Chapter 9
The Atmosphere in Motion: Air
Pressure, Forces, and Winds
Atmospheric Pressure
• Horizontal changes in
atmospheric pressure
generate wind
• Remember that the
vertical variation of
pressure is actually
much larger than the
horizontal variation.....
– In the horizontal
direction, pressure can
change by about 10 mb
in a distance of 100's of
kilometers
Pressure Fluctuations
Solar heating of ozone gasses in the upper atmosphere, and of water
vapor in the lower atmosphere, can trigger oscillating thermal tides
of sea-level pressure change.
Pressure Scale & Units
Many scales are
used to record
atmospheric
pressure, including
inches of mercury
(Hg) and millibars
(mb).
Figure 9.4
Pressure Defined
• Before we can understand
how winds are created, we
need to discuss the
concept of pressure in
greater detail...
– Recall that pressure is
defined as the force exerted
over some area P=F/a
– The atmospheric pressure
can be thought of as the
weight of the air above you
pushing down on some area
Ideal Gas Law
• How can pressure change?
• Air can approximately be regarded as an "ideal
gas"
• ideal gases obey the "ideal gas law":
– P = CρT
•
•
•
•
P = pressure exerted by the gas
C = constant
ρ = density of the gas = mass/volume
T = Temperature of the gas
– Or…. P = ρT (pressure increases if either the density
or the temperature increases)
Pressure changes due to density
variations
• From the ideal gas law:
P = CρT
– If T is constant, then P can
increase by increasing the
density of the gas
– Conversely, if the density
decreases, so does the
pressure
• What happens if the
temperature of the air
column changes?
Pressure changes due to
temperature variations
• Take a cold column of air
and warm it up:
– Which column of air has a
larger density?
– Which column of air
occupies a larger volume?
– Which will be larger, the
surface pressure for the
cold-air column or the
surface pressure for the
warm air column?
Pressure changes aloft due to
temperature variations
• Remembering that
pressure is a function of
how much atmosphere is
above a surface:
– At which location (1 or 2)
will the pressure be
higher?
– As a result, at 5 km will air
move from the cold to
warm column or from the
warm to cold column?
Pressure changes aloft due to
temperature variations
• What will happen
here at 500mb?
– Air will move from
the warm column to
the colder column
at 500 mb due to
the pressure
gradient force.....,
more on this later.
•
Measuring
Pressure
Pressure is usually measured with a
barometer ("bar" o "meter" - an
instrument that measures "bars")
• 1 atmosphere (14.7 lb/in²) is the
amount of pressure that can lift water
approximately 33.9 feet (10.3 m).
– Shallow pumps….
• Recall the different units of pressure:
– 1 bar = 1000 mb
– At sea level, 1013.25 mb = 29.92
inches of Hg = 76 cm of Hg
• Discuss the different types of
barometers....
– Mercury barometer
Measuring Pressure - Aneroid
Barometer
• Other types of
barometers....
– Aneroid barometer -->>
• Most common type of
barometer for home use
• The aneroid cell volume is
very sensitive to changes in
atmospheric pressure:
• The cell volume gets smaller
as the atmospheric pressure
increases
Measuring Pressure - Altimeters
and Barographs
• Altimeter:
– Measure pressure (aneroid barometer) but indicate altitude
– Used in aircraft, hand-held for tactical navigation
• Barograph
Measuring Pressure at the surface - the
surface pressure chart
• Surface pressure chart - isobars
(lines of constant pressure) are
plotted every 4 mb
• Maps of surface pressure are
very important:
– Give positions of highs and lows
– Can give information about the
direction and strength of the surface
winds
• Now will look at how a surface
(sfc) pressure chart is created
from observations around the
country
• Station elevation will dramatically
affect the sfc pressure reading
Measuring Pressure at the surface elevation differences
• One very important source of
error when generating a
surface pressure chart is that
not all stations are at sea
level....
