Figure 6-18 Atmospheres of the Terrestrial Planets In each graph the curve shows how temperature varies with altitude from 0 km to 100 km above the planet’s surface. The scale on the right-hand side of each graph shows how pressure varies with altitude. (a) Clouds in Earth’s atmosphere are seldom found above 40,000 ft (12 km). (b) Venus’s perpetual cloud layers lie at much higher altitudes. The atmosphere is so dense that the pressure 50 km above the Venusian surface is 1 atm, the same as at sea level on Earth. (c) By contrast, the Martian atmosphere is so thin that the surface pressure is the same as the pressure at an altitude of 20 mi (35 km) on Earth. Wispy clouds can be found at extreme altitudes.