As you rise up through the atmosphere, the temperature can vary greatly. The lowest level is the troposphere, which starts from the surface of the Earth. This level reaches to about 8 km high at the poles and 18 km high at the equator. The temperature gradually decreases to about −55°C until the tropopause is reached. This is the level where weather occurs. After passing through the tropopause, which is just a transition zone, the stratosphere starts. Here the temperatures rise to about 0°C at the stratopause. This is about 50 km above the Earth. Above the stratopause is the mesosphere. The temperatures begin to fall again to about −90°C at the mesopause, which is about 80 km in altitude. The thermosphere is the uppermost layer of the atmosphere, and the temperatures again begin to rise here. Because the air is dry, the temperatures can rise to over 100°C. The thermosphere continues upward to about 500 km above the Earth's surface.