Met’s Strale helmet is an everyday aerodynamic road helmet which won’t break the bank. It features a shell designed to smooth airflow over the top of the helmet, while also pulling it through the lid to keep the rider cool.
That effect is known as the ‘Venturi effect’, a theory which purports that air will accelerate when squeezed through a small hole. In a bike helmet, this means air can be guided into the air vents and out of the rear end, effectively dragging warm air with it, boosting cooling.