Specific Humidity and Evaporation Loss

Good read here...

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/evaporation

Once relative humidity reaches 100%, or when wet bulb temperature reaches dry bulb temperature, <...Evaporation...> stops; at this point <...Specific Humidity...>is irrelevant.

You can calculate Specific Humidity. Assuming you're talking about water in air, there are formulas giving saturated vapour pressure of water as function of temperature. One has been posted in this thread, another is the Antoine equation. Then Specific Humidity, gm water vapour/kg air

= RH*P_sat*18/29.3 where

RH = relative humidity, as a fraction

P_sat = saturated vapour pressure, mbar

18 = molecular mass of water. 29.3 = molecular mass of air.

The rate of evaporation at a given place is always dependent on the humidity of that place because if the air is already filled with water vapour, it will not have any place to hold excess vapour and therefore, evaporation will occur at an extremely slow rate.

Relative humidity is also sometimes defined as the ratio of actual vapor pressure of an area to the maximum vapor pressure. Vapor pressure is measured by the amount of water vapor in a given volume of air, and it increases as the vapor increases.

When the vapor pressure increases and reaches the maximum limit in a given volume of air, a state of equilibrium arises. In this state, an amount of water is getting evaporated and the same amount of vapor is getting condensed to form water again.

Thus, evaporation never really ceases to occur on any given surface of water. Depending on the relative humidity of the surrounding air, the evaporation rate increases or decreases or reaches an equilibrium state where evaporation and condensation are occurring at the same pace.

As humidity is dependent on temperature, wind and other factors, these factors affect evaporation too. There are a multitude of factors, including relative humidity, that govern the process of evaporation whether it is occurring at a small laboratory or over the surface of a huge water body like a lake or ocean.

There seems to be some confused thinking here.

I don't know what you mean by "....occurs only in the latent portion of heat transfer". Evaporation requires heat input due to the latent heat of evaporation. The heat input can be from an external source or by cooling of the remaining mass of water.

Also "......proportional to the change in specific humidity". I suppose you mean the difference in actual specific humidity of the air, and the saturation specific humidity at the air temperature. That sounds reasonable, but I wouldn't know offhand if it's true.