Phase Changes in Everyday Life .
Equilibrium occurs when the two rates, condensation and evaporation are the same.
The condensation rate depends on temperature AND pressure, which determines how often molecules collide with the surface. Air in general is below equilibrium vapor pressure. So evaporation from the birdbath exceeds condensation into it, eventually using up all the water. The Z-stuff in onions has a vapor pressure equation just like the one for water, but with different numbers. All such equations have higher vapor pressures at higher temperatures.