The beautiful blue halo of Earth's visible edge

100 kilometers high, the Earth's atmosphere thins to the virtual nothingness of space. At the very "top of the atmosphere," the incoming energy radiated by the sun matches the outgoing energy radiated as heat from the planet below. This determines the Earth's average temperature. See more fascinating details plus an amazing photograph at Earth Observatory: The Top of the Atmosphere. Also read the Earth Observatory story: Artic Reflection - Clouds Replace Snow and Ice as Solar Reflector.