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AC Geology Hurricanes & the Bermuda High Discussion

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Module 5 – Why the Wind Blows
Meteorology 10
Terrence J. Mullens
Lecture 20 – Air Pressure
This lecture’s goals
▪ Understand the definition of air pressure
▪ Explain how air pressure is measured
▪ Learn how air pressure varies in the atmosphere, and how to
identify it on a weather map.
What is air pressure?
▪ Air pressure is simply the weight of the air above you.
▪ From the top of the atmosphere to sea level, there is an average of
14.7 pounds of air per square inch of surface.
▪ Yes… that means that approximately 14.7 pounds of air is weighing
down on every square inch of your body right now!
▪ Two commonly used units are Inches of Mercury (inHg) and
Millibars (mb)
▪ Average sea level pressure in Inches of Mercury is 29.92 inHg
▪ Average sea level pressure in millibars is 1013.25 mb.
How is Air Pressure Measured
▪ Air pressure is measured using a
▪ The first barometer was invented
by Evangelista Torricelli, a student
of Galileo’s.
▪ The barometer consists of a pool
of mercury at the bottom with a
long tube that mercury could rise
▪ As air weighs down on the pool of
mercury, it is forced into the tube
rising up.
▪ Torricelli describe the pressure by
how high the mercury rose in the
tube… Inches of Mercury (inHg).
Air Pressure Decreases With Height
▪ As you rise in the atmosphere, you have less air above you…
thus air pressure decreases.
▪ However, like a pyramid, most air molecules crowd around the
surface of the earth.
▪ As a result, air pressure decreases exponentially with height!
However… air pressure also changes
▪ On the surface of the earth, there are areas of high and low
▪ These areas are typically weathermakers… so we care about them!
▪ Horizontal changes in air pressure are caused by horizontal
changes in air temperature.
▪ We’ll talk about this in the next lecture
Air Pressure on a Map
▪ Earth’s surface isn’t flat…
▪ Mountains, Hills, Valleys, Basins, etc. all have
different elevations, and thus have different
average surface pressures:
▪ Low pressure over high elevations (why your ears
▪ High pressure over lower elevations.
▪ This would make high/low pressure systems
very hard to identify on a weather map…
▪ Low pressure would be persistent over
mountains, high pressure would be persistent
over lowlands.
▪ To better identify weather systems, we
calibrate all pressure readings (called Station
Pressures) to Sea Level.
Air Pressure on a Map
▪ Surface air pressure data is displayed on a map using Isobars:
Lines of Constant Pressure.
▪ Isobars are usually drawn at 4mb intervals
Air Pressure on a Map
▪ Rather than looking
at a certain
meteorologists are
interested in looking
at weather patterns
aloft using pressure
▪ Height lines:
Altitude above sea
level that the air is
at that pressure.
▪ The map to the right
is a 500mb map.
Troughs: Cold Air
Ridge: Warm Air
Warm Regions have higher
Geopotential Heights, while Cold
Regions have lower Heights (Think of
the Two Columns!)
Lecture 21 – Pressure Gradient Force
This lecture’s goals
▪ Understand what forces act on the wind.
▪ Be able to explain the two-column model.
▪ Understand what affects the Pressure Gradient Force, and how
to calculate the Pressure Gradient Force.
Why the Wind Blows
▪ Consider Newton’s Laws of Motion:
▪ First Law: An object that is set in motion will stay in motion, unless
a force acts on it.
▪ Second Law: The NET force exerted on an object is equal to mass x
acceleration… ∑F = ma. (∑F means Sum of Forces)
▪ Third Law (we don’t need it today): Every action has an equal, but
opposite reaction.
▪ Using Newton’s Laws, we see that assuming the mass of the
atmosphere stays constant (this is good enough), forces acting
on it will accelerate it… creating wind.
▪ Forces either speed up, slow down, or change the winds direction.
Why the Wind Blows
▪ Forces that act on the wind are:
▪ The Pressure Gradient Force (Speeds up the Wind)
▪ This Lecture!
▪ The Coriolis Force (Changes the Wind’s Direction)… Next Lecture
▪ Friction (Slows down the Wind)… Next Lecture
Pressure Gradient Force
▪ The reason why we have wind!
▪ Air moves from high pressure to
low pressure in order to achieve
pressure balance.
▪ Think of two tanks of water with
a hose attached between them:
▪ Water will flow between the two
tanks until they both have the
same depth.
A model of the atmosphere
▪ Meteorologists use models to
make simple representations
of the atmosphere…
▪ Models eliminate
complexities allowing us to
focus on the main forces at
▪ An air parcel is also a model
of the atmosphere… by
eliminating complexities, we
were allowed to gain a
general view of how air cools
as it rises.
▪ We’ll see more models as we
progress this quarter.
A model Boeing 747 vs. a real 747
The air column model
▪ To understand how air pressure
changes, we use a column of air as
our model.