– So, many of the surface pressure
readings contain significant errors
due to altitude
• Pressure decreases with
height (elevation), so we must
correct this in order to produce
a meaningful surface weather
chart…
Surface Pressure vs. Sea Level
Pressure
Measuring Pressure at the surface - reducing
pressure to sea level
• Must reduce the pressure
measurements to sea level
using the following rule:
– In the lower part of the
atmosphere, pressure changes by
about 10 mb for every 100
meters of elevation change....
– Using this rule, we reduce all
pressure measurements to sea
level, producing a constant
elevation sea-level pressure
(SLP) chart ..., commonly referred
to as a surface weather map
• Now we will look at how
pressure is represented above
the surface…
Representing pressure above the surface
• First, the tropopause
height varies with
latitude since:
– Tropopause height is
proportional to the
mean tropospheric
temperature.
– Higher near the
equator (warm
troposphere
– Lower at the poles
(cold troposphere)
Representing pressure above the
surface - constant height chart
• What would the
pressure pattern
on a constant
height chart look
like if Z = 5km??
Representing pressure above the surface
- Z = 5 km
• Pressure would decrease
as you move northward
• This type of chart is not
used often in
meteorology
• Constant pressure
charts, however, are
used extensively....so
we’ll examine these in
greater detail.....
•
Isobaric (contant pressure)
Charts
First, recall that the
tropospheric thickness is
proportional to the mean
tropospheric temperature:
• As shown to the right--the
500mb surface will be
located at higher levels to
the south and at lower
levels further north
• Hence, on an isobaric chart
(e.g., 500mb) we plot
isopleths (contour lines) of
the height of the pressure
surface above sea level.
Ridges & Troughs
Upper level areas
with high
pressure are
named ridges,
and areas with
low pressure are
named troughs.
These elongated
changes in the
pressure map
appear as
undulating
waves.
Isobaric Charts - Example of 500mb
chart
• Notice that the
larger heights
are towards the
south where it is
warmer
• lower heights are
found further
north where it is
colder.....
Isobaric Charts - heights of the
commonly used surfaces
• The table to the
right gives
"approximate"
altitudes of the
common
isobaric charts
used in
meteorology
”High to Low, look out below!”
• Aircraft altimeter doesn’t measure distance from ground…it measures the
pressure outside the aircraft
– Aircraft altimeter assumes a standard pressure at an altitude
• Pilot sets plane’s altimeter at takeoff from station A, then flies towards B
– Altimeter tells him to fly along a constant P surface
– The closer the aircraft gets to B, the closer to the ground it gets
• If the aircraft was flying in bad wx in high terrain—would this be a good or
a bad situation?
• Plane drivers adapt by:
– En-route RAPCON updates
– Radio altimeter
– TF/TA
Isobaric Charts - Ridges and Troughs
• Notice that the height lines
are NOT oriented EW
• In fact, you should see a
wave-type pattern in the
height lines with:
– ridges
– troughs
• Is there warm or cold air
aloft associated with a
ridge? A trough??
• WE flow called zonal
• NS (SN) flow called
meridional (or high zonal)
Ridges and Troughs
Aloft - Highs and Lows
at the Surface
• Notice that ridges aloft
are associated with
Highs at the surface
• Troughs aloft are
associated with Lows at
the surface
• This association of
ridges/troughs with
Highs/Lows occurs
most of the time, BUT
NOT ALWAYS
What creates wind?
• Winds are the result of a balance of
physical forces acting on the air:
– Pressure gradient force
– Coriolis force
– Centripetal force
– Friction
• Let's now examine each of these forces,
and their effects on winds.......
Pressure Gradient Force
(PGF)
• Air will move from high
to low pressure.
• How strongly it flows
depends on the
differences in
pressure
• The pressure gradient
is a measure of that
pressure difference—
the change in
pressure over a given
horizontal distance
Isobar spacing and the magnitude of the
pressure gradient
• The magnitude of the
pressure gradient can
be assessed by noting
the spacing of the
isobars....
