A Cyclone on Mars: Observations in 1999

Observations of a cyclone in the Martian northern hemisphere from April 24 to April 27, 1999, are reported. The cyclone was revealed by the Hubble Space Telescope (HST), but later it was identified in CCD images obtained earlier by the author from April 24 to April 26 with the 0.6-m telescope of the Astrophysical Institute Observatory. The cyclone was situated close to the dawn limb at about 65° north latitude. As in the HST images, the cyclone was seen distinctly only at short (<550 nm) wavelengths. The cyclone drifted in the direction of the Martian rotation. The average drift velocity is estimated to be 19 km/h.