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Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than by air. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a heavy gas which is toxic and/or corrosive (the guys at chemistry stackexchange might have some nasty suggestions), so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.

Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than by air. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a heavy gas which is toxic and/or corrosive (the guys at chemistry stackexchange might have some nasty suggestions), so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.

Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than by air. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a heavy gas which is toxic, so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.

Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than by air. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a heavy gas which is toxic and/or corrosive (the guys at chemistry stackexchange might have some nasty suggestions), so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.

Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than airship. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a gas which is toxic or even corrosive, so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.

Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

• Hydrogen which is easy to manufacture in large quantities but has proven to be too unsafe for use in airships.
• Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
• Helium which is both safe (like air) and has a density which is almost (but not quite) as low as hydrogen. The main downside with it is that it is very expensive to manufacture.

For further reading, I recommend the Wikipedia article on lifting gas.

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of huge economical value.

To make air travel even more important in this world, it could have obstacles which can not be crossed by other means than by air. As we already established a layered atmosphere, why not make the lowest layers of the atmosphere consist of a heavy gas which is toxic, so that life is only possible on isolated islands which are on a height between the toxic layer and the helium layer. Travel by aircraft would then be the only feasible method to travel over the toxic chasms between these islands.