AIR PRESSURE AND WINDS

1
AIR PRESSURE AND WINDS
2
What is Air Pressure?

• Air Pressure is a measure of the weight of the
  air above a point of observation
• It is measured as a force/area
• The amount of force of a substance over a given
  area

3
Measurements of Air Pressure

• Baseline for Air Pressure is mean sea level
• In force/area, mean sea level equals 14.7 lbs/in2

• Pressure changes more quickly with vertical
  distance changes than horizontal distance changes
• Pressure decreases at a constant rate with
  increased elevation

4
(No Transcript)
5
First Measures of Air Pressure

• Evangelista Torricelli, 1643, invented the first
  instrument to measure air pressure
• Using a calibrated glass tube, inserted open end
  down, into a shallow dish of mercury (Hg),
  Torricelli noticed that the mercury would rise up
  into the tube

6
(No Transcript)
7
TORRICELLIS CONCLUSIONS

• Torricelli concluded correctly that pressure of
  the air on the mercury (Hg) forced it into the
  glass tube.
• Average height 760 millimeters (mm)
• The height of the mercury was a measure of
  atmospheric pressure
• Inches of mercury still used to day for measuring
  air pressure.
• 29.92 inches of mercury is air pressure at mean
  sea level.

8
Other measures of Air Pressure

• Another measure of air pressure is the Bar
  used mostly in meteorology.
• The Bar is based on the force of 1000 dynes/cm2
• a dyne is the force of acceleration of 1m/sec/sec
• One bar equals approximately 14.5 psi (pounds per
  square inch)
• One bar equals 100,000 Newtons/m2
• A Newton is the force required to accelerate 1
  Kilo_at_ 1meter/sec2
• Force of a small red apple falling under gravity

9
Using the Bar in Measuring Air Pressure

• A Bar can be divided into 1000 smaller sections
  called millibars
• Mean sea level pressure in millibars is 1013.25
  mb
• Equivalents 760 mm of mercury 29.92 inches of
  mercury 14.7 lbs/in2

10
Air Pressure Maps

• Connecting points of equal air pressure produces
  isobars (similar to contour lines on a
  topographic map)
• Pressure maps are used to identify different air
  pressure cells High or Low

11
(No Transcript)
12
Constant Pressure Charts

• Constant pressure (isobaric) chart are
  constructed to show height variations along an
  equal pressure surface.
• Any change in air temperature changes air density
  and air pressure.
• Using the USA as an example
• Air closer to the equator is generally warm,
  while air closer to the poles is generally cooler
• On an isobaric chart higher elevations
  correspond to higher pressures at any elevation
• Lower elevations on an isobaric chart correspond
  to lower pressures at any elevation
• Elongated highs bend into ridges
• Elongated lows bend into troughs

13
Constant Pressure Chart
14
Constant Pressure Chart Ridges and Troughs
High Pressure Ridge
Low Pressure Trough
Ridges
Ridges
Upper Atmosphere air flow over high pressure
ridges and under low pressure troughs
Northern Hemisphere
Troughs
15
Constant Pressure (Upper Level) Charts What do
they tell us?

• Show wind-flow patterns of importance to weather
  forecasting
• Tracks movement of weather systems
• Predict behaviors of surface pressure area
• A constant pressure chart helps pilot determine
  they are flying at correct altitude using an
  altimeter

16
What is Low or High Air Pressure?

• Low Air Pressure develops when there are fewer
  air molecules exerting a force.
• Pressure may be less than average sea level air
  pressure
• High Air Pressure develops when there are more
  air molecules exerting a force.
• Pressure may be more than average sea level air
  pressure

17
TYPES OF AIR PRESSURE

• There are two ways that high and low air
  pressure can develop in the atmosphere.
• Thermal Air Pressure
• Due to unequal heating of land and water
  conduction and convection
• Dynamic Air Pressure
• Upper atmospheric winds, earths rotation

18
How does Thermal Low Air Pressure Develop?

• Thermal Low Air Pressure develops over warm to
  hot surfaces through the process of conduction
  and convection.
• Air over the warm to hot surface becomes warmer,
  more buoyant and less dense than surrounding air
  it rises.
• The convection process reduces the number of air
  molecules close to the surface fewer air
  molecules exert a weaker force Low Air
  Pressure

19
Low Air Pressure
warm to hot surface heats air above it
conduction and convection warm air is less
dense, more buoyant than surrounding air and the
warm air starts to rise
20
How does Thermal High Pressure Develop?

