Air Pressure and Winds

Pressure FluctuationsPressure Fluctuations

Figure 9.3Figure 9.3

Solar heating of ozone gasses in the upper atmosphere, and of water Solar heating of ozone gasses in the upper atmosphere, and of water vapor in the lower atmosphere, can trigger oscillating thermal tides vapor in the lower atmosphere, can trigger oscillating thermal tides of sea-level pressure change.of sea-level pressure change.

Pressure Scale & UnitsPressure Scale & Units

Figure 9.4Figure 9.4

Many scales are used Many scales are used to record atmospheric to record atmospheric pressure, including pressure, including inches of mercury (Hg) inches of mercury (Hg) and millibars (mb).and millibars (mb).

The National Weather The National Weather Service uses mb, but Service uses mb, but will convert to metric will convert to metric units of hectopascals units of hectopascals (hPa).(hPa).

The conversion is The conversion is simply 1 hPa = 1 mb.simply 1 hPa = 1 mb.

Pressure MeasurementPressure Measurement

Changes in atmospheric pressure Changes in atmospheric pressure are detected by a change in are detected by a change in elevation of a barometric fluid or elevation of a barometric fluid or change in diameter of an aneroid change in diameter of an aneroid cell, which indicates changing cell, which indicates changing weather.weather.Average sea level pressure is 29.92 Average sea level pressure is 29.92 in Hg, or 1013.25 mb.in Hg, or 1013.25 mb.Figure 9.5Figure 9.5

Figure 9.6Figure 9.6

Pressure TrendsPressure Trends

Figure 9.7Figure 9.7

Barographs provide a plot of pressure with time, and are useful in Barographs provide a plot of pressure with time, and are useful in weather analysis and forecasting.weather analysis and forecasting.

Altimeters convert pressure into elevation, and are useful in steep Altimeters convert pressure into elevation, and are useful in steep terrain navigation or flying.terrain navigation or flying.

Both use aneroid cells.Both use aneroid cells.

Pressure Reading & ReportingPressure Reading & Reporting

Figure 9.8Figure 9.8

Increase terrain Increase terrain elevation and decrease elevation and decrease column of air above.column of air above.

To remove the effect To remove the effect of elevation, station of elevation, station pressure is readjusted pressure is readjusted to sea level pressure at to sea level pressure at ~10mb/100m.~10mb/100m.

Isobars show Isobars show geographic trends in geographic trends in pressure, and are pressure, and are spaced at 4 mb spaced at 4 mb intervals.intervals.

Single-Cell Circulation ModelSingle-Cell Circulation Model

Figure 11.1AFigure 11.1A

The basis for The basis for average air flow average air flow around the earth around the earth can be examined can be examined using a non-using a non-rotating, non-tilted, rotating, non-tilted, ocean covered ocean covered earth.earth.

Heating is more Heating is more intense at the intense at the equator, which equator, which triggers Hadley triggers Hadley cells to redistribute cells to redistribute rising heat from rising heat from the tropical low to the tropical low to the polar highs.the polar highs.

Ecclesiastes 16   The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.

ITCZ

ITCZ

ITCZ Limits

Three Cell Circulation ModelThree Cell Circulation ModelA rotating earth A rotating earth breaks the single cell breaks the single cell into three cells.into three cells.

The Hadley cell The Hadley cell extends to the extends to the subtropics, the reverse subtropics, the reverse flow Ferrel cell flow Ferrel cell extends over the mid extends over the mid latitudes, and the latitudes, and the Polar cell extends over Polar cell extends over the poles.the poles.

The Coriolis force The Coriolis force generates westerlies generates westerlies and NE trade winds, and NE trade winds, and the polar front and the polar front redistributes cold air.redistributes cold air.

Figure 11.2AFigure 11.2A

• HURRICANE IVAN DISCUSSION NUMBER 44 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT MON SEP 13 2004

• THE TRACK SCENARIO REMAINS THE SAME. THE HURRICANE IS FORECAST TO GRADUALLY TURN NORTHWARD OVER THE NEXT 72 HOURS INTO A WEAKNESS IN THE SUBTROPICAL RIDGE...AND THEN NORTHEASTWARD AS IT APPROACHES THE WESTERLIES.

2002

Observed Winds in JanuaryObserved Winds in JanuaryObserved average Observed average global pressure global pressure and winds have and winds have increased increased complexity due to complexity due to continents and the continents and the tilted earth.tilted earth.

