 Air flow can be characterized by a combination of five basic flow types. Translation, which is just a motion of air parcel with no change in area. Stretching deformation, which increases the parcel in one direction and decreases it in another. Shearing deformation, which shears the air parcel simultaneously in the X and Y directions, creating a diamond shape out of a square. Horticity, which spins the air parcel, and divergence, which grows the air parcel. The last four types can be represented by combinations of the partial derivatives of horizontal velocities, U and V, with respect to horizontal directions, X and Y. Note that if we know the wind velocity vectors in an Eulerian grid, then we can calculate these five wind types for an air flow by determining the changes in the velocities as functions of X and Y, and then combining these differentials that are shown here to find the actual values for stretching deformation, shearing deformation, Horticity, and divergence. The units for all of these motion types is per second, which is a frequency. These are the figures I have shown only those transformations that are positive. Negative translation goes to the left. Negative stretching deformation elongates the parcel in the Y direction. Negative shearing deformation elongates the parcel in the northwest-southeast direction. Negative Horticity is clockwise. Negative divergence causes the air parcel to shrink, which is called convergence. Prove it to yourself that these transformations shown here are all positive. We will use the divergence heavily in the next section or two of the lesson.