Characteristics of air are pressure and resistance, movement, and air compression. Air pressure is caused by the weight of air from the top of the atmosphere as it presses down upon the layers of air below. The upper atmosphere has less pressure than the air near the earth, because there is less air above it. Air pressure at sea level averages about 15 pounds on every square inch. This weight changes a little from day to day along with the weather.
Air is a little sticky and resists the motion of objects going through it.
The faster an object moves through the air, the more air resistance it meets. Air resistance also creates heat. The air moves constantly because of the motion of the earth, the gravitational attraction of the moon and sun, the heating of the sun and the vibration or movement of molecules of the gases that make up the air. Compressing air can alter the speed of molecules. The speed increases when the pressure becomes stronger and the air becomes warmer.