ANSWERS: 2

There is no way to know that. Even if we could accurately calculate the number of flights airborne at any given moment, not all seats on those airliners would be filled. In the USA alone, over 51,000 flights are tracked every day! According to the International Air Transport Association. it is estimated that 6 billion people fly annually worldwide. Try dividing that out by 24 hours in a day and see what you get for a number 'airborne at any hour'. That will be the closest you will get. Flight tracking software helps with finding the number of airborne flights over the United States but abroad coverage is very spotty and in some places it is nonexistent. Without that tool, even finding the number of planes airborne would be impossible much less number of passengers on board them which only the airline knows. Here is a map showing the number of flights airborne in the US at any time http://flightaware.com/live/

The statistics says that there was in total 18.000 commercial aircrafts in the world in the year 2005 and that until the year 2025 it is expected that the figure will double to 36.000. By a linear increase, we should have 900 additional aircrafts per year, giving 21.600 aircrafts today. Some big some small. Let us say that the average number of seats on a typical commercial aircraft is 140 and that the average cabin factor is 72%, which gives 101 actual passengers and an average of 5 crew members. That gives about 106 people on each aircraft. If at any given moment 30% of all planes are airborne, that would give a total of 21.600 x 0.3 x 106 = 686.880 people in the air at any given moment in the world. A consistency check with a figure of 6 billion passengers annualy in the entire world shows that every hour, we have an average of 6.000.000.000 / 365 / 24 = 684.932 people which is very close.
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