Deals with the concepts and quantities characterizing models of the air motions affecting the dynamic behaviour of the aircraft. The motion of the air with respect to the Earth is defined by the wind velocity in each point of the aircraft trajectory. In flight dynamic problems, it is usual to use mathematical models of the wind velocity field and its variation with time. The following models are defined: constant wind, wind gradients, discrete gusts, three-dimensional wind models, vortices. Other models can be defined by superposition of these models.