GOWC: an open tool for observing-conditions prediction and observation planning, tested against site monitors and an exposure-time calculator

GOWC (Global Observing-conditions Weather Catalogue) is an open web application that takes the user from a target to an exposure plan across 2,642 catalogued observatory sites. It computes seeing, airmass, extinction, precipitable water vapour, sky brightness and signal-to-noise from live Open-Meteo meteorology, including pressure-level profiles, and tests most of these outputs against an independent reference. The photometric SNR calculator, run with GOWC's own catalogue entries, reproduces the ING SIGNAL exposure-time calculator to a median 1.9 per cent over a 16-point magnitude-airmass grid on two instruments. Precipitable water vapour tracks the Paranal LHATPRO radiometer closely ($ρ=+0.95$, 606 nights) with a +43 per cent wet bias. Seeing is computed from the Tatarski $C_n^2$ chain integrated over the pressure levels above the site and calibrated to the ESO MASS free-atmosphere median at Paranal; matching it requires an effective Tatarski coefficient of 5.7, nearly twice the upper end of the published range, which quantifies how much turbulence four coarse pressure levels fail to resolve. Against ~20 months of ESO DIMM data the seeing forecast has a weak but significant night-to-night ranking skill ($ρ=+0.24$, 575 nights, at Paranal; +0.16 at La Silla). Surface wind alone does as well or better (+0.27, +0.41) and persistence does better (+0.42, +0.47), but the free-atmosphere term tracks MASS ($ρ=+0.35$) and carries information that surface wind does not (partial $ρ=+0.28$, +0.40). At held-out sites the absolute seeing scale is no better than a constant. GOWC therefore treats forecast seeing as indicative, shows measured values wherever a public site monitor exists, and states which outputs are measured, calibrated or unvalidated.