Astronauts took this photo of the Moon barely visible above Earth’s atmosphere. Earth’s blue halo appears because the atmosphere scatters blue light more than other wavelengths. At the top of the atmosphere, gases become so thin that they just cease to exist and then there is nothing but empty space. Since there is no easy way to define the top of the atmosphere, scientists say that it is 100 km above Earth’s surface. At that location, solar energy enters the Earth system mostly as visible light. Energy as reflected light and heat leave the Earth system there. If average global temperature remains the same, the incoming and outgoing energy are equal. If more energy is coming in than going out, global temperatures increase. If more energy is going out than coming in, global temperatures decrease. Increases or decreases in greenhouse gases can change this energy balance. Clouds appear in Earth’s atmosphere where there is water vapor. Clouds, along with snow and ice, reflect sunlight and play an important role in global climate. Where clouds reflect light back into space, they reduce the energy in the atmosphere. But water vapor is a greenhouse gas, so clouds can also trap heat. Scientists are interested in the effects of clouds on Earth’s heat balance.

^{Courtesy of NASA’s Earth Observatory. earthobservatory.nasa.gov/IOTD/view.php?id=7373. Public Domain.}