Positive (red) and negative (blue) buoyancy perturbations are plotted in a vertical plane. Observe that:

• The leading edges of the x-shaped region of disturbed fluid extend outward from the source at the group velocity.
• The lines of constant phase run parallel to these x-shaped arms.
• The phase speed is perpendicular to the group velocity in a sense such that the phase lines appear to propagate toward a horizontal line passing through the location of the source.

Zoom in to view fluid-parcel motions in the vicinity of the upper right arm.

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This material is based upon work supported by the National Science Foundation under Grant No. ATM-0225441.

Last edited 17 Nov 06