▪ A few simplifications and “rules”
apply to this column:
▪ It has uniform density… the air
doesn’t crowd near the surface.
▪ The column’s width does not
change with height.
▪ Air does not leave/enter the
column on its own.
The air column model
▪ Suppose you:
▪ Add air into the column… how does air pressure at the surface
▪ More air in the column… Higher pressure!
▪ Remove air from the column… how does surface air pressure
▪ Less air in the column… Lower pressure!
▪ Suppose you:
▪ Warm the air in the column?
▪ Warm air rises/expands… taller column!
▪ Cool the air in the column?
▪ Cooler air sinks/contracts… shorter column!
The Two Column Model of Air Pressure
▪ Two identical columns of
air pressure…
▪ Same surface pressure,
same air temperature,
same height.
▪ Pressure changes with
height are identical in
both columns.
The Two Column Model of Air Pressure
▪ Let’s change the temperature!
▪ Let’s cool the air over City 1
and warm the air over City 2.
▪ The column over City 1 shrinks.
The column over City 2 grows.
▪ Surface pressure remains the
same, but…
▪ Move up the column… how
does pressure change in each
column as you rise in each
▪ How has the pressure over the
“dot” changed?
▪ Hint: Count the dots…
The Two Column Model of Air Pressure
▪ Aloft = Overhead (Higher in
the atmosphere)
▪ Pressure aloft has
decreased over city 1…
▪ Cold air = Low Pressure
▪ Pressure aloft has increased
over city 2…
▪ Warm air = High Pressure
The Two Column Model of Air Pressure
▪ “Mother Nature” Hates
Imbalance… and Loves
▪ If air is allowed to travel, it
will travel from the higher
pressure aloft (City 2) to the
lower pressure aloft (City 1),
until the two become equal.
▪ The movement of air creates
a force called the pressure
gradient force.
The Two Column Model of Air Pressure
▪ So pressure aloft balances out…
all is well, right?
▪ Pressure aloft over City 1 and City
2 are equal, but you’ve:
▪ Added air to column 1… raising
the surface pressure!
▪ Remove air to column 2…
lowering the surface pressure!
▪ Since Nature craves equilibrium,
air at the surface begins to move
from City 1 to City 2…
▪ An Atmospheric Circulation is
The Two-Column Model “TL;DR
▪ Step 1: Two columns with same height, number of molecules,
surface pressure.
▪ Step 2: Warm one column… it becomes taller. Cool the other
column… it becomes shorter.
▪ Step 3: High Pressure aloft over warm column, Low Pressure
aloft over cool column.
▪ Step 4: Air Aloft moves from Warm Column to Cool Column.
This decreases surface pressure in Warm Column and Increases
surface pressure in Cool Column.
▪ Step 5 Air at the Surface moves from Cool Column to Warm
Pressure Gradient Force
▪ The strength of the wind is
determined by two things:
▪ The Difference between
High and Low pressure
▪ The Distance between the
High and Low pressure
▪ Pressure Gradient =
What is the Pressure Gradient between
Points 1 and 2?
Let’s do some examples
▪ Two stations: Station A and B.
▪ Example 1: Stations A and B are 50km apart. Station A has a surface
pressure of 1004mb and Station B has a surface pressure of
1006mb. What is their Pressure Gradient?
▪ Difference = 1006mb – 1004mb = 2mb
▪ Distance = 50km
▪ Pressure Gradient = Difference/Distance = 2mb/50km = 0.04mb/km
▪ Example 2: Stations A & B are 50km apart. Station A has a surface
pressure of 1004mb and Station B has a surface pressure of
▪ Difference = 1008mb – 1004mb = 4mb
▪ Distance = 50km
▪ Pressure Gradient = Difference/Distance = 4mb/50km = 0.08mb/km
Let’s do some examples
▪ Example 3: Stations A and B are now 100 km apart. Station A
has a reported pressure of 1004mb and Station B has a reported
pressure of 1006mb.
▪ Difference = 1006mb – 1004mb = 2mb
▪ Distance = 100km
▪ Pressure Gradient = 2mb/100km = 0.02 mb/km
▪ In Summary:
▪ Double the difference = Double the PGF
▪ Double the distance = Half the PGF
Pressure Gradient Force
▪ Is stronger when isobars are
closer together
▪ Large Difference over Small
▪ Is weaker when isobars are
farther together
▪ Small Difference over Large
Where in this Hurricane will the Winds
be Strongest?
Near the center…
isobars are very close!
Hurricane Sandy: What side is the
strongest winds?
Hydrostatic Balance
▪ Hydro = Fluid, Static = Not Moving
▪ Because air pressure decreases
with height, there is a vertical
pressure-gradient force…
▪ However, Gravity (a downward
force) balances out the Vertical
Pressure Gradient Force (an
upward force).
