Natural ventilation is a good way to vent cattle barns. However, on hot days with temperature above 20C, cows start having trouble to lose heat and additional air movement must be provided! This improves the heat release of the animals and thus reduces the risk of heat stress. It is important to start ventilating early; low air velocities already suffice to shift the lead-up to heat stress and while the temperature rises, the air velocity is increased proportionally.