CHAPTER 7
ATMOSPHERIC MOTIONS
CHAPTER 7
ATMOSPHERIC MOTIONS
Atmospheric Pressure is the force per unit area of a column of air above you
In other words, pressure is the weight of the column of air above you - a measure of how hard this column of air is pushing down
More fundamentally - atmospheric pressure arises from gravity acting on a column of air
1000 mb 1000 mb
500 mb level
1000 mb
500 mb
500 mb
1000 mb
The heated columnexpands
The cooledcolumn contracts
original 500 mb level
1000 mb
new 500 mblevel in warmair
new 500 mblevel in coldair
1000 mb
The 500 mb surface isdisplaced upward in the warmer column
The level corresponding to 500 mb is displaced downward in the cooler column
original 500 mb level
The surface pressure remainsthe same since both columnsstill contain the same mass of air.
1000 mb
new 500 mblevel in warmair
new 500 mblevel in coldair
1000 mb
The 500 mb surface isdisplaced upward in the warmer column
The 500 mb surface isdisplaced downward inthe cooler column
original 500 mb level
The surface pressure remainsthe same since both columnsstill contain the same mass of air.
A pressure difference in the horizontal direction A pressure difference in the horizontal direction develops above the surfacedevelops above the surface
HighLow
1003 mb 997 mb
original 500 mb level
Air moves from high to low pressure in middle Air moves from high to low pressure in middle of column, causing surface pressure to change.of column, causing surface pressure to change.
HighLowWarm air aloft = high pressure
Cold air aloft = low pressure
1003 mb 997 mb
original 500 mb level
Air moves from high to low pressure at the Air moves from high to low pressure at the surface…surface…
HighLow
High Low
Where would we haverising motion?
1003 mb 997 mb
original 500 mb level HighLow
High LowAir diverges around the surface high
Air converges around the surface low
Rising air above the surface low leads to clouds and storms◦ Low pressure centers
aka “cyclones” Sinking air above
the surface high leads to fair weather◦ High pressure centers
aka “anticyclones”
Cold air aloft means low pressure (heights), warm air aloft means high pressure (heights).
Above the ground, we typically look at maps showing the height of a given pressure level
If there are no horizontal variations in pressure, the pressure at a constant height level, or the height at a constant pressure level, are the same thing
When there are horizontal variations, we see high heights in warm air aloft; low heights in cold air aloft
Steeper slope means the contour lines are closer together!
Table 7.1, p. 181
The cause of the wind! Horizontal pressure gradients lead to winds PGF always directed from high to low
pressure The stronger the pressure gradient, the
stronger the wind ◦ Or, in other words, the closer the isobars are
together, the stronger the wind will be
The length of the red arrows indicate the strength of the PGF
We don’t actually see the wind blow straight across from high pressure to low pressure
There must be other force(s) at work…
“Apparent” force due to rotation◦An outside (nonrotating) observer
doesn’t experience it◦An observer on the rotating body (like
the earth, or the turntable) does experience it
Since we (and the atmosphere) are rotating with the earth, we are affected by this force
Coriolis force turns moving objects/air parcels to the right in the northern hemisphere, to the left in the southern hemisphere
The faster the motion, the stronger the Coriolis force
Coriolis force is zero at the equator, relatively strong at the poles
Pressure gradient force (PGF)◦ Always from high pressure to low pressure
Coriolis force◦ Always toward the right (in the northern
hemisphere) When these two are in balance, it is called
the geostrophic wind◦ Geostrophic = “earth turning”
If you’re traveling with the geostrophic wind, low pressure is always on your left!◦ “when the wind is at your back, lower pressure is
to your left (NH)”
The geostrophic wind blows parallel to straight isobars
But what if the isobars (or isoheights) aren’t straight?◦ (They’re usually curved – troughs and ridges)
When there is curvature, an observer (or an air parcel) in the rotating frame of reference experiences a force directed outward – the centrifugal force – think of being in a car going around a curve
Magnitude of centrifugal force is related to the velocity and the radius of curvature◦ Faster speeds = greater centrifugal force◦ Tight curves = greater centrifugal force
Involves the PGF, Coriolis, and Centrifugal forces – flow is parallel to curved isobars
This is a good estimate of the winds, except right near the ground
Stepped Art
Fig. 8-29, p. 214
There’s one more force that’s important for winds near the ground
Near the surface, the wind is slowed by drag from the ground, trees, buildings, etc.
What happens to force balance of geostrophic wind when the wind slows down?
When wind speed slows down, Coriolis force also is reduced
Therefore, PGF is stronger than Coriolis, and wind blows across isobars toward lower pressure
Wind blows in toward a surface low, and away from a surface high
Aloft – flow parallel to isobars or isoheights
Near surface – Near surface – flow in toward flow in toward low, away low, away
from highfrom high Cyclonic flow Cyclonic flow
(counterclockwise (counterclockwise in NH)in NH)
Anticyclonic flow Anticyclonic flow (clockwise in (clockwise in NH)NH)
Fig. 7.17, p. 189
Like a tornado, or your water in your bathtub
In these situations, the balance is between the PGF and centrifugal forces (Coriolis is unimportant)◦ This is called a
cyclostrophic wind The water flowing out
of your bathtub doesn’t change directions in different hemispheres!
