VHF signals are propagated within the range of the visible horizon in a straight line from one antenna to another - Line-of-sight propagation.
VHF or UHF signals going towards a metal-framed building can be easily reflected by the building,
Ultraviolet solar radiation is most responsible for ionization in the outer atmosphere. The region created is called the Ionosphere.
Two daytime ionospheric regions combine into one region at night - F1 and F2. During the day it separates into two regions.