Download Air Pressure, Forces and Winds

Air Pressure, Forces and Winds
Air Pressure, Forces and Winds
Aims
To understand the concept of air pressure
To understand the relationship between air pressure and winds
To understand how the various forces modify the wind
Objectives
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To describe the broad feature of the global mean annual winds near the surface and
near the top of the troposphere.
To describe the factors which affect air pressure as given by the Ideal Gas Law.
To describe factors impacting air pressure within the atmosphere.
To explain why gravity only influences vertical motion and not horizontal motion of air.
To explain causes for variations in air pressure within the atmosphere.
To briefly describe Newton's three laws of motion.
To identify the various forces that could act upon an air parcel to initiate atmospheric
motion.
To describe the formation of a sea/land breeze regime and discuss the principle it
illustrates.
To describe the relationship between the observed winds and the horizontal atmospheric pressure field pattern, relating the spacing of isobars to the speed of wind.
To describe the factors that influence the magnitude and direction of the pressure
gradient force and its components - the horizontal and vertical pressure gradient
force.
To describe the influence of the Coriolis effect upon free-moving objects or fluids and
list two factors that influence the magnitude of this effect.
To describe the effect of friction upon the speed and direction of the horizontal surface winds and identify two factors influencing the magnitude of the friction force.
To explain how the various forces are joined in the geostrophic wind, gradient wind
and surface winds.
To list the assumptions and properties of the geostrophic wind approximation.
To compare and contrast geostrophic winds and the observed surface winds.
To sketch a diagram (with isobars and wind arrows) showing the direction of the
winds associated with cyclones and anticyclones in both the Northern and Southern
Hemispheres.
To explain the relationship between horizontal and vertical motions.
To explain why cyclones are typically cloudy and stormy, while anticyclones are commonly fair weather systems.
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Air Pressure, Forces and Winds
Outline
Introduction
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Observed surface and higher altitude winds
Air pressure
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Ideal Gas Law or the Equation of State
Air pressure in the atmosphere, the pressure gradient force
Creation of areas of lower and higher pressure
Air pressure and winds
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The sea breeze analog
Global circulation
Newton’s Laws of motion
Factors affecting winds
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Pressure gradient and pressure gradient force
Coriolis effect
Centripetal force
Friction, gravity
Geostrophic flow
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Geostrophic balance, geostrophic flow
Curved wind and the gradient wind
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Cyclonic and anticyclonic flow
Centripedal acceleration
Surface wind
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Friction, convergence and divergence
How winds generate vertical air movement
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Surface convergence, divergence
Uplifting, subsidence
Factors that promote vertial air flow
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Changes in surface roughness, orography
North-South flow
Dynamical changes in flow
Wednesday 13.00-14.00 and by appointment
Tel.: (0116) 252 3848
GY1
Air Pressure, Forces and Winds
Topics
• Air pressure
• Relationship between air pressure and winds, forces
• Influences of the different forces on air movement and
wind
Outline
Introduction
Air pressure
• Ideal Gas Law or the Equation of State
• Air pressure in the atmosphere, pressure gradient force
Air pressure and winds
• The sea breeze analog
• Global circulation
Newton’s Laws of motion
Factors affecting winds
• Pressure gradient and pressure gradient force
• Coriolis effect, Centripetal force
• Friction, gravity
Geostrophic flow
• Geostrophic balance, geostrophic flow
Curved wind and the gradient wind
• Cyclonic and anticyclonic flow, Centripedal acceleration
Surface wind
• Friction, Convergence and divergence
How winds generate vertical air movement
• Surface convergence, divergence, Uplifting, subsidence
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Air Pressure, Forces and Winds
Bullets
Introduction
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Horizontal air movements observed in the atmosphere.
Wind is the movement of air measured relative to the Earth’s surface.
At the surface mostly zonal, but partially around centers.
Wind is a vector quantity; that is, it has both direction and magnitude - an acceleration
of the wind may consist of a change in speed or direction or both.
Aloft the wind is nearly only zonal, except in the tropics. In the high northern latitudes
more meandering than in the high southern latitudes.
Vertical motion of air can create horizontal motion? Example: sea breeze. This is an
example for a dynamically induced circulation - a circulation that is partially due to the
movement of the air itself.
