Clouds and the Energy Cycle       The sun's radiant energy is the fuel that drives Earth's climate engine. As shown in the figure, the Earth-atmosphere system constantly tries to maintain a balance between the energy that reaches the Earth from the sun and the energy that flows from Earth back out to space. Energy received from the sun is mostly in the visible (or shortwave) part of the electromagnetic spectrum. About 30% of the solar energy that comes to Earth is reflected back to space. The ratio of reflected-to-incoming energy is called "albedo" from the Latin word meaning whiteness. The solar radiation absorbed by the Earth causes the planet to heat up until it is radiating (or emitting) as much energy back into space as it absorbs from the sun. The Earth's thermal emitted radiation is mostly in the infrared (or longwave) part of the spectrum. The balance between incoming and outgoing energy is called the Earth's radiation budget.       The components of the Earth system that are most important to the radiation budget are the planet's surface, atmosphere, and clouds. Understanding clouds, where they occur, and their characteristics, is thought to be the key to understanding climate change. The effect of clouds on the Earth's radiation balance is measured