Venus's crushing atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen; Earth's more clement envelope comprises roughly 79% nitrogen, 20% oxygen and 1% argon (dry air), with significant traces of other gases, chief among them carbon dioxide. (Moist air contains a few per cent of water vapour.) In this comparison of the two atmospheres' vertical structures, the zero of altitude of the temperature profiles (solid lines: blue, Earth; green, Venus) is fixed where the pressure is equal to Earth's sea-level pressure, 1 bar. Black dots on the temperature profiles indicate where the pressure has increased or decreased by a factor of 10; Venus's lower atmosphere extends almost 50 km below the 1 bar level, such that at the surface the pressure is 92 bars. The temperature there averages 730 K, compared with Earth's average surface temperature of around 288 K (15 °C). Relative to the 1-bar level, clouds in both atmospheres — of water on Earth, of sulphuric acid on Venus — occupy roughly the same altitude range, and the lower atmosphere of Venus is relatively cloud-free. The tallest mountains rise 12 km above the average surface level on Venus and 9 km above sea level on Earth.