Differential Differential ocean-land ocean-land heating creates heating creates areas of semi-areas of semi-permanent high permanent high and low pressure and low pressure that guide winds that guide winds and redistribute and redistribute heat.heat.

Figure 11.3AFigure 11.3A

Observed Winds in JuneObserved Winds in June

Global pressure Global pressure and wind and wind dynamics shift dynamics shift as the Northern as the Northern Hemisphere Hemisphere tilts toward the tilts toward the sun, bringing sun, bringing the inter-the inter-tropical tropical convergence convergence zone, the Pacific zone, the Pacific high, and high, and blocking highs blocking highs in the southern in the southern oceans oceans northward.northward.

Figure 11.3BFigure 11.3B

North American Winter WeatherNorth American Winter Weather

Figure 11.4Figure 11.4

Semi-Semi-permanent permanent highs redirect highs redirect North North American American winds, such as winds, such as cold interior cold interior southerly flow southerly flow from the from the Canadian high.Canadian high.

The Polar front The Polar front develops a develops a wave like wave like pattern as air pattern as air flows around flows around lows.lows.

Global Precipitation PatternsGlobal Precipitation Patterns

Global low pressure Global low pressure zones around the zones around the equator and 60° equator and 60° latitude generate latitude generate convergence at the convergence at the surface, rising air surface, rising air and cloud and cloud formation.formation.

Zones of high Zones of high pressure at 30° and pressure at 30° and the Poles experience the Poles experience convergence aloft convergence aloft with sinking, drying with sinking, drying air.air. Figure 11.5Figure 11.5

http://www.eoascientific.com/campus/earth/multimedia/coriolis/view_interactive

Coastal Summer WeatherCoastal Summer Weather

The semi-permanent Pacific high blocks moist maritime winds and The semi-permanent Pacific high blocks moist maritime winds and rain from the California coast, while the Bermuda high pushes rain from the California coast, while the Bermuda high pushes moist tropical air and humidity over the eastern states.moist tropical air and humidity over the eastern states.

Figure 11.6Figure 11.6

Coastal Winter WeatherCoastal Winter WeatherDuring winter During winter months, the months, the Pacific high Pacific high migrates migrates southward and southward and allows for allows for maritime winds maritime winds with moisture with moisture and rains to and rains to reach California.reach California.

On the east On the east coast, coast, precipitation is precipitation is rather even rather even throughout the throughout the year.year.

Figure 11.7Figure 11.7

January Winds AloftJanuary Winds Aloft

Land-sea Land-sea temperature temperature differences differences trigger ridges trigger ridges and troughs in and troughs in the isobaric the isobaric surface.surface.

Figure 11.8AFigure 11.8A

June Winds AloftJune Winds Aloft

Horizontal Horizontal temperature temperature gradients gradients establish establish pressure pressure gradients that gradients that cause westerly cause westerly winds in the winds in the mid latitudes.mid latitudes.

Figure 11.8BFigure 11.8B

Jet StreamJet Stream

Figure 11.9Figure 11.9

High velocity Polar and subtropical jet stream winds are located High velocity Polar and subtropical jet stream winds are located in the lower tropopause, and they oscillate along planetary ridges in the lower tropopause, and they oscillate along planetary ridges and troughs.and troughs.

Figure 11.10Figure 11.10

300 mb Winds & Jets300 mb Winds & Jets

Figure 11.11Figure 11.11

300 mb pressure 300 mb pressure surface maps surface maps illustrate lines of illustrate lines of equal wind speed equal wind speed (isotachs) as the (isotachs) as the jets meander.jets meander.

Jet streaks are Jet streaks are the maximum the maximum winds, exceeding winds, exceeding 100 knots.100 knots.

Polar Jet FormationPolar Jet Formation

Steep gradients of Steep gradients of temperature temperature change at the change at the Polar front Polar front trigger steep trigger steep pressure pressure gradients, which gradients, which then forces then forces higher velocity higher velocity geostrophic geostrophic winds.winds.

This is the trigger This is the trigger for jet stream for jet stream flow.flow.

Figure 11.13AFigure 11.13A

Winds & Angular MomentumWinds & Angular Momentum

Angular momentum Angular momentum is the product of is the product of mass, velocity, and mass, velocity, and the radius of the radius of curvature and it must curvature and it must be conserved.be conserved.

As northward-As northward-flowing air flowing air experiences a smaller experiences a smaller radius, it increases in radius, it increases in velocity and velocity and augments the jet augments the jet stream flow.stream flow.

