"Since the middle of the 20th century, the airplane has been one of the chief means of long-distance transport in the world. Nearly every major metropolitan area today has at least one airport, with facilities to handle passengers and ticketing, as well as facilities for refueling and repairing the aircraft that land there. Beginning in the 1940s, the growing role of airplanes as combat vehicles led to the widespread construction of military air bases. Nearly 400 of these facilities in the U.S. alone act as bases and maintenance facilities for aircraft assigned to all branches of the military."
"Airports add an additional trade route, provide an experience bonus to all Air Units and increase the number of Air Units you may house in a city. A city with an Airport can also airlift one unit per turn to any of your other cities and house four extra Air Units for a total of eight. Airports also add unhealthiness in cities."
In the city screen press the following icon to start Airport construction: