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. $\endgroup$ Dec 5, 2017 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! $\endgroup$ Dec 5, 2017 at 20:20
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    $\begingroup$ 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, 2017 at 16:59
  • $\begingroup$ A very broad question that really can't be answered without some better idea on how you're thinking. Are you talking about in a day or over climatic timescales? For an individual location (near water or not?) or over the whole globe? Are you saying something helps maintain air temp (airmass advection?) or we're interested in just latent heat (basically magically turning off sensible heat from the land and even the warming ocean surface layer)? What about indirect factors like changes in clouds, horizontal wind, cloud formation, and vertical mixing since energy transfer to the air is changed? $\endgroup$ Aug 12, 2021 at 7:11
  • $\begingroup$ But overall, solar energy helps increase evaporation by providing energy to the water, as long as there's still "room" to store the moisture [i.e. the air isn't saturated... no fog]... though over long timescales equilibriums will be reached. Consider that a steaming hot cup of water will evaporate much more quickly than a cup of cold water. However this evaporation is also transferring latent heat into the atmosphere, so it's truly impossible to remove temp change from the air unless you counteract them using with other factors (which themselves have consequences on the energy\water budget) $\endgroup$ Aug 12, 2021 at 7:22

2 Answers 2


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.


When the density of atmospheric carbon increases, so too does the amount of water vapor in the atmosphere. What is being overlooked is the effect that the water vapor has on the sunlight shining through it, as it serves to intensify it, much like a magnifying glass.

  • $\begingroup$ do you have any sources for your statements,i think you are oversimplifying this a bit in your answer.we are a science site i think we can handle it. $\endgroup$ Aug 12, 2021 at 12:30

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