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# NATS 101 Lecture 12 Vertical Stability - PowerPoint PPT Presentation

NATS 101 Lecture 12 Vertical Stability. Ball speed leaving. Racket velocity before hit. Forehand:. Ball speed approaching. Ball speed leaving. Racket velocity before hit. Drop Shot:. Ball speed approaching. Tennis Basics. Piece of cake, right?. Boundary velocity. Molecule speed leaving.

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### NATS 101Lecture 12Vertical Stability

Racket velocity before hit

Forehand:

Ball speed approaching

Ball speed leaving

Racket velocity before hit

Drop Shot:

Ball speed approaching

Tennis Basics

Piece of cake, right?

Molecule speed leaving

Molecule speed approaching

Molecule speed leaving

Boundary velocity

Molecule speed approaching

Air Molecules Act Similarly

Contracting Boundary:

Expanding Boundary:

Expansion requires work against outside air

Air molecules rebound from “walls” at a slower speed, resulting in a cooler temperature

Assuming no transfer of heat across parcel walls (adiabatic expansion), cooling rate is 10oC/km

Rising Air Cools-Sinking Air Warms

no heat transfer

494 m/s

10oC

502 m/s

502 m/s

494 m/s

8 m/s

1 km

Rising Expanding

Sinking Contracting

20oC

502 m/s

Ahrens, Fig 5.2

Dew point decreases with height at a rate of2oC/km b/c DP varies less with Pressure than Temp.

The rate is much less than cooling rate for air.

Thus, unsaturated air can become saturated IF it rises far enough.

10C10C

20C 12C

30C14C

Ahrens, Fig 5.2

Red=TemperatureBlue=Dew Point

As a saturated parcel rises and expands, the release of latent heatmitigates the adiabatic cooling

Cooling for saturated air varies with mixing ratio.

We will use an average value of6oC/kmfor moisture lapse rate

Note:sinking air always warms atdry lapse rate

Rising, Saturated Air Cools Less

no heat transfer

14oC

497 m/s

502 m/s

latent heating

497 m/s

502 m/s

5 m/s

1 km

Rising Expanding

Sinking Contracting

20oC

502 m/s

Ahrens, Fig 5.2

+10C +2C

DAR

-6C -6C

MAR

saturated

-6C -6C

MAR

unsaturated

+10C +2C DAR

unsaturated

-10C-2C

DAR

+10C +2C

DAR

Ahrens, Fig 5.12

These concepts can be applied to understand Temp and DP changes for moist flow over a mountain

Rising unsaturated air, and all sinking air

Temp changes at Dry Adiabatic Rate (DAR) of 10oC/km

Dew point changes at rate of 2oC/km

Rising saturated air

Temp cools atMoist Adiabatic Rate(MAR) of6oC/km

Dew point decreases at rate of6oC/km

StableRock always returns to starting point

UnstableRock never returns to starting point

Conditionally UnstableRock never returns if rolled past top of initial hill

Ahrens, Fig 5.1

• Archimedes' principle is the law of buoyancy.

It states that"any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body."

• The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If the density of an object is greater/less than the density of water, the object will sink/float.

• Demo: Diet vs. Regular Soda.

http://www.onr.navy.mil/focus/blowballast/sub/work2.htm

Stable air strongly resists upward motion

External force must be applied to an air parcel before it can rise

Clouds that form in stable air spread out horizontally in layers, with flat bases-tops

Ahrens, Fig 5.3

Unstable air does not resist upward motion

Clouds in unstable air stretch out vertically

Absolute instability is limited to very thin layer next to ground on hot, sunny days

Ahrens, Fig 5.5

Ahrens, Fig 5.7

ELR is the Temp change with height that is recorded by a weather balloon

6.5o C/km

6.0o C/km

ELR is 6.5o C/km, on average, and thus is conditionally unstable!

10.0o C/km

ELR is absolutely unstable in a thin layerjust above the ground on hot, sunny days

Ahrens, Meteorology Today 5th Ed.

Rising unsaturated air, and all sinking air

Temp changes at DAR of 10oC/km

DP changes at rate of 2oC/km

Saturation occurs with sufficient lifting

Rising saturated air

Temp and DP cools at MAR of 6oC/km

Note that MAR is always less than DAR

Vertical Stability Determined by ELR

Absolutely Stable and Unstable

Conditionally Unstable

Temp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability Modulates

Vertical Extent and Severity of Cumulus

• Topic - Precipitation Processes