Figure 11.14Figure 11.14

                                              

14 September 04 http://www.wunderground.com/US/Region/US/JetStream.html

http://weather.unisys.com/upper_air/ua_500.html

Smoothed Isobar MapsSmoothed Isobar Maps

Figure 9.9AFigure 9.9A

Continental maps of Continental maps of station recorded station recorded sea-level pressure sea-level pressure are often smoothed are often smoothed and simplified to and simplified to ease interpretation.ease interpretation.

Smoothing adds Smoothing adds error to those error to those already introduced already introduced by error in by error in instrument instrument accuracy.accuracy.

Constant Height ChartConstant Height Chart

Maps of atmospheric Maps of atmospheric pressure, whether at pressure, whether at sea level or 3000 m sea level or 3000 m above sea level, show above sea level, show variations in pressure variations in pressure at that elevation.at that elevation.

Figure 9.10Figure 9.10

Constant Pressure ChartConstant Pressure Chart

Figure 9.11Figure 9.11

Maps of Maps of constant constant pressure pressure provide another provide another means for means for viewing viewing atmospheric atmospheric dynamics.dynamics.

In this example, In this example, there is no there is no variation in variation in elevation for a elevation for a pressure of 500 pressure of 500 mb.mb.

Variation in HeightVariation in Height

Isobaric (constant pressure) surfaces rise and fall in elevation Isobaric (constant pressure) surfaces rise and fall in elevation with changes in air temperature and density.with changes in air temperature and density.A low 500 mb height indicates denser air below, and less A low 500 mb height indicates denser air below, and less atmosphere and lower pressure above.atmosphere and lower pressure above.Contour lines indicate rates of pressure change.Contour lines indicate rates of pressure change.

Figure 9.12Figure 9.12Figure 9.13Figure 9.13

Ridges & TroughsRidges & Troughs

Upper level areas with high pressure are named ridges, and areas Upper level areas with high pressure are named ridges, and areas with low pressure are named troughs.with low pressure are named troughs.

These elongated changes in the pressure map appear as undulating These elongated changes in the pressure map appear as undulating waves.waves.

Figure 9.14Figure 9.14

Surface & 500 mb MapsSurface & 500 mb MapsSurface maps chart Surface maps chart pressure contours, pressure contours, highs and lows, and highs and lows, and wind direction.wind direction.

Winds blow clockwise Winds blow clockwise around highs, called around highs, called anticyclones.anticyclones.

500 mb maps reveal 500 mb maps reveal patterns that on patterns that on average are 5600 m average are 5600 m above the surface, above the surface, where westerly winds where westerly winds rise and fall across rise and fall across ridges and troughs.ridges and troughs.

Figure 9.15AFigure 9.15A

Forces & MotionForces & Motion

Pressure forces are Pressure forces are only one influence on only one influence on the movement of the movement of atmospheric air.atmospheric air.

Air responds Air responds similarly as water to similarly as water to this force, moving this force, moving from higher pressure from higher pressure to lower pressure.to lower pressure.

Centripetal, friction, Centripetal, friction, and apparent and apparent Coriolis are other Coriolis are other forces, however, forces, however, determining winds.determining winds.

Figure 9.16Figure 9.16

Pressure Gradient ForcePressure Gradient Force

Figure 9.17Figure 9.17

Change in pressure per change in distance determines the Change in pressure per change in distance determines the magnitude of the pressure gradient force (PGF).magnitude of the pressure gradient force (PGF).

Greater pressure changes across shorter distances creates a Greater pressure changes across shorter distances creates a larger PGF to initiate movement of winds.larger PGF to initiate movement of winds.

Figure 9.18Figure 9.18

PGF vs. Cyclonic WindsPGF vs. Cyclonic WindsPressure Pressure gradient force gradient force (PGF) winds (PGF) winds acting alone acting alone would head would head directly into directly into low pressure.low pressure.

Surface Surface observations of observations of winds, such as winds, such as the cyclonic the cyclonic flow around flow around this low, reveal this low, reveal that PGF winds that PGF winds are deflected by are deflected by other forces.other forces.

Figure 9.19Figure 9.19

Apparent Coriolis ForceApparent Coriolis Force

Earth's rotation transforms Earth's rotation transforms straight line motion into curved straight line motion into curved motion for an outside viewer.motion for an outside viewer.

The Coriolis force explains this The Coriolis force explains this apparent curvature of winds to apparent curvature of winds to the right due to rotation.the right due to rotation.

Its magnitude increases with wind Its magnitude increases with wind velocity and earth's latitude.velocity and earth's latitude.

