Atmospheric Forces

AOS 101 Discussion Sections 302 and 303

Why Does the Wind Blow? What makes the wind blow? We need to think about Newton's Laws 1st Law

An object at rest will remain at rest; an object in motion will remain in motion as long as no force is exerted on the object.

2nd Law The total force exerted on an object is equal to the

acceleration of the object times its mass

Pressure Gradient Force Compels fluids to move from high pressure to

lower pressure

PGF and Wind

Isobars and PGF

PGF

Coriolis Force An apparent force

Results from the constant rotation of the Earth

Northern Hemisphere Acts at a 90°angle to the

right of the object in motion (such as the wind)

This means that a wind from the south would have a CF acting toward the east

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

Missile travels toward Winnipeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s

Geostrophic Balance

X

Geostrophic BalancePressure Gradient Force

Geostrophic Balance

Coriolis Force

Pressure Gradient Force

Geostrophic Balance

Coriolis Force

Geostrophic Wind

Pressure Gradient Force

Upper Level Flow

PGF

CF

Geopotential Height

Geopotential Height

PGF/ CF/ Centripetal

Friction Force

WindFriction

Friction Force This throws the wind out of geostrophic balance

There is now a net force acting on the wind in the direction opposite its motion

WindFR

PGF

CF

Friction Force Upper Level Wind

Balance: PGF/ CF Lower Level Wind

Balance: PGF/ CF/ Friction Friction causes wind to

cross isobars at ~30°angle at surface

Front Collapse Experiment

Front Collapse Experiment

Rotating Tank Experiment

Atmospheric Fronts

AOS 101 Discussion Sections 302 and 303

Warm Front

Warm Front

Drawn as a red line with red semi-circles pointing in the direction of the front’s movement

Cold Front

Drawn as a blue line with blue triangles pointing in the direction of the front’s movement

Stationary Front Stalled

No movement of the temperature gradient

Convergence of wind Drawn as alternating

segments of red semicircles (warm front) and blue triangles (cold front) in opposite directions

Occluded Front• A region where a faster

moving cold front has caught up to a slower moving warm front.

• Generally occurs near the end of the life of a cyclone

• Drawn with a purple line with alternating semicircles and triangles

Cold Occlusion The type most

associated with mid-latitude cyclones

Cold front "lifts" the warm front up and over the very cold air

Associated weather is similar to a warm front as the occluded front approaches

Once the front has passed, the associated weather is similar to a cold front

Vertical structure is often difficult to observe

Warm Occlusion Cold air behind

cold front is not dense enough to lift cold air ahead of warm front

Cold front rides up and over the warm front

Upper-level cold front reached station before surface warm occlusion

Fronts

Identifying FrontsWe know that we need to look for low pressure

and a boundary of cold and warm air.

To pinpoint the parts of our cyclone, look for specifics in the observation maps

• Find the center of cyclonic rotation

• Find the large temperature gradients

• Identify regions of wind shifts

• Identify the type of temperature advection

• Look for kinks in the isobars