Final answer:
The Stevenson screen's slatted construction is what enables the free flow of air, necessary for accurate meteorological measurements.
Step-by-step explanation:
The characteristics of the Stevenson screen that allow free flow of air are its slatted construction for ventilation. The slats in the Stevenson screen enable air to circulate freely while still protecting the meteorological instruments inside from direct sunlight and precipitation. This design prevents artificial heating from the sun and allows the instruments to measure the air temperature more accurately.