Figure 9.20Figure 9.20

Figure 9.21Figure 9.21

Geostrophic WindGeostrophic Wind

Figure 9.23Figure 9.23

Winds have direction and magnitude, and can be depicted by Winds have direction and magnitude, and can be depicted by vectors.vectors.

Observed wind vectors are explained by balancing the pressure Observed wind vectors are explained by balancing the pressure gradient force and apparent Coriolis force.gradient force and apparent Coriolis force.

These upper level geostrophic winds are parallel to pressure These upper level geostrophic winds are parallel to pressure contours.contours.

Wind Speed & Pressure ContoursWind Speed & Pressure Contours

Just as a river speeds and slows when its banks narrow and expand, Just as a river speeds and slows when its banks narrow and expand, geostrophic winds blowing within pressure contours speed as geostrophic winds blowing within pressure contours speed as contour intervals narrow, and slow as contour intervals widen.contour intervals narrow, and slow as contour intervals widen.

Figure 9.24Figure 9.24

Isobars & Wind PredictionIsobars & Wind Prediction

Figure 9.25AFigure 9.25A

Upper level pressure Upper level pressure maps, or isobars, maps, or isobars, enable prediction of enable prediction of upper level wind upper level wind direction and speed.direction and speed.

Centripetal Acceleration & CyclonesCentripetal Acceleration & Cyclones

Acceleration is defined Acceleration is defined by a change in wind by a change in wind direction or speed, and direction or speed, and this occurs as winds this occurs as winds circle around lows circle around lows (cyclones) and highs (cyclones) and highs (anticyclones).(anticyclones).

Centripetal force is the Centripetal force is the term for the net force term for the net force directing wind toward directing wind toward the center of a low, and the center of a low, and results from an results from an imbalance between the imbalance between the pressure gradient and pressure gradient and Coriolis forces.Coriolis forces.

Figure 9.26AFigure 9.26A

Northern & Southern Hemisphere FlowNorthern & Southern Hemisphere Flow

Figure 9.27AFigure 9.27A

Winds blow Winds blow counterclockwise counterclockwise around low pressure around low pressure systems in the systems in the Northern Northern Hemisphere, but Hemisphere, but clockwise around clockwise around lows in the Southern lows in the Southern Hemisphere.Hemisphere.

Meridional & Zonal FlowMeridional & Zonal Flow

Wind direction and speed are indicated by lines, barbs, and flags, Wind direction and speed are indicated by lines, barbs, and flags, and appear as an archer's arrow.and appear as an archer's arrow.

Upper level winds that travel a north-south path are meridional, Upper level winds that travel a north-south path are meridional, and those traveling a west-east path are zonal.and those traveling a west-east path are zonal.

Figure 9.28Figure 9.28

Friction & Surface WindsFriction & Surface Winds

Surface objects Surface objects create frictional create frictional resistance to wind resistance to wind flow and slows the flow and slows the wind, diminishing wind, diminishing the Coriolis force the Coriolis force and enhancing the and enhancing the effect of pressure effect of pressure gradient forces.gradient forces.

The result is The result is surface winds that surface winds that cross isobars, cross isobars, blowing out from blowing out from highs, and in highs, and in toward lows.toward lows. Figure 9.29AFigure 9.29A

Surface Flow at Lows & HighsSurface Flow at Lows & Highs

Southern Hemisphere flow paths are opposite in direction to Southern Hemisphere flow paths are opposite in direction to Northern Hemisphere paths, but the same principles and Northern Hemisphere paths, but the same principles and forces apply.forces apply.

Figure 9.30Figure 9.30

Figure 9.31Figure 9.31

Sensing Highs & LowsSensing Highs & Lows

The location of The location of high and low high and low pressure centers pressure centers are estimated by are estimated by detecting surface detecting surface wind direction and wind direction and noting pressure, noting pressure, Coriolis, and Coriolis, and friction forces.friction forces.

This figure This figure illustrates the illustrates the procedure when procedure when standing aloft and standing aloft and at the surface.at the surface.

Figure 9.32AFigure 9.32A

Vertical Air MotionVertical Air Motion

Winds converging into a low pressure center generate upward Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules.winds that remove the accumulating air molecules.

These updrafts may cause cloud formation.These updrafts may cause cloud formation.

Likewise, diverging air molecules from a high pressure area are Likewise, diverging air molecules from a high pressure area are replenished by downward winds.replenished by downward winds.

Figure 9.33Figure 9.33