How has Earth's atmospheric water been in previous eras and periods? How has partial pressure and net amount of water vapor and colloidal water been in previous eras and periods on Earth?
 A: As noted in the comments, the Earth's atmospheric water content is strongly dependent on atmospheric temperature, with an expected absolute increase in atmospheric water vapour of about
$7\%/\textrm{K}$.  The relative humidity - the ratio of atmospheric moisture to storage capacity - may, however, decrease with increasing temperature.
For example, the total absolute water content of the atmosphere (in all forms: vapour, clouds, water droplets, ice crystals, …) will be lower in ice ages, but the Earth's average relative humidity  may be higher. This is because although there is less oceanic evaporation, the cooler air also cannot hold as much water and convection driven precipitation generating mechanisms may be less efficient, so water stays in the atmosphere longer.
This is consistent with what has been observed in recent decades as the Earth's has warmed; the amount of moisture in the atmosphere has increased while the Earth's average relative humidity has decreased.
These relative humidity changes are small, however, because it is difficult for the Earth's relative humidity to change too much. If the air get's too dry, precipitation slows down and water builds up in the atmosphere until eventually it rains or snows. For example, when modelling the Precambian atmosphere, it was assumed that the average relative humidity would be similar to today.
