The Atmosphere in Motion: Air Pressure, Forces, and Winds

1
Chapter 9

• The Atmosphere in Motion Air Pressure, Forces,
  and Winds
•  

2
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

3
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.
4
Pressure Scale Units
Many scales are used to record atmospheric
pressure, including inches of mercury (Hg) and
millibars (mb).
Figure 9.4
5
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 PF/a
• The atmospheric pressure can be thought of as the
  weight of the air above you pushing down on some
  area

6
Ideal Gas Law

• How can pressure change?
• Air can approximately be regarded as an "ideal
  gas"
• ideal gases obey the "ideal gas law"
• P C?T
• P pressure exerted by the gas
• C constant
• ? density of the gas mass/volume
• T Temperature of the gas
• Or. P ?T (pressure increases if either the
  density or the temperature increases)

7
Pressure changes due to density variations

• From the ideal gas law P C?T
• 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?

8
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?

9
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?

10
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.

11
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?

12
Measuring Pressure - Aneroid Barometer

• Other types of barometers....
• Aneroid barometer --gtgt
• 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

13
Measuring Pressure - Altimeters and Barographs

• Altimeter
• Measure pressure (aneroid barometer) but indicate
  altitude
• Used in aircraft, hand-held for tactical
  navigation
• Barograph

14
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

15
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

16
Surface Pressure vs. Sea Level Pressure
17
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

18
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)

19
Representing pressure above the surface -
constant height chart

• What would the pressure pattern on a constant
  height chart look like if Z 5km??

20
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 well examine these in greater
  detail.....

21
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.

22
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.
23
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.....

24
Isobaric Charts - heights of the commonly used
surfaces

• The table to the right gives "approximate"
  altitudes of the common isobaric charts used in
  meteorology

25
High to Low, look out below!

• Aircraft altimeter doesnt measure distance from
  groundit measures the pressure outside the
  aircraft
• Aircraft altimeter assumes a standard pressure at
  an altitude
• Pilot sets planes 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
  terrainwould this be a good or a bad situation?
• Plane drivers adapt by
• En-route RAPCON updates
• Radio altimeter
• TF/TA

26
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)

27
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

28
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.......

29
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 differencethe change in pressure over a
  given horizontal distance

30
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)

31
Next--the Coriolis Force

• The Coriolis Force (CF) arises due to the fact
  that the earth is rotating
• Coriolis force2mWVsinf
• mmass
• Vspeed
• Wearths angular rotation
• Sinfsine of latitude

32
Equation!

• Coriolis force2mWVsinf
• mmass
• Vspeed
• Wearths angular rotation
• Sinfsine of latitude

33
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
  earths 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

34
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?  

35
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)

36
Geostrophic windUpper level, straight height
contours

• Parcel at A moves toward lower pressure (PGF)
• Once parcel starts moving, CF begins to pull to
  the right
• Eventually, PGFCF, and geostrophic wind
  develops.
• Parallel to height lines/isobars
• Strength dependent on magnitude of PGF

37
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.....

38
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 ForceV2/r Becomes important for
faster flow and smaller radius (such as hurricane
or tornado).
39
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)

40
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!

41
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 PGFCFFriction
• 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

42

• Notice winds crossing isobars, flowing towards
  lower pressure
• Notice strength of winds in comparison with
  isobar density

43
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?

44
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

45
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

46
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 oclock

47
Surface Winds

• Stand with surface winds blowing at your back
• Turn 30o clockwise
• Surface Low pressure center will be at your 9
  oclock

Figure 9.32b
48
Measuring winds

• Two common types of instruments used to measure
  winds are
• Aerovanes?
• Anemometers
• Others??

49
Windsock
Weathervanes
50
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)

51
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.

52
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

53
Key Concepts and Facts cont

• 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.