WIND

LAWS GOVERNING WIND

VELOCITY, ACCELERATION, FORCE AND PRESSURE

Any object that moves has a particular speed which is defined as the distance traveled per unit of time

Velocity incorporates direction as well as speed

Acceleration is the change in velocity (either speed and/or direction) with respect to time

In meteorology the acceleration due to gravity which is 9.8 m/sec/sec is very important and is fairly constant with a slight decrease in gravity from equator to pole and from surface to upper atmosphere

Newton's Second Law

-relates force(F) to mass (m) and acceleration (a)

-says that the acceleration of an object is proportional to the force acting upon it and inversely proportional to its mass

expressed as a=F/m or F=ma

Keep in mind that the F above is the net force acting on the object. Atmospheric motion-caused by an imbalance of forces

Newton's Laws of Motion (refer to an inertial (static) system while we live in a noninertial system)

2nd Law

-when a net force acts on a body of mass m the body accelerates (a) in the direction that the net force acts

-acceleration is directly proportional to magnitude of net force and inversely proportional to mass of body

-Net Force= ma

1st Law

-if forces acting are balanced (net F= 0) a body initially at rest will remain at rest and a body in motion will continue in motion in a straight line at constant speed.

3rd Law (action-reaction law)

-forces come in pairs and forces act on different bodies

-for every action there exists an equal and opposite reaction; moreover, the forces producing these actions always exist in pairs and act on different bodies in opposite directions.

INERTIAL VERSUS NONINERTIAL COORDINATE SYSTEMS

The validity of Newton's laws depend utterly on the coordinate

system to which they are referred. Since the earth is accelerating

in several ways it may seem strange to us that Newton was able

to arrive at laws describing inertial events when he lived in

a noninertial system.

-for small scale effects the acceleration of a coordinate system

attached to the earth can be considered negligibly small.

-we cannot, in studying the atmosphere ignore these accelerations

because much of the atmospheric motion is large scale

FORCES OF THE ATMOSPHERE

Major components

a. pressure gradient force (real)

b. coriolis effect (apparent)

c. centrifugal effect (apparent)

d. frictional force (real)

e. gravity (real)

GENERAL WIND PRINCIPLES

1. a horizontal pressure gradient accelerates the air away from regions of high air pressure and towards areas of low air pressure

2. the coriolis effect causes a synoptic and global scale winds to swerve to the right of their initial direction in the N. Hemis and to the left in the S. Hemis

3. frictional resistance slows winds that are within 1 km of the earth's surface

4. centripetal and centrifugal forces influence curved air motion

5. gravity accelerates air downward but does not modify horizontal winds

Forces involved in large-scale atmospheric circulation

Pressure gradient influences hydrostatic equilibrium, geostrophic wind, gradient wind and surface winds

Centripetal force influences gradient and surface winds only

Coriolis influences geostrophic, gradient and surface winds only

Friction just influences surface winds

Gravity just influences hydrostatic equilibrium

WIND FORCES

PRESSURE GRADIENT

Pressure differences cause a net force to act in a given direction

-force/unit mass is proportional to pressure differences and

inversely proportional to the distance between them

-ratio of pressure diff/distance is pressure gradient

-can act in any direction and affects both magnitude and direction.

-can occur because of changes in temperature, humidity, convergence and divergence

-isobars

CORIOLIS

Earth's rotation introduces a modification in the flow of air in

response to the pressure gradient force

-known as the coriolis effect

-named after GG Coriolis, a French mathematician who developed

the concept fully in l835

Causes all matter in motion to be deflected toward the right of its

path in the northern hemisphere and to the left of its path in the

southern hemisphere.

Any particle in motion in a straight line tends to maintain that path

with respect to space coordinates.

-however, a particle in motion at the earth's surface is constrained

by gravity to follow a curved path parallel with the earth's surface.

Because the earth rotates on an axis, carrying with it the atmosphere

and oceans, a particle in motion is subjected to the equivalent

of a force acting always at right angles to the path of motion

-really a fictitious force.

Magnitude depends on latitude, velocity and speed of rotation

Coriolis force increases in intensity in direct proportion to

the linear speed of motion

-is proportional to rotational speed of earth and sine of latitude

C = 2vw sin lat

w = rate of rotation 7.29 x 10 (-5) radiant/sec

v=velocity of wind

In mid latitudes of northern hemisphere a bullet fired with a velocity

of 800 feet per second at a target 400 feet away will drift one tenth

of an inch to the right.

A battleship gunner who takes dead aim at a target 20 miles

away and fires a shell at 2500 feet per second will miss

his target by 200 feet.

During WWI the shells of the giant German gun called Big Bertha which

bombarded Paris from a firing site some 70 miles away took three

minutes to reach their destination and they underwent a coriolis drift

to the right which was almost a full mile which was corrected for by the

gunners.

Pilot's corrections for coriolis are obscured by the

jockeying necessary to compensate for wind drift

Drains in northern hemisphere

the Coriolis "Force" does NOT cause rotation in a draining sink,

bathtub, or toilet!!!

Draining water on these scales can rotate in EITHER direction - clockwise

or counter-clockwise depending upon the "residual" momentum of the

"still" water and the drain design.

As far as toilets, you shouldn't be looking at them after you're done

using them , but this water is being forced down the drain anyway

GEOSTROPHIC WIND

The geostrophic wind is that which results from the balance between pressure

gradient and coriolis in the case of straight or broadly curving isobars.

The geostrophic wind is a steady horizontal wind along straight parallel

isobars in an unchanging pressure field.

When wind motion becomes established the coriolis effect causes continuous

deflection further to the right of the pressure gradient in the northern

hemisphere. Deflection will cease when the wind direction becomes parallel

to the isobars, otherwise it will blow against pressure gradient.

Idealized geostrophic flow predicts winds parallel to the isobars with

speeds dependent on isobaric spacing.