Next Time…
▪ Wind!
Lecture 22 – Other Wind
This lecture’s goals
▪ Understand the Coriolis and Friction Forces.
▪ Explain how Upper-Level Winds are different from Surface
▪ Understand Cyclonic and Anti-Cyclonic Motion.
▪ Understand how wind is measured.
Quick Review
▪ Newton’s Laws:
▪ Inertia (an object in motion stays in motion unless acted upon by a
net force).
▪ Net Force = mass x acceleration…
▪ Forces either speed up, slow down, or change the wind’s direction.
▪ Pressure Gradient Force:
▪ Winds flow from high to low pressure
▪ Strength of Pressure Gradient = Difference / Distance
Coriolis Force
▪ Caused by the rotation of the Earth.
▪ Once wind starts blowing, the Coriolis force
begins to steer the wind:
▪ To the right of the wind’s path in the Northern
▪ To the left of the wind’s path in the Southern
▪ Think of two people playing catch on a merrygo-round:
▪ When the merry-go-round is not moving, a
person can throw the ball to another person in a
straight path.
▪ If the merry-go-round is moving, the ball
“deflects” from a straight path.
▪ Actually, the ball still travels straight, but the
people on the merry-go-round move, which
makes it seem that the ball has deflected.

Coriolis Force
Coriolis Force
▪ The strength of the Coriolis Force
depends on:
▪ The latitude: The higher the latitude, the
stronger the force.
▪ Weak near the equator, stronger near the
▪ The speed of the wind: The faster the
wind, the stronger the force.
▪ This is why strong cyclones (like
Hurricanes) have tight spirals.
▪ The size of the motion: Large scale
motions have a stronger influence, while
small scale motions have little, if no
Coriolis influence.
▪ No… your toilet bowl isn’t large enough to
feel the Coriolis force.
Geostrophic Balance
▪ Geo = Earth, Strophic = Turning.
▪ Balance between the Pressure Gradient Force and Coriolis Force
in the Upper Atmosphere.
▪ As a result: Winds blow parallel to isobars. These are called Straight
Lined Winds.
Geostrophic Balance
Geostrophic Balance
▪ When the isobars are far apart, the wind blows gently… when
the isobars are close together, the wind blows quickly.
Centripetal Acceleration
▪ Suppose those isobars are curved…
▪ The wind still flows parallel to
isobars, but is forced to change
directions (because the isobars
aren’t straight)…
▪ This is called a Gradient Wind.
▪ Changes in directions are a type of
acceleration, called centripetal
Cyclonic and Anticyclonic Flow
▪ Cyclonic Flow (Low Pressure): Winds blow counter-clockwise
around a low in the Northern Hemisphere.
▪ Anti-cyclonic Flow (High Pressure): Winds blow clockwise
around a high in the Northern Hemisphere.
Upper Level Winds
▪ Zonal Winds: Winds that move in a West-East direction.
▪ Meridional Winds: Winds that move in a North-South direction.
Winds at the Surface
▪ In the upper-atmosphere, the
Pressure Gradient and Coriolis
forces balance each other out…
▪ Near the surface of the Earth,
Friction acts to slow the wind down,
throwing off the balance between
the Pressure Gradient and Coriolis
▪ Friction is only important in the
lowest 1000 meters of the
atmosphere… this is called the
Planetary Boundary Layer.
▪ Friction slows the wind down,
weakening the Coriolis force…
Winds at the Surface
▪ Friction slows down the wind, weakening the Coriolis force…
making the Pressure Gradient Force stronger… as a result:
▪ At the surface, winds blow Counterclockwise and Into Low
▪ At the surface, winds blow Clockwise and out of High Pressure.
In the Southern Hemisphere…
Convergence and Divergence.
▪ At the surface:
▪ Winds converge and rise into Low Pressure… this produces
Divergence Aloft.
▪ Winds diverge and sink into High Pressure… this produces
Convergence Aloft.
Wind Direction
▪ Meteorologists care about
where the wind is coming
FROM, using a compass.
▪ 0° means due North
▪ 90° is due East
▪ 180° is due South
▪ 270° is due West.
Offshore vs. Onshore flow
A prevailing wind is a wind direction
most often observed over a given
time period at a given location.
Prevailing Winds in Colorado cause
trees to be shaped like this.
Measuring Winds
▪ Anemometers are the primary instrument for measuring wind
▪ Wind Socks are also used at airports for a more visual guide.
▪ Wind Vanes give wind direction.
That would be cool!
▪ Pressure is based on the weight of the air above you
▪ Average Sea Level Pressure = 1013.25mb or 29.92 inHg
▪ Air moves from High Pressure to Low Pressure
▪ There are three forces that affect the wind…
▪ Pressure Gradient Force (PGF)
▪ Coriolis Force (CF)
▪ Friction (only important in the Boundary Layer!)
Next Time…

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