Despite the enormous progress made in atmospheric research during the past 20 years, scientists still lack a basic understanding of a number of issues in the higher regions of the Earth's atmosphere. These issues include the reasons for the dryness of the stratosphere, whether or not there is evidence of the greenhouse effect in the upper atmosphere, and various questions connected with the air motion and transport mechanisms in the transition layer between the troposphere and lower stratosphere. The troposphere extends from the ground to altitudes of 8 km (5 mi) in polar regions and to 19 km (12 mi) at equatorial latitudes. The tropopause separates the troposphere from the stratosphere, which extends above the troposphere to altitudes of roughly 50 km (31 mi).