Winds are labeled by the direction from which they blow
Air pressure
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The pressure (force per unit area) exerted by the atmosphere depends on the pull of
gravity and the mass and kinetic energy of the gas molecules that compose air.
At any specified point within the atmosphere, air pressure has the same magnitude in
all directions.
Air pressure and an density decrease rapidly with altitude in the lower atmosphere
and then more gradually aloft. The atmosphere has no clearly defined upper boundary. Rather, the atmosphere of Earth gradually merges with the highly rarefied hydrogen/helium atmosphere of interplanetary space.
About 99% of the mass of the atmosphere is situated below an altitude of 32 km (20
mi).
Air pressure readings are reduced to sea level in order to remove the influence of station elevation.
Ideal Gas Law relates pressure, volume and temperatures; also called the equation
of state as it describes the state of a parcel of air.
Within the atmosphere, air density is inversely proportional to air temperature. Hence,
all other factors being equal, cold air masses are more dense and exert higher pressure than do warm air masses.
Within the atmosphere, air density is also inversely proportional to water vapor concentration. Hence, at equivalent temperatures, dry air masses are denser and exert
higher pressure than do humid air masses.
As a rule, temperature has a much more significant influence on air pressure than
does humidity.
Pressure gradient force (PGF) - reflects the tendency of a liquid to move to areas of
lower pressure. At right angles to the isobars.
Air pressure plays a crucial role: reflects density of air and its gradients the energy
available to drive winds.
Air pressure may fluctuate in response to divergence or convergence of air, which is
produced by changes in wind speed or direction.
Areas of lower and higher pressure can be created for example by uneven heating
On Earth, the observed vertical pressure distribution results from the fact that in the
tropics the air is warmer and therefore less dense and so occupies a larger volume.
Thus, at the same height in the atmosphere there is still more air above than near the
poles, thus the pressure is higher at the same height.
Air Pressure, Forces and Winds
Air pressure and winds
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Sea breeze analog: In a sea breeze first the isobars are tilted -> horizontal motion.
An air flow aloft is created due to the tilted isobars drawing air up.
The rising air draws air in along the surface -> horizontal motion.
The horizontal motion creates a vertical motion.
The vertical wind creates a horizontal wind.
The air flows from higher to lower pressure.
PGF is the force which initiate winds.
If the PGF were the only force then the winds would blow directly from high to low
pressure according to the observed North South directions of the pressure gradients.
Global circulation looks different, often winds are more or less parallel to the isobars
and not at right angles. Sea- breeze analog only works for a non-rotating Earth.
Observations suggest a three cell global circulation.
Newton’s Laws of motion
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If there is no net force then a body at rest will remain at rest and a body in motion will
move in a straight line at constant speed.
The acceleration of an object is directly proportional to the net force acting on that
body and inversely proportional to the mass of the body.
Actio = Reactio
Factors affecting winds
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Pressure gradient force (PGF) - reflects the tendency of a liquid to move to areas of
lower pressure. At right angles to the isobars proportionally to the pressure gradient.
The pressure gradient force initiates air motion and arises in part from spatial variations in air temperature and, to a lesser extent water vapor concentration.
Areas of lower and higher pressure can be created for example by uneven heating.
If the PGF were the only force then the winds would blow directly from high to low
pressure according to the observed North South directions of the pressure gradients.
However, often winds are more or less parallel to the isobars and not at right angles.
The centripetal force is an imbalance of actual forces and operates whenever the
wind describes a curved path.
The Coriolis effect - due to rotation of Earth - results from movements in North-South
and East-West (except movements on the equator) direction. More precisely it results
from the fact that the used frame of reference (lat, long, height) is rotating independently from movements of objects.
The Coriolis effect due to movement in North South results from the fact that the
Earth rotates under an object moving in the air, e.g. an air parcel.
The Coriolis effect due to movements in East West direction has the same reason.
The Coriolis deflects the wind to the right of its initial direction in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflective force is zero at the
equator and increases with latitude to a maximum at the poles.
The Coriolis effect is important only in large-scale (planetary and synoptic-scale) circulation systems.
The Coriolis force, f, necessary to account for the effect is proportional to the speed
of the moving object and the sin of the latitude, f = 2 Ω v sinφ
Friction at the surface - results from that roughness of the surface.
