Contrail cirrus is a major contributor to aviation's radiative forcing. Avoiding persistent contrail formation has been suggested as a measure to reduce the climate impact of aviation, requiring accurate forecasts of ice supersaturated conditions, where the relative humidity over ice (RHi) exceeds 100 %. Numerical weather prediction models and reanalysis products often underestimate or do not account for ice supersaturation. This study evaluates ice supersaturated regions (ISSRs) in the ECMWF ERA5 reanalysis dataset using In-service Aircraft for a Global Observing System (IAGOS) measurements over tropical and extratropical regions in the upper troposphere and lower stratosphere from 2011 to 2022. It considers seasonal and vertical differences, and how cloudy and clear-sky conditions affect ERA5’s ability to predict ISSRs. ERA5 underestimates ISSR occurrence due to a dry bias in RHi; the equitable threat score (ETS) is 0.2–0.4, indicating a weak to mediocre skill. Lowering the ERA5 RHi threshold improves ISSR prediction, with the largest improvements for RHi between 85 % and 95 %, although the optimal threshold varies with distance to the tropopause, region and season. Clear-sky conditions result in an ETS of 0.05–0.18, while the ETS is mostly below 0.1 in cloudy conditions, indicating an almost random relationship. In clear-sky conditions, lowering the threshold to 85 % increases the ETS by approximately 0.1. In cloudy conditions, lowering the threshold shows little benefit because increases in correctly predicted ISSRs are offset by increases in false positives. These findings improve our understanding of ISSR variability and has implications for accurate assessment of persistent contrail formation.