evaporation is the process that allows liquid water to return to the earth from the atmosphere.
what is the process that allows liquid water to return to the earth from the atmosphere?
evaporation
estimates for the habitable zone within the solar system range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. however their atmospheric conditions vary substantially. the aphelion of venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to at which water can only exist as vapour. the entire orbits of the moon, mars, and numerous asteroids also lie within various estimates of the habitable zone. only at mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. at hellas basin, for example, atmospheric pressures can reach 1,115 pa and temperatures above zero celsius (about the triple point for water) for 70 days in the martian year. despite indirect evidence in the form of seasonal flows on warm martian slopes, no confirmation has been made of the presence of liquid water there. while other objects orbit partly within this zone, including comets, ceres is the only one of planetary mass. a combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossible for these bodies to sustain liquid water on their surface. despite this, studies are strongly suggestive of past liquid water on the surface venus, mars, vesta and ceres, suggesting a more common phenomena than previously thought. since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of earth or venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.
circumstellar habitable zone
liquid water path









































liquid water path - in units of [g/m²] is a measure of the total amount of liquid water present between two points in the atmosphere. 









































lwp is an important quantity in understanding radiative transfer in the atmosphere. it is defined as the integral of liquid water content between two points in the atmosphere. for nadir observations and whole atmospheric column we have 









































where is the liquid water mixing ratio and is the density of air (including water loading).









































the atmosphere is in approximate hydrostatic equilibrium and hydrostatic equation for atmospheric pressure is given by
liquid water path