If the air temperature remains the same, will adding more sun reduce the amount of water vapor in the air - that is, does the sun have any impact on water vapor in the air when temperature is removed as a variable?

  • $\begingroup$ I would assume that we have more water vapor because we need to store the solar energy somewhere (through the liquid-gas phase transition of water we "consume"/convert energy; see e.g. latent heat). But that is just a guess. $\endgroup$ – daniel.neumann Dec 5 '17 at 20:18
  • $\begingroup$ Could you maybe clarify why do you expect the amount of water vapor to decrease in this situation? Maybe I am wrong or misunderstand your question. And welcome to EarthScience.SE! $\endgroup$ – daniel.neumann Dec 5 '17 at 20:20
  • $\begingroup$ Sunlight doesn't remove water-vapor from the air in any meaningful way. UV light high in the atmosphere can remove hydrogen from water-vapor molecules, but in very tiny amounts relative to the total amount of water-vapor in the atmosphere (explained in the answer. below). Sunlight does play a role in evaporation.of surface water, as does wind and for rivers, terrain and vertical drop. In short, Sunlight has more effect on liquid water on the surface than water in the air. More sunlight would evaporate more water. $\endgroup$ – userLTK Dec 6 '17 at 16:59

You need to define what you mean by 'adding more sun'. I'm asuming you mean 'increase the solar constant'.

The amount of water vapour in the air is, as a first order approximation, related to temperature - every degree of temperature rise gives us 7% more water vapour. So if 'adding more sun' increases the temperature then it automatically increases water vapour.

Now, to add more sun without increasing the temperature, you'd have to compensate by doing something like painting large areas of land white; this would increase the Earth's albedo, reflecting the extra solar radiation. No temperature change overall, so no water vapor change.

The only direct effect of the sun on water vapor is something entirely different - through photo-dissociation. This is where the sun's UV rays directly break up water molecules. However, in order to have an appreciable effect on the day to day concentration of water vapour, you'd need to replace the Sun with a much bigger and hotter star that blasted out far more UV radiation. The problem here is that everyone would die.

So as far as the question can be answered, it would be no under any realistic scenario.


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