• Thermal High Pressure develops over cool to cold
  surfaces
• Cooler air is less buoyant and more dense than
  surrounding air cool air sinks
• As more air sinks to the surface, it adds more
  and more air molecules, which creates a stronger
  force High air pressure

21
High Air Pressure
Air over a cool to cold surface slowly sinks
toward the ground cool air is more dense, less
buoyant than surrounding air
22
High and Low Air Pressure Air Flow

• As warm air is lifted away from the surface in a
  Thermal Low Air Pressure, fresh air is pulled
  into the center of the Low to replace the lifted
  air (surface air convergence).
• Warm rising air cools as it rises cloud
  formation possible
• As cooler air sinks toward the surface in a
  Thermal High Air Pressure, the sinking air is
  pushed out from the center of the low at the
  surface to make room for new falling air (surface
  air divergence).
• Cool sinking air warms slightly as it sinks no
  cloud formation possible

23
Air Flow in Surface Low and High Air Pressures
Surface high pressure
Cool sinking air
Warm rising air
Surface low pressure
24
DYNAMIC AIR PRESSURE

• Air pressure systems created by upper level winds
  and the earths rotation are called Dynamic Air
  pressure.
• Dynamic Highs have a core of warm descending air
• Air is still sinking, but under a dynamic high,
  the air warms considerably as it descends
• Dynamic Lows have a core of cool rising air
• Air is still rising, but under a dynamic low,
  even cool air is pulled up

25
Wind Horizontal Air Movement Due to a Difference
in Surface Pressures

• Air Movement based on two of Newtons Laws of
  Motions
• (1) An object in motion or at rest will tend to
  stay in motion or at rest until a force is
  exerted on it (INERTIA)
• (2) The force on an object is equal to the mass
  of the object times the acceleration produced by
  the force
• F ma

26
What are the forces involved with Air Movement?

• Pressure Gradient Force
• Controls both Wind Direction and Wind Velocity
• Coriolis Force/Effect
• Controls Wind Direction, only
• Friction
• Controls Wind Velocity, only
• Acts to slow wind down close to surface

27
Pressure Gradient Force

• Pressure Gradient is the rate of pressure change
  that occurs over a given distance
• Pressure Gradient Force (PGF) is the net force
  produced when differences in horizontal air
  pressure exist
• PGF is always directed from High Pressure to Low
  Pressure and moves at right angles to the isobars

28
Pressure Gradient Force

• Isobars close together indicate a rapid change in
  air pressure producing a steep Pressure Gradient
  Force
• Result Strong, high speed winds
• Isobars far apart represent a slow change in air
  pressure producing a gentle Pressure Gradient
  Force
• Result Weak, low speed winds

Green arrows represent the same horizontal
distance between two points
29
Coriolis Force

• Coriolis Force is an apparent force due to the
  rotation of Earth on its axis.
• This force appears to deflect any free-moving
  object (plane, ships, rockets, bullets, air,
  currents) from its original straight-line path.
• The deflection is to the right in the Northern
  Hemisphere and to the left in the Southern
  Hemisphere

30
Coriolis Force
Blue arrows indicate the direction of
deflection To the Left of original path in
southern hemisphere To the Right of the original
path in northern hemisphere
31
Coriolis Force

• Coriolis Force varies with speed, altitude and
  latitude of a moving object.
• Coriolis Force is almost zero at equator and
  greatest near the poles
• The higher the velocity of the moving object, the
  stronger Coriolis affects the object.
• Coriolis affects only wind direction

32
Friction

• The effect of friction is observed closest to
  Earths surface Boundary Layers
• Friction slows down wind speed
• The friction layer varies in height across the
  Earth, but for the most part lies within about a
  kilometer of the surface.

33
Friction
Wind speeds slow the closer to the surface.
High altitude winds do not experience friction
and are much faster than surface winds
No friction in upper air
34
Forces and Wind Direction

• Pressure Gradient Force, Coriolis Force, and
  Friction affect the movement of air into and out
  of Air Pressure systems.
• Air always moves into the center of a Low
  cyclonic air flow.
• Air always moves out of the center of a High
  anticyclonic air flow

35
Forces and Air Flows (Northern Hemisphere)
36
Cyclonic Air Flow (Surface Lows)

• Northern Hemisphere
• Counterclockwise and into the center of a Low
• Southern Hemisphere
• Clockwise and into the center of a Low

37
Cyclonic Air Flow Northern Hemisphere
38
Anticyclonic Air Flow (Surface Highs)

• Northern Hemisphere Anticyclonic Air Flow
• Clockwise and out of the center of a High
• Southern Hemisphere Anticyclonic Air Flow
• Counterclockwise and out of the center of a High

39
Anticyclonic Air Flow Northern Hemisphere
40
Geostrophic Winds

• A theoretical horizontal wind that blows in a
  straight path at a constant speed, parallel to
  the isobars.
• Jet Streams are a close approximation to a
  Geostrophic Wind
• Wind exists at approximately 1000 meters above
  the ground above the boundary layer (friction
  layer)
• It develops when the Pressure Gradient Force and
  Coriolis Force are in a dynamic balance.