Obstacles on the Earth’s surface slow the wind by breaking it into turbulent eddies.
Water is in general fairly smooth and land rougher.
Air Pressure, Forces and Winds
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The force due to friction depends on the surface type and the terrain in general, the
roughness of the surface, and the speed of the movement. It generally influences
wind speed and direction within about 1km of Earth’s surface (the friction layer).
Gravity accelerates all objects with mass downwards and perpendicularly to Earth’s
surface. It does not modify horizontal winds but it is important for vertical motion of air.
Geostrophic flow
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Results from the balance of the pressure gradient force and the Coriolis effect
As a parcel of air is accelerated due to the PGF the Coriolis effect increases.
Coriolis force becomes larger and larger as long as the parcel accelerates until finally
the North-South components of the pressure gradient and Coriolis force balance and
the flow is parallel to the isobars. Otherwise there would be a force towards the lower
pressure, thus acceleration and a further increase in the Coriolis force.
The winds generated in this way are called geostrophic winds. They are unaccelerated, horizontal winds blowing at constant speed in a straight path parallel to isobars
at altitudes above the friction layer.
The closer the isobars - the larger the pressure gradient - the higher the wind speed
Idealization!
Thus, in the case of geostrophic winds, the wind direction and speed are directly
related to the prevailing pressure pattern.
Buys Ballot’s Law: in the Northern Hemisphere: If you stand with your back to the
wind, low pressure will be found to your left and high pressure to your right.
Curved wind and the gradient wind
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Wind flow is rarely straight, but often following curved isobars.
Another force is required to generate the curved flow.
Gradient winds result from the spatially (longitudinally) changing pressure patterns,
e.g. around a low. They reflect the impact of centrifugal forces in curved flow: Consider a circular low pressure system - as the flow of air parcels is directed towards a
low on the Northern Hemisphere, the Coriolis force leads to a deflection to the right
until the flow is straight and parallel to the isobars. As the parcels now continue on a
straight path they move away from the center and are thus experiencing less of a
centrifugal force, additionally possibly increasing the pressure gradient. Then the
PGF becomes slightly more dominant. In the subsequent correction towards the low
pressure center the parcel moves back to the original radius (experiencing a centripetal acceleration) which results in a curved flow.
Thus, this curved flow, the gradient wind, is characterized by a modification of the
geostrophic balance due to centrifugal forces, centripetal accelerations, which point
in the same direction as the Coriolis force.
So, the gradient wind is a horizontal wind that parallels curved isobars at altitudes
above the friction layer. The centripetal force operating in the gradient wind and is the
result of an imbalance between the horizontal pressure gradient and the Coriolis force.
For lows the resulting curved flow around the pressure center is cyclonic - the same
direction as the rotation of the Earth (counterclockwise on the Northern and clockwise
on the Southern Hemisphere).
The flow around cyclones is often slower than the geostrophic wind (subgeostrophic),
except in the case of a steep pressure gradient, as the centrifugal force points in the
same direction as the Coriolis force balancing the pressure gradient force.
The reverse of the above true for anticyclones, high pressure systems.
Near the surface friction is much more important than the centrifugal force, except for
rapidly rotating storms.
Air Pressure, Forces and Winds
Surface wind
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Near the surface, within the first very few kilometers of the atmosphere, friction acts
to slow the air movement.
Friction reduces wind speed and thus the Coriolis force.
Leads to a change in wind direction towards low pressure, as the pressure gradient
force is not affected.
Flow is now across the isobars (in contrast to geostrophic and gradient winds!) at an
angle which depends on the friction.
This causes net inflow, convergence, in cyclones and net outflow, divergence, around
anticyclones.
Thus, in large-scale (synoptic and planetary-scale circulation systems, friction slows
the wind and interacts with the Coriolis effect to shift the wind direction across isobars
and toward low pressure so that within the friction layer, horizontal winds blow clockwise and outward in Northern Hemisphere anticyclones and counterclockwise and
inward in Northern Hemisphere cyclones.
How winds generate vertical air movement
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Surface lows result in horizontal convergence requiring uplifting and divergence aloft
in order to maintain surface low.
Divergence aloft can intensify low pressure systems, storm centers. Inadequate
divergence aloft leads to weakening of the cyclone.
