What percentage of water vapor is found in the air?
It may vary from 0 to 10.3% vol/vol depending on the region of the earth.
The psychrometric chart is the document used to find the particular amount of water in the air under particular conditions. A hygrometer is used to make measurements of humidity directly.
NOTE that “humidity” as commonly used is NOT “percent of water vapor in the air”, but the relative amount compared to the maximum that could be held in the given conditions. For example, 90% relative humidity is much different in the desert or on the ocean coast.
How to use a psychrometric chart is here:
Absolute humidity is the total mass of water vapor present in a given volume of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 83 grams per cubic meter when the air is saturated at 50°C.