1972 United Nations Airmails
As with most airmail stamps, this set shows the link between flight and the mail. The 17¢ shows cloud formations, and the 21¢ has a multicolored Jet Stream.
Airmail plays a large part in the modern postal system. Large trucks carry the mail to airports, where it is loaded onto planes. Since 1975, nearly all first class mail going more than 200 miles has traveled by plane.