Air pressure wind dynamics are simulated by the displacement of a sealed 2.4-m diameter piston, which forms part of the back of the apparatus. The displacement of the piston (up to 75 mm) causes the volume of the space between it and the test specimen assembly to increase or decrease, thus varying the air pressure difference across the specimen. The movement of the piston can be programmed to produce sinusoidal, triangular or square wave forms of air pressure at frequencies ranging from 0.1 to 10Hz and amplitudes up to 2KPa. A secondary blower generates the steady-state component of the air pressure dynamics. The apparatus also contains a water spray system that simulates the action of rain.