generation of broadband microcombs in the
visible range. We experimentally demonstrate combs pumped at 1060~nm (283~THz)
that reach wavelengths as short as 680~nm (440 THz), and combs pumped at 780~nm
(384 THz) that reach wavelengths as short as 630 nm (475 THz). We further show
through simulations that microcombs extending to wavelengths as low as 461 nm
(650 THz) should be accessible in this platform.
