As we move higher in the atmosphere, the air becomes less dense. This is because the weight of the air above a given point decreases as we move upward. This is why balloons rise and airplanes fly.

Pressure is directly related to density. More specifically, pressure is equal to the product of the density and the acceleration due to gravity (P = ρg). Therefore, as the air becomes less dense, its pressure also decreases.

The average sea level atmospheric pressure is about 14.7 pounds per square inch (psi). At 18,000 feet, the atmospheric pressure is only about half of what it is at sea level (7.35 psi). This means that the air at 18,000 feet is about half as dense as the air at sea level.

Temperature also plays a role in air density. Warm air is less dense than cold air. This is because warm air molecules move more quickly than cold air molecules, and therefore have more energy. As a result, they spread out more and take up more space, making the air less dense.

Therefore, the density of air decreases with height due to the following reasons:

- Decreased air pressure

- Increased temperature