Atmosphere of Earth (see the temperature section, below). Because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. From highest to lowest, the five main layers are: The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere). It extends from the exobase, which is located at
A Town Called Mercy called the Gunslinger. However, the Gunslinger is a product of experiments by Jex to win a civil war on his planet, and the Doctor is unsure of what is the right thing to do. Showrunner Steven Moffat pitched the Wild West theme to Whithouse when thinking of ways to give each episode a distinct theme. Whithouse further developed the theme, including classic Western tropes and a sympathetic villain. "A Town Called Mercy" and the previous episode "Dinosaurs on a Spaceship" were the first to enter production for the seventh series. Much of the episode was filmed in March 2012 in