We tend to take air for granted. After all, it works just fine for us in the environment we live in. There's plenty of it, it keeps us alive, and it doesn't change much as we move from one location to another.

As divers, we need to understand the physical characteristics of air in order to predict changes that occur as we descend in the water. In this lesson, you'll learn the composition of air, its density, and other characteristics that affect us as divers.

After this lesson, you will be able to:

• List the main components of air.
• State the density of air in pounds per cubic feet.
• Calculate the weight of air in various scuba cylinders.