The definition is pretty much arbitrary, but it's generally considered the point at which the atmosphere becomes too thin to support conventional flight. In Europe it's considered 62 miles (100km), NASA considers it to be 50 miles (about 80km).

Geography would have a very negligible effect at that altitude. It's a fixed altitude above sea level and doesn't fluctuate

We have defined it precisely for ease of convention and discussion but its really fuzzy

FAI (International Aeronautical Federation) defines it as 100 km up above mean sea level. NASA and the US Armed Forces use 80 km above sea level.

These are just round convenient numbers to have a threshold. The atmosphere thins steadily as you get higher and higher up so there's no magic line where you're definitely in atmosphere below it and definitely in space above it. The 100 km definition for space comes from an estimate that above that height you'd need to be traveling at orbital velocities to generate enough lift to stay aloft so at that point you're orbiting not flying through the air so its a spacecraft not an airplane.

Aside from a couple test planes like the X-15, most things either operate well under that level (U2 and SR71 could hit about 25km) or well over that level (Low Earth orbit is generally 300-2000km)

There are lots of definitions.

The atmosphere drops off exponentially with height. That's math for "it gets small really quick as you go higher". You can go to the top of a mountain and notice the change. The change being that pressure is lower, and the density is lower.

You are more likely to notice a lower pressure. Fun story: I drove up pikes peak in Colorado, which goes from around 5000 feet elevation up to 14,000 feet elevation. I had a bag of potato chips (salt and vinegar) in the car. As we were going up, the bag expanded, and you could see it. It looked like it was about to burst. That is because the atmosphere pressure was lower, but the pressure in the bag (it is sealed) stayed the same. By the time we reached the top, it actually popped.

So, where does the atmosphere stop?

Hard to say, because it just gets smaller and smaller (i.e. less dense and less dense). So some people pick 80 km altitude as one point, because the air is so thin you can mostly ignore it.

At 100 km, molecules tend to not collide into each other much at all, so that is another good altitude to call the top.

At 10,000km (the exosphere) the molecules there are 50/50 chance of just escaping earth altogether.

So those are the definitions of boundaries, take your pick.

Does it vary with topography? No. It can vary with subtle changes in gravitational forces, but the dynamics of the atmosphere are a much larger effect.

Vary with tides? Yes, but surprise, not so much the moon, but the sun. The sun heats the atmosphere, every day in a nice 24 hour cycle. That makes tides in the atmosphere, where the heating is a strong effect that the gravitational pull. So that is different than with water, where gravitational pull is the main effect. The sun can cause 24 hour tides, but also 12 hour tides, and some 8 hour tides.

US Air Force says it's 50 miles.

FAI says it's 100 km (62 miles).

Both are just "round numbers" that kind of make sense, but have no precise physical reason to be exactly that.

There is of course no clear line between atmosphere and no atmosphere, it's continuous reduction in pressure. The usual demarcation used is Karman line. As altitude increases and pressure decreases, the velocity required for fixed wing craft to stay aloft increases, Karman line is altitude where speed of of an hypothetical ideal aircraft would have to match orbital speed to stay flying, which of course would make it a spacecraft instead. So that is considered the end of aeronautics and start of astronautics.

And yeah it fluctuates, mostly depending on solar wind conditions. So actual used altitudes are convenient round numbers, 50 miles by US and 100km by rest of the world.

> Does it fluctuate due to earth’s topography or relative moon position (like tides)?

No, the definition of "space" is based on altitude above sea level, not local topography. In other words, there's not an extra "air mountain" above Mount Everest.

The Moon does pull on the atmosphere as it does the water, but this effect is negligible and also does not affect where space is defined to begin.

Good question. The atmosphere just gets thinner and thinner the higher one goes.

The US defines space as above 50 miles. The rest of the world defines it as 100km, which is ~60 miles.

Not too far off.

*this is the older definition, I might be temporally wrong.

It is basically arbitrary but a consensus is the metric century (62 miles for those of us in the USA), 100Km, is pretty standard.

Thank you all for the answers!

For a jurisdictional boundary the commonly accepted point is the Karman Line 100 kilometers above sea level. The idea is that manmade objects above that line are spacecraft and certain agreements apply to them, and below that are aircraft and different agreements (like territorial and airspace sovereignty) apply to them.

From a more sciency answer, the atmosphere thins out and there is a measure where they say it's not really atmosphere anymore. However, it is not uniform, in that kind of like waves on the sea, the "surface" of the atmosphere isn't flat but fluctuates.