The troposphere is the lowest layer of Earth's atmosphere, and is where all weather phenomena occur. It extends from the Earth's surface to an altitude of about 10 kilometers (6.2 miles). The troposphere is characterized by decreasing temperature with increasing altitude, at a rate of about 6.4 degrees Celsius per kilometer (3.5 degrees Fahrenheit per 1,000 feet). This temperature gradient is what drives weather patterns, as warm air rises and cold air sinks. The troposphere contains most of the Earth's atmospheric mass and aerosols, and is where clouds form.