ForceForce DirectionDirection MagnitudeMagnitude Important Important when…when…
Pressure Pressure Gradient Gradient (PGF)(PGF)
From high to low From high to low pressurepressure
Stronger when Stronger when pressure pressure differences are differences are greatergreater
Pressure Pressure varies varies horizontallyhorizontally
CoriolisCoriolis
To the right of To the right of wind in NH, to the wind in NH, to the left of motion in left of motion in SH – always at SH – always at 90º angle to wind90º angle to wind
Increases from Increases from equator toward equator toward pole, increases pole, increases with increasing with increasing wind speedwind speed
Earth is Earth is rotating, rotating, system is system is large and large and lasts a long lasts a long timetime
CentrifugaCentrifugall
Outward from Outward from center of center of curvaturecurvature
Increases with Increases with increasing increasing speed, increases speed, increases with sharper with sharper curvecurve
Flow/motion Flow/motion is curvedis curved
FrictionFrictionIn the opposite In the opposite direction of the direction of the wind wind
Increases with Increases with increasing increasing speed, increases speed, increases for rough for rough surfacessurfaces
Near the Near the earth’s earth’s surface surface (lowest 1000 (lowest 1000 m)m)
NameName Forces Forces involvedinvolved ResultResult Valid when…Valid when… ExampleExample
GeostrophiGeostrophicc
PGF & PGF & CoriolisCoriolis
Flow Flow parallel to parallel to straight straight isobarsisobars
Isobars are Isobars are straight, no straight, no frictionfriction
Upper-Upper-level zonal level zonal windwind
GradientGradient
PGF, PGF, Coriolis, Coriolis, CentrifugCentrifugalal
Flow Flow parallel to parallel to curved curved isobarsisobars
Isobars are Isobars are curved, no curved, no friction, friction, system is largesystem is large
Upper-Upper-level low level low pressure pressure centercenter
Surface / Surface / Boundary Boundary layerlayer
PGF, PGF, Coriolis, Coriolis, FrictionFriction
Flow Flow toward low, toward low, away from away from highhigh
Isobars are Isobars are straight, straight, friction friction importantimportant
Wind near Wind near surfacesurface
CyclostropCyclostrophichic
PGF & PGF & CentrifugCentrifugalal
Flow Flow parallel to parallel to curved curved isobars isobars (can be (can be either either direction direction around a around a low)low)
Isobars are Isobars are curved, system curved, system is small is small (Coriolis (Coriolis unimportant)unimportant)
Tornado, Tornado, draining draining sinksink
Fig. 7.13, p. 185
Mercurial (Fortin) Aneroid
◦ Recording: Barograph Electronic (Pressure
Transducer)
◦ Wind vane◦ Cup anemometer◦ Aerovane (Wind
Monitor by R.M. Young)◦ Sonic◦ Rawinsonde (lifted by
Weather Balloon) Wind soundings Wind Profiler
A small increase in wind speed can greatly increase the wind force on an object◦ F ~ V2
◦ Turbulent whirls (eddies) pound against the car’s side as the air moves past obstructions, such as guard railings and posts
◦ Similar effect occurs where the wind moves over low hills paralleling a highway
◦ Weird Stuff: Wind erosion, desert pavements, sand ripples, snow ripples, snow dunes, snow rollers, snow fences, windbreak, shelter belt
Table 7.2, p. 196
Fig. 7.24, p. 196
Fig. 7.25, p. 197
Fig. 7.26, p. 197
Fig. 7.27, p. 198
Fig. 7.28, p. 198
Winds which are more likely to come from a general direction can have a large influence on climate.
Wind sculptured trees (even in BCS, my backyard)
Wind also influences water Waves forming by wind blowing over the
surface of the water In general, the greater the wind speed, the
greater the amount of energy added, and the higher the waves will be◦ Wind speed◦ Length of time wind blows ◦ Fetch (distance of straight wind over water)
As waves travel across the open ocean into areas of weak winds, their crests become lower and more rounded, forming swells
https://www.fnmoc.navy.mil/wxmap_cgi/index.html
https://www.fnmoc.navy.mil/ww3_cgi/index.html
Seiches◦ Sloshing back and forth of a semi-enclosed body
of water (Great Lakes, bays)
Fig. 7.29, p. 199
The windiest region in the US is the Central Plains
Other windy spots include Alaska, Hawaii, and Atlantic and Pacific coasts
Mountaintops and passes tend to be windy
Fig. 7.30, p. 201
Table 7.3, p. 201
The estimated maximum speed at which wind can blow at sea level is 200 to 225mi/hr
Above this speed, friction with the earth’s surface creates such a drag on the wind that it cannot blow any faster
Wind speeds in excess of 225 mi/hr are possible on mountaintops, narrow valleys, and tornadoes.
Few locations in the world that have in place anemometers capable of measuring wind speeds over 200 mi/hr
Many instruments are simply blown away by winds of this magnitude
National Hurricane Center – anemometer died at 164 mph.
Fig. 7.31, p. 203