The 4-day rotation of the upper atmosphere of venus

Near u.v. photographs of Venus have been analysed. Evidence is presented that so-called Y- and ψ-shaped markings accelerate from local sunrise to some time during local afternoon; mean horizontal velocities are 83 msec^?1 (morning) and 122 msec^?1 (afternoon). Possibly these configurations are more stable than a normal cloud system. ‘Irregular’ features do not seem to share this acceleration; they circulate at higher altitudes, sometimes covering or distorting the stable configurations.