41
Geostrophic Wind
Northern Hemisphere
42
Air Flow Across Isobars
Upper Air Flow
Surface Air Flow
NORTHERN HEMISPHERE AIR FLOWS
43
Jet Streams

• A jet stream is a swift river of air found in the
  upper troposphere
• Two are usually found in each hemisphere
• Polar jet stream
• Subtropical jet stream
• Each jet stream is formed by different processes

44
Polar Front

• Air sinks at the Poles, creating the Polar highs
• Air flows from the Poles down towards the Equator
• Coriolis force deflects the air to the right,
  resulting in the polar easterlies
• The boundary between the polar easterlies and the
  westerly winds of the midlatitudes is called the
  polar front
• The polar front separates cold polar air from
  more temperate air to the South

45
Polar Front
46
Polar Jet Stream

• It resembles a stream of water moving west to
  east and has an altitude of about 10 kilometers.
• Its air flow is intensified by the strong
  temperature and pressure gradient that develops
  when cold air from the poles meets warm air from
  the tropics.
• Strong winds exist above regions where the
  temperature gradient is large
• The polar jet stream forms because of this
  temperature gradient
• The polar jet stream is found above the polar
  front at approximately 600 N and 600 S

47
Subtropical Jet Stream

• The subtropical jet stream is located
  approximately 13 kilometers above the subtropical
  high pressure zone.
• The reason for its formation is similar to the
  polar jet stream.
• However, the subtropical jet stream is weaker.
  Its slower wind speeds are the result of a weaker
  latitudinal temperature and pressure gradient.

48
Jet Streams
49
Polar Jet Stream Seasonal Shifts
50
Global Semi-Permanent Air Pressure Systems

• There are a number of air pressure systems that
  are considered semi-permanent due to their
  consistency in location.
• Most of these pressure systems are found over the
  worlds oceans both in the northern and
  southern hemisphere.

51
Pacific Ocean Semi-Permanent Air Pressure Systems

• Pacific High
• Located at approximately 300 N, off the coast of
  California
• Seasonally shifting
• Shifts to the South (closer to Baja California)
  during the winter (winter storms to southern
  California)
• Shifts to the North during the summer (no
  precipitation in southern California)
• Aleutian Low
• Located at approximately 600 N, in the Gulf of
  Alaska
• Seasonally shifting
• to the south in winter (sending winter storms to
  southern California)

52
(No Transcript)
53
Atlantic Ocean Semi-Permanent Air Pressure
Systems

• Bermuda-Azores High
• Located approximately 300N
• Shifts seasonally south in winter, north in
  summer
• Icelandic Low
• Located approximately 600 N, near Iceland
• Noreaster

54
Pacific and Atlantic Ocean Air Pressure Systems
55
Ocean Currents

• Ocean currents are generated by winds blowing
  across the surface of the waters
• Ocean currents in both the Atlantic and Pacific
  Oceans flow in a clockwise gyre (a semi-circular
  flow), responding to air flow out of the Pacific
  High and the Bermuda-Azores High

56
Pacific Ocean Currents

• California Current
• A south-flowing cold current, flowing parallel to
  the west coast of North America
• Equatorial Currents
• A series of westerly-flowing warm currents,
  flowing from eastern to western tropical Pacific
  Ocean basin
• Kuroshio Current
• Northerly-flowing warm current, flowing along the
  east coast of Asia
• North Pacific Drift
• An easterly-flowing, somewhat warm current,
  flowing towards North America

57
Atlantic Ocean Currents

• Gulf Stream
• A northerly-flowing warm current, flowing
  somewhat parallel to east coast of North America
• Labrador Current
• A southerly-flowing cold current
• North Atlantic Drift
• An easterly-flowing somewhat warm current,
  flowing from western to eastern Atlantic Ocean
  basin
• Canary Current
• A southerly-flowing cool current, flowing almost
  parallel to west coast of Europe and Africa
• Equatorial Currents
• A series of westerly-flowing warm currents,
  flowing from eastern to western tropical Atlantic
  Ocean basin

58
Atlantic and Pacific Ocean Currents
59
GLOBAL CIRCULATION

• Energy from the Sun heats the entire Earth, but
  this heat is unevenly distributed across the
  Earth's surface.
• Equatorial and tropical regions receive far more
  solar energy than the midlatitudes and the polar
  regions.
• The tropics receive more heat radiation than they
  emit, while the polar regions emit more heat
  radiation than they receive. 
• If no heat was transferred from the tropics to
  the polar regions, the tropics would get hotter
  and hotter while the poles would get colder and
  colder.
• This latitudinal heat imbalance drives the
  circulation of the atmosphere and oceans. 
• Around 60 of the heat energy is redistributed
  around the planet by the atmospheric circulation
  and around 40 is redistributed by the ocean
  currents.

60
ATMOSPHERIC CIRCULATION

• One way to transfer heat from the equator to the
  poles would be to have a single circulation cell
  where air moved from the tropics to the poles and
  back.  This single-cell circulation model was
  first proposed by Hadley in the 1700s.

61
HADLEY CELL CIRCULATION
62
ATMOSPHERIC CIRCULATION

• Since the Earth rotates, its axis is tilted and
  there is more land in the Northern Hemisphere
  than in the Southern Hemisphere, the actual
  global air circulation pattern is much more
  complicated.
• Instead of a single-cell circulation, the
  global model consists of three circulation cells
  in each hemisphere.
• These three cells are known as the tropical cell
  (also called the Hadley cell), the midlatitude
  cell and the polar cell.

63
Three-cell Circulation Ferrel
64
GLOBAL WINDS AND AIR PRESSURE SYSTEMS