"on a serious note, does anyone have a reasonable explanation as to how the atmosphere doesnt simply get siphoned off into the vacuum of space?...
...gravity doesnt seem to be a good explanation. i know theres a pressure gradient. near the surface, an air pressure gradient results in a wind blowing in the direction of lowest pressure as the high tries to equalize with the low. why not the same from the surface to space" - Robo
Gravity does help insofar that it sets a speed limit that an element has to achieve in order to escape the gravitational pull.. For an element to escape the gravitational pull of the Earth its speed has to exceed @11km per second. The Sun can excite elements in the high atmosphere to an extent that they go fast enough to escape. We would lose a lot more if the Earth wasn't protected by the magnetic field because that handily deflects a lot of energy that would otherwise be absorbed by these elements.
Density also plays a part. Denser elements/compounds are closer to the surface and lighter ones further up in the atmosphere (think stones and corks in a bowl of water). There are also a lot more elements/compounds at sea level than, say, at the edge of the atmosphere. That also helps slow down the speed of these components of our atmosphere by giving them something to 'bump' into that slows their relative speeds. Higher up, with less things to bump into, allows far greater speeds and we do see losses.*
The Earth does lose a lot of mass to this bleeding into space. It loses about 140000 kg per day. However a lot of these are replaced by the various geological processes going on in the Crust and suchlike.
It is interesting how the atmosphere gets all mixed up by the various forces in action but that in itself is another topic and requires a knowledge of the Coriolis Effect which in itself requires an acceptance of a globe shaped Earth.
*I understand what you meant about the pressure gradients and that the greater pressure should move to the lower pressure to stabilise the system however the force of gravity overcomes this and, of course, it is gravity in the first place that is causing the greater pressure to be at sea level.
