The sky is a common term for Earth's atmosphere, which is approximately 60 miles thick, according to NASA. The atmosphere is composed of different levels, with the exosphere marking the boundary of space. According to University at Albany, the exosphere observable from space extends 60,000 miles minimum from Earth's surface.
In addition to the exosphere, the levels of the atmosphere include the troposphere, stratosphere, mesosphere and thermosphere. All weather occurs in the lowest level of the atmosphere, the troposphere, which reaches an altitude of between five and 11 miles. The highest visible clouds in the sky form within the troposphere, the region that holds almost 99 percent of the water vapor within the Earth's atmosphere. The exosphere is the highest level of the Earth's atmosphere, which borders on interplanetary space.