Atmospheric Pressure:
Atmospheric pressure refers to the force exerted by the weight of the atmosphere on a particular area. It is the pressure that the Earth's atmosphere exerts on objects within it. This pressure decreases with increasing altitude, as the weight of the overlying air decreases.

Friction of Air:
Friction of air, also known as air resistance or drag, is the force exerted by the air on an object moving through it. It is caused by the collision of air molecules with the surface of the object. The amount of friction depends on the shape, size, and speed of the object.

Differences:

1. Definition:
- Atmospheric pressure is the force exerted by the weight of the atmosphere on a particular area.
- Friction of air is the force exerted by the air on an object moving through it.

2. Cause:
- Atmospheric pressure is caused by the weight of the overlying air.
- Friction of air is caused by the collision of air molecules with the surface of the object.

3. Direction:
- Atmospheric pressure acts in all directions, exerting equal force on all surfaces of an object.
- Friction of air acts opposite to the direction of motion of the object.

4. Effects:
- Atmospheric pressure affects various phenomena like weather patterns, wind, and the behavior of gases.
- Friction of air affects the motion of objects through the atmosphere, slowing them down and causing them to lose energy.

5. Measurement:
- Atmospheric pressure is measured using instruments like barometers, which quantify the force per unit area.
- Friction of air is typically measured indirectly by observing the change in motion or speed of an object.

6. Altitude Dependence:
- Atmospheric pressure decreases with increasing altitude, as the weight of the overlying air decreases.
- Friction of air does not depend on altitude but is influenced by factors like air density and speed.

7. Applications:
- Atmospheric pressure is important in various fields like meteorology, aviation, and scuba diving.
- Friction of air is considered in designing vehicles, aerodynamics, and parachute jumps.

In conclusion, atmospheric pressure is the force exerted by the weight of the atmosphere, while friction of air is the force exerted by air molecules on objects moving through it. Atmospheric pressure acts in all directions and affects weather patterns, while friction of air acts opposite to the object's motion and slows it down. Both these factors play crucial roles in different aspects of our daily lives and scientific applications.