It's estimated that there are around 5,000 planes in the air over the United States at any given time. There are probably a couple of thousand more airplanes flying in other parts of the world, and about 500,000 people are in those planes.
Most of the planes are in flight over the U.S., Europe and East Asia; fewer fly over Africa, South America and Australia. There are more planes in the air during daylight hours than during the night.
During the course of a day, about 87,000 flights are undertaken in the U.S. skies, and of those, probably one-third are commercial flights. On an average day, U.S. air traffic controllers handle 28,537 commercial flights, 27,178 private plane flights, 24,548 planes for hire flights, 5,260 military flights and 2,148 cargo flights.
The amount of fuel used by all these planes is staggering. For example, it's estimated that on a flight from London, U.K., to Singapore in Southeast Asia, a distance of 6,741 miles, a Boeing 747 will use approximately 1,523 barrels of jet fuel.
Another factor of air flight is the CO2 emissions from each plane. According to the Air Transport Action Group, air flights worldwide produced 705 million tons of CO2 in 2013. People are being encouraged to fly only when it's absolutely necessary and to use a reputable and recognized carbon offset scheme when traveling by air.