Most measurements of air temperature and humidity originate from Stevenson-type thermometer screens, which can produce erroneous measurements in light winds owing to insufficient ventilation of the in-screen sensors. A field experiment to measure airflow within a Stevenson screen found mean airflow to be only 0.2 m s⁻¹, well below the 1 m s⁻¹ minimum normally assumed, and only 7 % of 10 m mean wind speeds. Implications for air temperature and humidity measurement uncertainties are discussed.