Air at velocity of 2 m/s is flowing over a tray of water. The temperature of air and water is 25 oC. The width of the tray is 45 cm and its length along the direction of air flow is 20 cm. The diffusivity of water vapour in air is D = 0.26 x 10-4 m2/s. The relative humidity of air is 50%. Kinematic viscosity of air at 25oC = 16.14 x 10-6 m2/s. The density of saturated water vapour at 25oC is 0.02298 kg / m3