"We detect these objects at a rate that suggest Earth is being bombarded by five to 30 small comets per minute, or thousands per day." Comets are known to contain frozen water and are sometimes called "dirty snowballs"
http://www.panspermia.org/streaks.htm
Well, I believe that water molecules do escape into space but not nearly at the rate of hydrogen or helium. These are much lighter than H2O so can reach escape velocity with less energy.
Also, the Earth's magnetic field protects the atmosphere from solar winds reducing the loss rate and gravity keeps most of the atmosphere close to the surface.
So all in all, we're in a pretty good situation.
When water reaches the dew point, it generally precipitates out of the air and forms little droplets that form clouds and falls back to the planet. Obviously some of the water will remain in the atmosphere even after most of it is removed in the form of precipitation. That water vapor will be retained for the reasons provided above.
As James said. Its a combination of Gravity and the earths geomagnetic field. Point in fact the water that we believe Mars once had was lost in the solar winds because of weaker gravity and a failed or decayed geomagnetic field.
Gravity
gravity and the planetary magnetic field
If Al Gore stops sucking maybe it would.
it could be a curse'