– If the isobars are far
apart, the pressure
gradient is small
– If the isobars are close
together, the pressure
gradient is large (steep)
Next--the Coriolis Force
• The Coriolis Force (CF) arises due to the
fact that the earth is rotating
• Coriolis force=2mWVsinf
–
–
–
–
m=mass
V=speed
W=earth’s angular rotation
Sinf=sine of latitude
Equation!
• Coriolis force=2mWVsinf
–
–
–
–
m=mass
V=speed
W=earth’s angular rotation
Sinf=sine of latitude
Properties of Coriolis Force (CF)
• Acts on objects not rigidly attached to the earth
• Always acts to deflect an object to the right (left) of it's
direction of motion in the northern (southern) hemisphere
• Magnitude is zero at the equator; maximum at the poles
• Magnitude depends on the rotation rate of the earth - the
magnitude would increase if the earth’s rotation rate increased
– If the earth were not rotating, CF would be zero
• CF is larger for things moving at faster speeds; zero if the
object is not moving
– CF is negligible for slow-moving objects, or for those moving over short
distances
• Such as, flushing of a toilet...or the movement of a snail
CF
• The Coriolis Force is an "apparent" force that
arises solely due to the fact that the earth is
rotating. Therefore, it can only change a parcel's
direction, it CAN NOT affect its speed.
• If a high speed train travels from LA to NY, will
the coriolis force act on the train?
• Does the coriolis force act on a baseball thrown
from a pitcher to the catcher during a major
league game?
• Does the coriolis force have an affect on ocean
currents?
Definition of Geostrophic Flow
• When the isobars are straight, parallel
lines, and the only two forces acting on a
parcel are the PGF and the CF, then the
wind is called the geostrophic wind:
• PGF and CF are equal in strength
(magnitude) and opposite in direction
• The geostrophic wind is always parallel to
the isobars (height lines on an isobaric
chart)
Geostrophic wind—Upper level, straight
height contours
• Parcel at A moves
toward lower pressure
(PGF)
• Once parcel starts
moving, CF begins to
“pull” to the right
• Eventually, PGF=CF,
and geostrophic wind
develops.
– Parallel to height
lines/isobars
– Strength dependent on
magnitude of PGF
Introduction to Gradient Wind Flow
• Wind flow is geostrophic when the PGF
and the CF are in balance
– This occurs when the isobars (height lines)
are relatively straight
– What about the case when the isobars have
curvature, as around highs and lows????
• When there is curvature in the flow, we
must also consider the centrifugal force
acting on a parcel.....
The Centrifugal Force
• If you attach a string to a ball and swing it in a circular
manner, the force that is required to keep the ball moving
in the circular path is called the centripetal force
– The centripetal force is directed inward, towards the axis of
rotation
• As you swing the ball with the string, you feel the string
tug on your hand...., this is called the centrifugal force
and is equal and opposite to the centripetal force
Centripetal Force=V2/r
Becomes important for faster flow and
smaller radius (such as hurricane or
tornado).
Gradient flow around highs and lows
• So, the gradient
wind is due to a
combination of:
– pressure gradient
force
– coriolis force
– centrifugal force
(pointing out)
Boundary Layer Winds
• Above approximately 850 mb (5000’), the
wind flow is either in geostrophic or
gradient wind balance
• From the surface up to about 3500’ above
ground level (AGL), we must include the
effect of friction
– Oops….the flow is no longer in geostrophic or
gradient wind balance!
•
Winds
near
the
surface
of
the
earth
Below the boundary layer
(about 3500 feet AGL),
friction “drags” and slows
wind speed
– The friction force is in the
opposite direction as the
wind direction
• Dominant forces involved:
PGF+CF+Friction
• Causes wind to blow across
isobars toward lower
pressure
• 10-40o shift from geostrophic
wind
– 10o over open water; up to 40o
over mountainous terrain
• Notice winds
crossing isobars,
flowing towards
lower pressure
• Notice strength
of winds in
comparison with
isobar density
Effect of friction on flow around lows
and highs
• How does friction affect
flow around lows and
highs near the surface?
