Complex sentence: Since the 1960s a puzzling phenomenon has been observed in the atmosphere of Venus whereat the atmosphere above the cloud base is seen to travel around the planet about 50 times faster than the rotation of the planet surface, or in only four to five Earth-days.
Split into simpler sentences: Since the 1960s a puzzling phenomenon has been observed in the atmosphere of Venus. The atmosphere above the cloud base is seen to travel around the planet about 50 times faster than the rotation of the planet surface, or in only four to five Earth-days.