Weather professional question

Question

When the relative humidity is0, (air is saturated), what is the amount of water (H2O) in relation to air volume?

Answer 1

The answer to your question depends upon the temperature of the air.  More water will be found in a saturated, warm volume of air than a cool, saturated volume of air.  Therefore, as air temperature increases, the amount of water vapor that evaporates into the air before it becomes saturated will increase.

The reason for this relationship is NOT due to warmer air being able to hold more water vapor than cooler air.  Contrary to popular belief, air has no temperature-dependent holding capacity for water.  So, what is actually at work here?

When air is "saturated," the number of water molecules evaporating into the gaseous state is equal to the number of water molecules condensing into the liquid state.  This condition is known as equilibrium.  Warmer air temperatures permit more water molecules to evaporate into the air before equilibrium is reached, because the warmer air imparts these molecules with a higher kinetic energy (higher kinetic energy translates to water molecules that move faster and that can evaporate more readily).  That is why more water is found in warmer volumes of air.

How much water in relation to total volume of air plus water?  The link below contains a graph that plots the amount of water at saturation versus air temperature.  It is typically between zero and four percent of the total volume.