Announcements

1
Announcements
Midterm exam 1 will be handed back at the end of
class. Will go over written questions and give
distribution of scores. Homework assignment 1
due next Monday. Groups 3A and 4A pick up labs
on Monday. Athlete grade forms will be filled
out today and turned in by me. Apologize for
lateness on this, but needed to have exam grades
for this.
2
Summary of Force BalancesWhy the wind blows
Force Balance Forces Involved Where it happens
Geostrophic Pressure gradient and Coriolis Winds at upper levels (with no curvature)
Gradient Pressure gradient, Coriolis, and centripetal (or centrifugal) Winds at upper levels with curvature.
Cyclostrophic Pressure gradient and centrifugal Smaller-scale, tight rotations like tornadoes and hurricanes
Gradient Friction Pressure gradient, Coriolis, centripetal, and friction Surface winds
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NATS 101 Section 4 Lecture 17

• Turbulence and Local Winds

4
Couple of important loose ends from Chapter 8
5
Hydrostatic Balance(One more) force balance in
the vertical
Mathematically
Solution to above equation
Vertical pressure gradient force
gravity 0
An exponentially decaying solution of pressure
with height
6
FlashbackChange in density and pressure with
height
Density and pressure decrease exponentially with
height. For each 16 km in altitude, the pressure
decreases by a factor of 10. Explained by
hydrostatic balance.
7
Buys-Ballots Law
If you stand with your back to the wind (in
Northern Hemisphere) Low pressure on your
left High pressure to your right. Crude
approximation, assuming the winds are in
geostrophic balance.
8
Scales of Atmospheric Motion vs. Lifespan
The general circulation Atmospheric oscillations
Weve already started to investigate some of the
synoptic-scale features
Topics for todays discussion
Basically heres our roadmap for the rest of the
course.
9
The smallest type of atmospheric motion is
turbulence.

• Its what the airline pilot cheeringly calls
• bumpy air or choppy air on your flight.
• It seems like to me they never really seem to
  know exactly when its going to happen!

Just a few bumps, folks! Nothing to be worried
about!
Jay Jay the Happy Jet Plane
Turbulence Irregular atmospheric motion
characterized by currents up and down, or eddies.
A departure from smooth (or laminar) flow.
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Mechanical Turbulence (Forced Convection)
Mechanical turbulence is ultimately due to
changes in wind speed over a distance, or wind
shear. The change can be due to (for
example) Surface friction Obstructing objects
or terrain features. Changes in wind speed above
the ground.
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Billow CloudsA visual indicator of wind shear
STRONG WINDS ABOVE CLOUD
TURBULENT EDDIES
WEAK WINDS BELOW CLOUDS
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Terrain-Induced Turbulence
Where Jay Jay the Happy Jet Plane SHOULD NEVER
FLY In the vicinity of a mountain wave induced
rotor!!
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Lenticular Cloud on Mt. FujiIndicative of
mountain lee waves
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BOAC Flight 911 March 5, 1966Crashed on Mt.
Fuji due to lee wave turbulence
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Thermal Turbulence(Free Convection)
Formed by rising thermals due to surface heating
during the day. These may form convective clouds
by afternoon. So if you want the smoothest
ride, take the morning flight!
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So Jay Jay the Happy Jet Plane decides to go from
Tucson to Denver
Just a few bumps, folks! Nothing to be worried
about!

• Why is he likely to have a bumpy ride?

17
Percentage of positive reports of turbulence on
U.S. commercial jet flights
SUMMER
WINTER
COLORADO
COLORADO
(UCAR RAP)
Mostly mechanical turbulence due to mountain waves
Mostly thermal turbulence due to summer
thunderstorms.
COLORADO HAS THE MOST TURBULENT AIR AROUND!
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Flight Simulators
Todays pilots are well trained how to deal with
turbulence, as well as other adverse weather
phenomena, in sophisticated flight
simulators. Modern jet aircraft also have
instrumentation to indicate pilots of severe wind
shear, largely through the efforts of research
labs like National Center for Atmospheric
Research.
19
Local WindsSea-Land BreezeMountain-Valley
Circulation

• Basically the same physical idea for each one
• A direct thermal circulation which occurs a small
  enough scale that the pressure gradient is the
  dominant force which drives the wind.
• Driven by the diurnal cycle of solar heating.

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Sea-Land Breeze
DAYTIME
NIGHTIME
PGF
PGF
Water is relatively cool Land is relatively
warm Onshore flow at the surface Offshore flow
aloft.
Water is relatively warm Land is relatively
cool Offshore flow at the surface Onshore flow
aloft.
Occurs because water has a high heat capacity, so
it heats and cools more slowly than the land.
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Sea Breeze Circulation and Sea Breeze Front
Sea breeze forces air to converge over land,
leading to upward motion and formation of clouds
and storms.
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Sea Breeze Front North Carolina Example
EARLY MORNING
MID AFTERNOON
LATE AFTERNOON
MID MORNING
(University of Wisconsin)
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Florida Peninsula
Sea breeze is blowing the smoke from a forest
file away from the Gulf of Mexico side (FL Dept.
of Forestry).
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Mountain-Valley Winds
NIGHTTIME
DAYTIME
Air is heated along mountain slopes Warmed air
is locally less dense than the surrounding air
above the valley. Upslope flow.
Air is cooled along mountain slopes Cooled air
is locally more dense than the surrounding air
above the valley. Downslope flow.
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Local Topography of Tucson, AZ
CATALINA MTNS.
Were surrounded by mountains on three sides, so
mountain valley circulations play a BIG role in
our weatherespecially during the monsoon!
TUCSON MTNS.
X
University of AZ
RINCON MTNS.
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Summary of Lecture 17
Hydrostatic balance is the force balance in the
vertical. Gravity balances the upward pressure
gradient force. This explains the exponential
decrease in pressure with height. Buys ballot
law gives a crude approximation to where high and
low pressure are relative to your
position. Various scales of atmospheric motion
are used to classify weather and climate
phenomena. Be familiar with them. Turbulence is
irregular atmospheric motion characterized by
eddies. Mechanical Due to wind shear Thermal
Due to differential heating leading to
thermals. The local circulations mentioned today
are direct thermal circulations driven by the
diurnal cycle of solar heating. Sea-land
breezes Occur because of the difference in heat
capacity between water and land. Mountain-
valley winds Occur because of the heating and
cooling of elevated terrain. Important for
weather in the western U.S., especially in
summer.
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Reading Assignment and Review Questions
Reading Remainder of Ch. 9 Chapter 9
Questions Questions for Review (8th ed.)
1,3,4,5,6,8,20,21
(9th ed.) 1,3,4,5,6,7,9,21,22 Questions
for Thought 1,5,8,10,11