Earth's atmosphere is divided into five main layers stacked from lowest to highest: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Most weather occurs in the troposphere, which extends from the surface to roughly 12 km altitude. Above the troposphere lies the tropopause, a boundary where temperature stops decreasing with altitude, marking the transition into the stratosphere. The stratosphere is critically important for life because it contains the ozone layer, which absorbs most of the Sun's harmful ultraviolet radiation. Continuing upward, the mesosphere is the coldest layer, with temperatures falling to about −90°C as altitude increases. Beyond these layers lie the thermosphere and finally the exosphere, where the atmosphere gradually thins into space.
The atmosphere's composition is roughly 78% nitrogen (N₂), 21% oxygen (O₂), and about 1% other gases including argon and carbon dioxide. This mixture, along with water vapor and trace compounds, sustains life and drives the physical processes that produce weather. Weather refers to the short-term state of the atmosphere — temperature, precipitation, wind, and humidity at a particular place and time — whereas climate is the long-term average of weather patterns, typically calculated over 30 years or more. This distinction is fundamental: a single hot day is weather, while a long-term warming trend is climate.
Understanding these atmospheric layers, their compositions, and the weather-versus-climate distinction provides the foundation for studying how energy, air, and water move through Earth's systems.