A Midsummer Night’s screen

The frequency of summer nights whose minimum temperature remains above a rarely-exceeded threshold – such the “Tropical Night” 20 °C requirement– provides one measure of climate warming. Unfortunately, relying on nocturnal temperature minima for this may select conditions when naturally ventilated thermometers, such as within Stevenson screens, poorly reflect true air temperature due to calm or light winds. This is most critical for marginal cases near the threshold. To reduce such variability, force-ventilated (aspirated) thermometers are preferable. Parallel use with existing methods will help preserve climatological continuity, whilst ensuring threshold-based indices are less affected by summer night screen temperature representativity.