Troposphere
The troposphere is the lowest layer of Earth's atmosphere, where nearly all weather occurs and temperature decreases with altitude.
Definition
The troposphere is the bottom layer of the atmosphere, the one we live in, extending from the surface to the tropopause at about $8\ \text{km}$ near the poles and $16$ to $18\ \text{km}$ at the equator, and it holds about $75$ to $80\%$ of the atmosphere's mass along with nearly all of its water vapor, so this is where clouds, rain, and wind all happen. Temperature drops with height at an average lapse rate of about $6.5^\circ C$ per kilometer because the ground heats the air from below, and convective overturning keeps the layer well mixed. The tropopause itself is a temperature minimum that strongly resists vertical mixing, so pollutants and aerosols linger in the troposphere for only days to weeks, and the jet streams sit near breaks in the tropopause between tropical, mid-latitude, and polar air.
Example
On top of Mount Mitchell, the highest peak in North Carolina, temperatures run about $10^\circ C$ colder than in Charlotte because the summit reaches higher into the troposphere, and thunderstorm clouds over the Carolinas can tower all the way to the tropopause, flattening into an anvil shape when they hit the stable stratosphere above. A parcel of summer air rising from Charlotte cools at the dry adiabatic rate of $9.8^\circ C$ per kilometer until it condenses into a cloud, then cools more slowly as latent heat is released, which is why afternoon cumulus clouds form with flat bottoms at nearly the same height.
Key Insight
The layer is thickest where the air is warmest, since hot tropical air expands and pushes the tropopause higher, making the troposphere roughly twice as deep at the equator as at the poles. Greenhouse warming heats the troposphere while cooling the stratosphere above it, and that pattern, warming below and cooling above, is one of the clearest fingerprints that recent warming comes from greenhouse gases rather than a brighter Sun, which would warm both layers together.