• Due to the frictional
turning of the wind such
that it crosses the
isobars, what can you
infer about the vertical
motions in the vicinity of
a:
– surface low?
– surface high?
Effect of friction on flow around lows and
highs - associated weather
• At the center of a surface low, the air converges, and then must rise
– We expect to see clouds and precip near a surface low
• At the center of a surface high, the air is diverging, and must be coming from
aloft due to sinking motion
– We expect to see clear, dry weather near a surface high
Pressure Systems
•
•
High: center of pressure surrounded on all sides by
lower pressure
Air moves clockwise (anticyclonically) around the
center (in the Northern Hemisphere)
–
–
–
•
•
Also called an anticyclone
Area of sinking air
Often suppresses clouds and precipitation
Low: center of pressure surrounded on all sides by
higher pressure
Air moves counterclockwise (cyclonically) around the
center
–
–
–
Also called a cyclone
Area of rising air
Often produces cloudy skies and precipitation
Buoys-Balot Law
• Upper level
winds:
– Stand with the
upper level
winds to your
back
• (Note cloud
drift)
– Upper level Low
will be at “your 9
o’clock”
Surface Winds
• Stand with surface
winds blowing at
your back
– Turn 30o clockwise
– Surface Low
pressure center will
be at your “9 o’clock”
Figure 9.32b
Measuring winds
• Two common types of
instruments used to
measure winds are:
– Aerovanes
– Anemometers
– Others??
Windsock
Weathervanes
Wind Direction
• The speed of the wind is
given by the number of barbs
on the wind flag (also see
appendix A-5)
• Wind direction is reported :
– 0° - from the north (northerly)
– 90° - from the east (easterly)
– 180° - from the south
(southerly)
– 270° - from the west (westerly)
Test yer learnin’
• 1. If the earth were not rotating, how would the wind blow with respect
to centers of high and low pressure?
• 2. Why are surface winds that blow over the ocean closer to being
geostrophic than those that blow over the land?
• 3. If you live in the Northern Hemisphere and a region of surface low
pressure is directly west of you, what would probably be the surface
wind direction at your home? If an upper-level low is also directly west of
your location, describe the probable wind direction aloft and the direction
in which middle-type clouds would move. How would the wind direction
and speed change from the surface to where the middle clouds are
located?
• 4. Consider wind blowing over a land surface that crosses a coastline
and then blows over a lake. How will the wind speed and direction
change as it moves from the land surface to the lake surface?
• 5. With your present outside surface wind, determine where regions of
surface high- and low- pressure areas are located. If clouds are moving
overhead, locate the locations of high- and low- pressure aloft.
Key Concepts and Facts
• Pressure decreases most rapidly w/ elevation in cold
column of air
• Cold air aloft is normally associated w/ low pressure,
while warm air is associated w/ high pressure
• We use a barometer to measure air pressure
• The amount of pressure change that occurs over a given
horizontal distance is the pressure gradient
• Horizontal differences in pressure create a pressure
gradient force (PGF). This force causes winds to blow
• On a weather map, closely spaced isobars (or height
contours) represent a steep (large) PGF and strong
winds, and visa versa
Key Concepts and Facts con’t
• The Coriolis Force (CF) causes the wind to bend to the right of
its path in the Northern Hemisphere, and to the left in the
Southern Hemisphere
• The CF only influences the direction of the wind and not its
speed
• The winds on an upper-level weather chart tend to blow parallel
to contour lines in a more or less WE direction in both
hemispheres (mid and high latitudes)
• Sinking air (subsidence) occurs above a surface high pressure
system; rising air occurs above a surface high pressure system
• Surface winds tend to cross isobars, towards lower pressure, at
an angle that averages 30o.
• In the Northern hemisphere, surface winds blow clockwise and
outward from the center of the High, and counterclockwise and
inward toward the center of the Low. Opposite directions in the
Southern Hemisphere.