Friction is an extremely important force to meteorology. Not only does friction decrease the wind speed, it also changes the direction of the wind. Two types of friction occur in the atmosphere. One is molecular friction (friction between individual air molecules (called viscosity)) and friction between two surfaces (i.e. friction between air and land). Since air molecules are free to move about, friction by viscosity is much less significant than friction between two surfaces.
The frictional force causes the wind to slow down about 20% (i.e. a 20-knot surface wind will be slowed to 16 knots). It is at the earth's surface where the air interface comes in contact with the land / water interface. The air rubbing against the earth's surface causes friction. Friction is the force that allows your car to stop when you put on the brakes. If there was no friction, the car would just keep moving forward like it was on pure ice. Since the air is forced to slow down, the wind energy is transformed into heat or mechanical energy (primarily mechanical energy in meteorology). An example of friction creating heat energy is rubbing your hands together quickly. You will feel your hands begin to warm up as you quickly rub your hands together back and forth. An example of friction creating mechanical energy is the wind blowing down a billboard sign. When friction forces the wind to slow down, the energy used to slow down the wind is transformed into mechanical energy which (blows tree limbs around, creates ocean waves, blows leaves about, supplies a pressure on objects at the earth's surface).
Friction is far more important near the earth's surface (PBL) then higher up in the atmosphere. When higher up in the atmosphere there are no longer trees, buildings, land surface, water surface, etc. to slow the wind down. As a result, upper level winds tend to be stronger than surface winds. Take for example a hurricane. The strongest winds associated with a hurricane are not at the surface, they are above the PBL (between 850 and 700 mb). The wind associated with the jet stream and any wind above the PBL is not influenced by friction to any great extent. The winds above the PBL are not gusty like they are at the earth's surface. The winds above the PBL are more uniform and smoothly flowing. It is surface friction that causes the phenomenon of "gusty winds" at the surface. The higher the wind speed, the gustier the winds will be at the surface.
Friction is important also for these reasons: (1) It causes air in the PBL to converge toward low pressure and diverge from high pressure (friction causes PBL air to flow toward low pressure) (2) friction is a little stronger over land surfaces than over water surfaces since land surfaces have a more rough and vertical texture (3) During strong winds, it is the retarding force of friction that creates damage at the earth's surface (kinetic energy converted to mechanical energy).