Complex sentence: Kelley and Stout at NASA define a "hot tower" as a rain cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere, which extends approximately nine miles (14.5 km) high at the tropics.
Split into simpler sentences: Kelley and Stout at NASA define a "hot tower" as a rain cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere. It extends approximately nine miles (14.5 km) high at the tropics.