I've noticed that an airplane appears to have more lift when it's almost touching the ground then it has 100 feet or more in the air. What causes this to occur?
This is known as Ground Effect. Not to be confused with flaring, which is a technique used by pilots to gain lift by increasing the angle of attack as airspeed decreases.
Technicality, you can flare an aircraft at any altitude. The higher the altitude, the faster the airspeed of which you can flare an aircraft before stalling due to air thinning as altitude increases.
Ground effect is caused by the increased pressure under the wing because the vortex at the end of the wing which normally just twists behind the wingtips for long distances runs into the ground itself thereby increasing the pressure under the wing as a whole.
You can think of why the vortex forms if you think of the end of the wing as deflecting air downward. Right past the end of the wing, the air is not deflected at all. This makes some of the air deflected down move sideways outward from the wing because the pressure is lower there. This is what creates the spinning action of the vortex. The wing comes through the air and presses some of the air down but the air right next to it is not pressed at all, so there is a downward AND sideways movement.
In ground effect the downward effect is increased while the sideways effect is reduced. When you are close to the ground, the air pressure under the wing is higher and at the same time there is less room for the air to flow sideways because of the distance. Big vortices can't form as easily so less of the planes energy goes into spinning air and more into keeping it up.
Although I agree that some of the physical phenomena described in the posts above are real effects, I think they are tertiary. If you are you are observing this from a commercial airplane, two main things explain the lion's share of the higher lift near ground:
1.) The airplane maintains a higher angle of attack. This means the nose (and therefore the wing) tilts up a few degrees, greatly increasing the lift at a given airspeed. This also greatly increases drag, which is why you will hear the engines INCREASE thrust, just before landing. So three observable things happen: The nose tilts up; the engines increase thrust (you hear increased engine noise) and the airplane slows down.
2.) In addition to the nose tilt, modern large commercial airplanes have adjustable flaps and slats, which extend to greatly increase the wing area; which greatly increases lift at slower airspeeds (that is their purpose for existence). These are extended on approach, to give the airplane the ability to maintain lift at the slow airspeeds desired during landing.
When the wing is brought near the ground, two things happen:
1) The downwash velocity of the wing's trailing vortices is reduced (the ground literally "blocks" the downwash). The effects are:
1a) The effective angle of attack increases, so for a given geometric angle of attack the lift increases. But in a gentle landing the lift is almost fixed (equal to the weight), so in this case the actual effect is to reduce the geometric angle of attack of the airplane so as to cancel the downwash reduction.
1b) The induced drag decreases for a given lift. Or equivalently, the span efficiency increases, sometimes above unity. This has the same effect as increasing the thrust a bit, and it's what makes the airplane seem to want to "float" above the runway during the flare before finally settling down.
Effects 1a) and 1b) depend on the dimensionless ratio height_above_ground/wing_span.
2) The 2D-like flowfield near the wing's airfoil is modified so as to increase the pressure difference between its top and bottom, thus increasing the lift for a given angle of attack. Again, if the lift is fixed, the actual effect will be to reduce the geometric angle of attack. This effect simply adds to 1a) above. It has no effect on the induced drag, so it doesn't influence 1b) above.
This effect depends on the dimensionless ratio height_above_ground/airfoil_chord, so it's only significant in low-wing airplanes.