What is the main difference between airports of the early 1920s an international airports of the 1930s

Early airfields and airports grew from the use of airplanes for military, agricultural, airmail, and aerial observation purposes. Equipped with grass or dirt runways, they were not pleasant places for the early passengers that flew once U.S. scheduled airline service began in 1926. Arriving and departing aircraft blasted dirt, pebbles, and grit into the passengers' faces as they walked exposed through all types of weather and climbed open stairways into their plane. Eventually, procedures changed so that aircraft could warm up away from the passengers, then be pushed to the boarding area by hand with the engine off, boarded, and restarted with the doors closed. Airlines also built facilities in city centers. Before a flight, passengers would wait downtown at airline lounges, then take an airline bus to the field just before departure.  

As aircraft grew larger, equipment was built to connect the terminal with the aircraft. Early planes had their own built-in steps at the side or rear door or could be reached by small stairways pushed up to the plane on wheels. The first movable stairways connecting passengers on the surface to the side of an aircraft were built. Airports introduced rollup, ground-level canopies to make the connection more pleasant in bad weather. The larger airliners had doorways more than a full story above ground, so enclosed airbridges were introduced to connect elevated terminals directly with the aircraft. By the 1930s it was clear that grass runways had become inadequate as aircraft became heavier and their landing gear more rigid, requiring a smooth and predictable paved surface. Runways were oriented based on local conditions such as the prevailing wind direction and the size, height, and position of obstructions in the area.  

an expansion is one of two basic business cycle phases; the other is contraction. the transition from expansion to contraction is a peak, and the changeover from contraction to expansion is a trough.

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