What tells you where the statosphere begins?

The stratosphere begins at the tropopause, which is the boundary between the troposphere and stratosphere. This boundary is characterized by a temperature inversion, where temperatures increase with altitude, contrasting with the cooling temperatures of the troposphere below. Typically, the tropopause is located at about 8 to 15 kilometers (5 to 9 miles) above sea level, varying with latitude and weather conditions.