Water vapor varies by volume in the atmosphere from a trace to about 4%. Therefore, on average, only about 2 to 3% of the molecules in the air are water vapor molecules. The amount of water vapor in the air is small in extremely arid areas and in location where the temperatures are very low (i.e. polar regions, very cold weather). The volume of water vapor is about 4% in very warm and humid tropical air.

Why can't the amount of water vapor in the air be greater than 4%? The answer is because temperature sets a limit to how much water vapor can be in the air. Even in tropical air, once the volume of water vapor in the atmosphere approaches 4% it will begin to condense out of the air. The condensing of water vapor prevents the percentage of water vapor in the air from increasing. If temperatures were much warmer, there would be a potential to have more than 4% water vapor in the atmosphere. Think about the steam trapped in a tea kettle. The very warm temperatures and higher pressures allow for a large amount of water vapor to exist in the air within the tea kettle. Just from watching the steam leave the tea kettle, one can get an idea of the water vapor density within that kettle. The amount of water vapor within the air in the kettle is greater than 4%. If the earth's oceans were placed on the planet Venus, the ocean water would boil into the atmosphere and produce a very dense steam (current surface temperatures on Venus are 900 degrees Fahrenheit with an average sea level pressure of 92,000 millibars (92 times that of Earth)). Under this amount of enormous heat and pressure (hot enough to melt lead), water vapor would well exceed 4% of the atmosphere by volume. As a note, Venus does not have any significant amounts of water vapor; the atmosphere of Venus is 96% carbon dioxide and 3.5% nitrogen. In summary, temperature determines the maximum amount of water vapor that can exist in the air. The higher the temperature, the greater the potential percentages of water vapor in the air.