and the angular velocity of rotation may itself be changing. Our strategy is to begin with an inertial frame . K. 0, then go to a frame . K ′ having linear acceleration relative to . K. 0.then finally to a frame. K. rotating relative to . K ′ We will construct the Lagrangian in . K, and from it the equations of motion in that noninertial frame. In a mass of continuum that is rotating like a rigid body, the vorticity is twice the angular velocity vector of that rotation. This is the case, for example, in the central core of a Rankine vortex. As vorticity is a property of the stream flow, namely the amount of rotation, a short overview of the height field at 500 hPa is presented. The following structures can be summarized: In the middle of the image a pronounced, broad westerly stream prevails; The null condition and global existence for nonlinear wave equations on slowly rotating Kerr spacetimes, Journal Eur. Math. Soc., 15(5):1629-1700, 2013. A vector field method approach to improved decay for solutions to the wave equation on a slowly rotating Kerr black hole, Analysis and PDE, 5(3):553-625, 2012.