[scientist@work (OP)]

Why does water stick to the earth.
It cant just be the atmosphere.
There must be a bigger result for this!

[Kryptid]

Gravity.

[Janus]

Gravity.

Which is also why the atmosphere clings to the Earth, and why we aren't flung into space by its rotation.

[OokieWonderslug]

. It doesn't. Water and the other gases routinely escape our gravity. Back in the dinosaur days air pressure was twice what it is now. We've already lost more than half of our atmosphere. If you wait long enough we will wind up like Mars.

[CliffordK]

Earth can't hold the lightest of molecules (Hydrogen, Helium, etc).

One would expect a greater abundance of lighter inert gases in our atmosphere (higher prevalence in galaxy), but what we find is:
Helium (Atomic mass 3 or 4), 5ppm, almost all ⁴He which is a nuclear decay product, and  ³He is very rare.
Neon (Atomic mass 20), 18 ppm
Argon (Atomic mass 40), 0.9% (9000 ppm)
Krypton (Atomic mass 85) 1 ppm
Xenon (Atomic mass 131) 0.09 ppm

So, Earth is pretty good at holding onto Argon (Atomic mass 40), and heavier gases, but loses the lighter ones.

Atmospheric compounds:
H₂O, Molecular mass: 18
N₂, Molecular mass: 28
O₂, Molecular mass: 32
CO₂, Molecular mass: 44

So, one would expect any H₂O that evaporated to go the way of Neon and escape our atmosphere.

HOWEVER, water tends to be very sticky when cold.  So, it will slowly evaporate around room temperature, but it will strongly form water droplets and ice as temperatures are dropped.



So, as one gets up into the upper troposphere/lower stratosphere, it gets mighty cold, and condenses out the water and it falls back to earth as rain, hail, and snow.

Note that the temperature zigs and zags a bit.  It may be that our Ozone layer helps keep it hotter in the stratosphere, and cooler at the cloud level, and is vital to keeping our water.

The other thing that happens is that Hydrogen will float away as H₂, but strongly binds to oxygen (H₂O), and various carbon compounds, and other elements.  Thus keeping our Hydrogen on Earth.