Venus had been considered as a "twin planet" of the Earth due to its physical
similarities, e.g., radius and mean density. Only with the Space Age, its hostile
environment has been known, 92 bar pressure and ~750 K temperature at the surface, which
is very different from the Earth. As more spacecraft visit Venus, more details of the
clouds and atmospheric structures have been understood, such as 20-km thick sulfuric
acid cloud layer, a decreasing trend of cloud top level from equator to a pole (67-63
km), strong temperature inversion layer at high latitudes, polar vortex, an unidentified
UV absorber in the clouds, and strong zonal winds near the cloud top level (>100
m/sec). In the meantime, clear temporal variations have been noticed. UV albedo of
clouds shows its drastic changes in morphology day-to-day. Interestingly, its
latitudinal contrast is well correlated with SO2 abundance, which has a significant
decreasing trend over time. This implies that the cloud formation through sulfur
photochemical reactions may experience temporal variations. In this talk, I would like
to introduce the clouds of Venus in its structure and variations, using data acquired by
ESA's Venus orbiter (Venus Express, 2006-2014) and its successor, JAXA's orbiter
(Akatsuki, Dec 2015-), as well as radiative transfer model calculations.