Final answer:
Ascending unsaturated air expands and cools at a rate of 10 Celsius degrees per 1000 meters due to the adiabatic cooling process.
Step-by-step explanation:
Ascending unsaturated (clear) air expands and cools at about 10 Celsius degrees per 1000 meters of ascent (5.5 Fahrenheit degrees per 1000 feet). This phenomenon occurs because as air rises, the pressure decreases, which allows the air to expand. When air expands, it does so by doing work on the surrounding air, which requires energy. This energy comes from the internal energy of the air itself, resulting in a decrease in temperature, known as the adiabatic cooling process.