|THE IMPORTANCE OF FRICTION
METEOROLOGIST JEFF HABY
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
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
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).