Lift is a force on a wing (or any other solid object) immersed in a
moving fluid (such as air), and it acts perpendicular to the flow of the fluid.
Depending on the speed and direction of the airplane and the air, lift can be
positive (moving the airplane up), negative (down), or zero (steady elevation).
The process of lift creation consists of four parts: compressing and dividing
the approaching air, creating low pressure above the wing, creating high
pressure underneath the wing, and lifting the wing as the result of pressure
differences.
Source: Brain,
http://travel.howstuffworks.com/airplane3.htm
As the air approaches the front tip of the wing its pressure increases due to the wing pushing forward (figure 1). Here the air divides into two parts: one part goes over the top of the wing (figure 1 A), creating an upwash (figure 2), and the other part goes underneath the wing (figure 1 B).
Source: Denker, John S. 2003. See How It Flies. May 1. Internet:
Figure 2 analyzes the air flow around a wing using an air tunnel simulation with lines of colored smoke in the air.
As the top surface of the wing curves downward and away from the air stream, a low-pressure area develops (figure 1, in red) and the air speeds up and flows downward toward the back of the wing, creating a downwash (figure 2).
The air flowing under the wing does not develop low pressure like above the wing. Instead, the pressure under the wing is slightly higher since the air does not speed up and it was initially compressed at the front of the wing. As the air underneath nears the end of the wing, its speed and pressure gradually match those of the air coming over the top.
The sum of all the pressures acting on the wing (all the way around) creates a net force on the wing (figure 1 D). A portion of this force goes into slowing the wing down (drag component, figure 1 E), and the rest goes into lifting the wing (lift component, figure 1 C).