Thus horizontal movement resulted into vertical movement. The net upward movement is generally slow, less than 1 km day-1.
Nevertheless important: cloud formation and precipitation.
Divergence aloft can even create surface lows.
Anticyclones have to be maintained from aloft as well.
Convergence aloft means subsidence of the air column.
The subsiding air masses are heated adiabatically and thus cloud formation is
reduced and a tendency to fair weather results.
Pressure trend gives indication of “good” or “bad” weather.
Factors that promote vertial air flow
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Change in surface roughness leads to change in wind speed.
Reduction in wind speed (e.g. wind moving onto land) leads to convergence
upstream and thus ascending air accompanies flow off the water surface.
Contributes to cloudy conditions over land during a sea breeze.
Conversely, a decrease in friction leads to an increase in wind speed and subsidence
Mountains lead to horizontal divergence aloft as the winds pass over the mountains.
On the lee side the air expands vertically resulting in horizontal convergence aloft.
Examples for dynamical creation of winds, i.e. winds create pressure changes which
in turn create winds.
Links
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http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter7/deluxe.html
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
Air Pressure, Forces and Winds
Introduction
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
Air pressure
The Ideal Gas Law
p⋅V = K⋅T
p - pressure
V - volume
T - temperature
K - a constant
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Air Pressure, Forces and Winds
Differential heating
Land warms more than
water
Air rises more over land
than over water
- in fact the air might be
sinking over the ocean
Clouds over land,
not over the ocean
Source: Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison
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Air Pressure, Forces and Winds
Heating
density drops
air rises
Vertical motion
surface pressure drops
Horizontal motion?
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Heating
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Air Pressure, Forces and Winds
Air pressure and winds
Example: Seabreeze
Isobars
980 mb
lines of equal pressure
1000 mb
980 mb
980 mb
p1
p2
l
GY1003 - Earth: A Dynamic Planet A, Autumn 2007, Lecture 15, Jörg Kaduk
Pressure gradient
force
p1 – p2
1
F = – --- ------------------l
ρ
Source: Lutgens, F.K. and E.J. Tarbuck, 1998.
The Atmosphere
Air Pressure, Forces and Winds
980 mb
Establishment of
Seabreeze
980 mb
980 mb
980 mb
1004 mb
1000 mb
Source: Lutgens, F.K. and E.J. Tarbuck, 1998.
The Atmosphere
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Air Pressure, Forces and Winds
Global circulation on a non-rotating Earth
Sea breeze analog
Not observed
- other forces acting
not only pressure
gradient force
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The
Atmosphere
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Air Pressure, Forces and Winds
Newton’s Laws of motion
First law
If
x=?, v=?, a=?
ΣF = 0 (no net force), then a=0, v=c
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Second law
a = ( ΣF ) ⁄ m , a is directly proportional to the
net force, a , is inversly proportional to the
mass m
Write: ΣF = ma
Third law
actio=reactio
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F2
F3
F force,
unit: Newton (N)
1N = 1 kg m s-2
Air Pressure, Forces and Winds
Geostrophic flow
The Coriolis effect
The Coriolis force
in North-South
direction
F = 2Ωv sin φ
v wind speed
φ latitude
Ω rotation speed
Non-rotating Earth
Slower
Faster
Rotating Earth
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Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Air Pressure, Forces and Winds
The Coriolis Effect in the West-East Direction
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
The Geostrophic Wind
Balance of:
pressure
gradient force
and
Coriolis force
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
Curved wind and the gradient wind
Cyclonic flow
(Northern hemisphere)
Anticyclonic flow
(Northern hemisphere)
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
Surface wind
At surface: Additional effect:
Friction
Friction slows down the wind
Coriolis force reduced
Upper level wind - no friction
Wind turns to direction of
Pressure gradient force
Wind crosses isobars
Surface wind - effect of friction
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Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The
Atmosphere
Air Pressure, Forces and Winds
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
Airflow associated with surface cyclones and anticyclones
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Air Pressure, Forces and Winds
Sumary
• Air pressure caused by weight of air column above and air movement
• Ideal gas law, relating pressure, temperature and density
• Pressure gradient force
• Sea breeze
• Earth rotation and the Coriolis force
• Geostropic wind
• Gradient wind
• Surface effects
• Surface convergence, divergence, Uplifting, subsidence
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