Features velocities: a) zonal wind profiles HST data in September 2012 at two wavelengths: 763 nm (red line) and at 275 nm (blue line). The black line is a reference profile from García-Melendo & Sánchez-Lavega (2001). The × are the measured speed of Y1 and Y2; b) differences in velocity between profiles. Blue line and dots: HST profile at 275 nm minus HST profile at 763 nm (indicates vertical wind shear); red line and dots: García-Melendo & Sánchez-Lavega (2001) profile minus HST 763 nm profile (indicates long-term behaviour); the cross (×) represents the velocity difference between Y1 and Y2 relative to the 763 nm profile at their latitudes (which suggests wave phase speed for Y features).