Intercomparison of air temperature measurements in aspirated and naturally ventilated thermometer screens

In a naturally ventilated wooden thermometer screen, such as the Stevenson-type screen, which is still used in many weather stations, anomalous temperature and/or humidity readings may occur, particularly in calm conditions and when exposed to direct sunlight. This is due to insufficient ventilation and the resulting relatively high thermal inertia caused by the microclimate that develops within the housing. In accordance with WMO recommendations, forced ventilation using a fan has become standard practice when measuring temperature and relative humidity in a thermometer screen.

The ventilation system at weather stations is continuously monitored during data collection, and repairs are arranged should any problems arise. To assess the urgency of the repairs required, comparative measurements were carried out at the GeoSphere Austria weather station in Vienna, comparing an actively ventilated thermometer screen with a naturally ventilated one. Over a period of more than two years, between June 2023 and October 2025, measurements of air temperature and humidity inside the screens, as well as other meteorological parameters in the vicinity of the screen’s location, were carried out, including global radiation, duration of sunshine and wind speed.

The screens have been compared for different weather conditions (wind regime, radiation conditions, etc.), magnitude of the difference between aspirated and naturally ventilated screens is estimated, and the relationship with other parameters is investigated. Essentially, the known characteristics, like proportional bias – the increase in the temperature as solar radiation increases and/ or wind speed decreases – are generally confirmed. Weather-related variations are examined in greater detail, and the resulting conclusions for operational purposes are drawn.