FRICTION

The primary effect of surface friction is a reduction in the rate of

flow in the lowest layers of the atmosphere called the 'friction layer'.

-thickness is variable

-roughness of surface, density of air and temperature variation with

height and speed of air itself are main contributing factors

-at increasing altitudes above surface frictional effect becomes

smaller and wind speeds generally increase in magnitude

Turbulence and gustiness are characteristics of atmospheric motion,

especially when free air speed exceeds a certain value

For a given fluid and type of boundary the flow is laminar or smooth

at low fluid velocities. Above a certain limit, the flow becomes

unstable and breaks down into turbulent flow, with eddies forming

near the boundaries.

In normal circumstances friction is the retardation of motion and is

caused by the interlocking of surface irregularities and the adhesion

of the molecules of the two surfaces that are in contact.

There is frictional drag within gas when there are velocity differences

This retardation of motion is called VISCOSITY

When the random, thermal motion of the molecules is responsible for

this slowing up, the retardation-sometimes called molecular viscosity

is rather low. The decelerating effect of the molecular agitation

can be explained as follows

-if a stream of air is directed along a smooth, solid surface, the

air molecules in contact with the surface will have no motion

other than the usual random agitation because they adhere to

the surface. These surface molecules will in turn retard the flow

of these molecules adjacent to them because there is a

continuous exchange of the non-moving molecules at the surface

with those in the next tier.

-there is a progressively lesser retardation with distance from

the surface

-the molecular viscosity is so small that if it alone were

responsible for frictional drag in the atmosphere its effect

would diminish to nothing about one meter from the surface

Eddy viscosity is far more important which is about ten thousand

times more effective in retarding air motion

-acts through the transfer of momentum between layers of air by

eddies rather than by molecules

-eddies are parcels of various sizes that leave their normal positions within an otherwise smooth, orderly flow

-marked by irregular variations in wind direction and speed i.e. turbulence

Friction Layer Wind

-near the surface is always opposed to the direction of air motion and therefore tends to decrease wind speed

-in slowing down the wind in gradient flow the coriolis and centrifugal force are decreased but not the pressure gradient force , therefore this tends to cause the wind to cross isobars in an oblique fashion

-the effect of friction is to decrease coriolis force without influencing pressure gradient force so wind crosses isobars at an oblique angle toward the low

Fluid flow that is characterized by eddy motion is known as turbulence

1. mechanical turbulence-generated by irregularities on surface or different wind velocities

2. thermal turbulence-generated by thermal differences of surface i.e. convection