ITU-R Study Group 3 deals with propagation effects in all regions of the atmosphere and free space, including the Earth's ionosphere. Most of the Earth's “weather” (precipitation) occurs in the troposphere, which is the non-ionized region from the surface up to a height of about 15 km (at the equator) above the surface. The thickness of the troposphere decreases towards the poles. Propagation effects in the troposphere tend to increase in importance as the frequency increases above 1 GHz. For mobile-satellite systems, the effects of reflection from the Earth's surface are critically important at even lower frequencies.