The troposphere is the lowest layer of Earth's atmosphere, extending from the surface up to about 8 to 18 kilometers (5 to 11 miles) high, with thickness varying by latitude—thicker at the equator (up to 18 km) and thinner at the poles (around 6 km in winter)

. It contains about 75-80% of the atmosphere's total mass and nearly all of its water vapor, making it the densest atmospheric layer

. This layer is where almost all weather phenomena occur, including clouds, rain, storms, and wind, because it holds the air we breathe and the water vapor that forms weather systems

. The air composition in the troposphere is roughly 78% nitrogen, 21% oxygen, and 1% other gases such as argon, water vapor, and carbon dioxide

. Temperature in the troposphere generally decreases with altitude at an average rate of about 6°C per kilometer, due to heating primarily from the Earth’s surface. This temperature gradient causes turbulent mixing and convection, which drives weather patterns

. The upper boundary of the troposphere is the tropopause, a layer where temperature stops decreasing and remains constant or increases slightly, marking the transition to the stratosphere

. Because the troposphere is in direct contact with Earth's surface, it is influenced by surface processes like evaporation, photosynthesis, respiration, and human activities, making it critical for life and climate on Earth

. It is also shielded from the Sun’s harsh ultraviolet radiation by the ozone layer in the stratosphere above, which helps maintain the stability of many atmospheric molecules and supports life

. In summary, the troposphere is the atmospheric layer closest to Earth, essential for weather, climate, and supporting life by containing breathable air and water vapor