Atmospheric pressure and windsAtmospheric pressure and winds

Review of last lectureReview of last lecture Thickness of the atmosphere: less than 2% of EarthThickness of the atmosphere: less than 2% of Earth’’s s

thicknessthickness Definition of temperature. 3 units. Definition of temperature. 3 units. Vertical distribution of temperature: 4 layers, what Vertical distribution of temperature: 4 layers, what

separate them?separate them? Horizontal distribution of temperature. Horizontal distribution of temperature. 6 factors.6 factors.

1.1. Latitudinal variations in net radiationLatitudinal variations in net radiation

2.2. Land-Water ContrastsLand-Water Contrasts

3.3. Atmospheric CirculationAtmospheric Circulation

4.4. Ocean CurrentsOcean Currents

5.5. AltitudeAltitude

6.6. Local EffectsLocal Effects

Pressure EssentialsPressure Essentials Pressure – force exerted/unit area (weight above you)Pressure – force exerted/unit area (weight above you)

units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa)units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa) Average surface pressure over globe: 1013.2 mb.Average surface pressure over globe: 1013.2 mb.

Atmosphere is mixture of gases -> partial pressure.Atmosphere is mixture of gases -> partial pressure. DaltonDalton’’s Law: sum of partial pressures equals total s Law: sum of partial pressures equals total

pressure pressure

Pressure gradient (pressure difference between two Pressure gradient (pressure difference between two locations/distance) gives rise to a force (pressure locations/distance) gives rise to a force (pressure gradient force), which sets the air in motion. gradient force), which sets the air in motion.

The Equation of State (Ideal Gas Law)The Equation of State (Ideal Gas Law)

Describes relationships between pressure, temperature, and density (Start w/ molecular movement in sealed container Pressure proportional to rate of collisions between molecules and walls).

At constant temperatures, an increase in air density will cause a pressure increase (Add more molecules increase density increase rate of collisions raise pressure)

Under constant density, an increase in temperature will lead to an increase in pressure (Raise temperature increase speed of molecules increase rate of collisions raise pressure)

Pressure = density x temperature x 287 J kg-

1 K-1

[ p = ρTR]

Pressure decreases with height

Why? Because downward gravity force is balanced by vertical pressure gradient (called hydrostatic equilibrium)

ΔΔp/p/ΔΔz = z = ρρgg

Vertical pressure distribution: Hydrostatic Vertical pressure distribution: Hydrostatic equilibriumequilibrium

ρρgg

ΔΔp/p/ΔΔzz

Pressure decreases non-linearly w/ height

(Why? Because air is compressible, so denser near the surface)ΔΔp/p/ΔΔz = z = ρρgg

Vertical pressure distribution (cont.)Vertical pressure distribution (cont.)

VideoVideo Weather: Weather: WindWind

Horizontal pressure distribution and horizontal pressure gradient Pressure maps depict isobars, lines of equal pressure Through analysis of isobaric charts, pressure gradients are apparent

Steep pressure gradients are indicated by closely spaced isobars Typically only small gradients exist across large spatial scales (4%

variation at continental scale), smaller than vertical gradients

Surface pressure chart

H – high pressure areaL – low pressure area

• Upper air pressure distributions are best determined Upper air pressure distributions are best determined through the heights of constant pressure due to density through the heights of constant pressure due to density considerations (constant pressure surfaces of warmer considerations (constant pressure surfaces of warmer air will be higher in altitude than those of cooler air)air will be higher in altitude than those of cooler air)

• From the hydrostatic equilibrium,From the hydrostatic equilibrium, ΔΔp/p/ΔΔz=z=ρρgg SoSo ΔΔz=z=ΔΔp/(p/(ρρg) (1)g) (1)This means that This means that for the same amount of air, its for the same amount of air, its thickness is determined by its densitythickness is determined by its density..

• From the equation of stateFrom the equation of state P=ρTRρTR SoSo ρ = P/(TR) (2)ρ = P/(TR) (2) This means that This means that warmer air has a lower densitywarmer air has a lower density

• Combine (1) and (2), we get: ΔΔz=(z=(ΔΔp)TR/P p)TR/P This means that This means that for the same amount of air, for the same amount of air, warmer air warmer air has a larger thicknesshas a larger thickness

Horizontal pressure distribution: Horizontal pressure distribution: Upper AirUpper Air

Horizontal Pressure Gradients: Upper Air cont’d

Figure from our book

Figure from different book showing same concept

500 mb height contours for May 3, 1995

• Height contours analogous to the pressure gradient

• So this chart is kind of like the surface pressure chart we looked at earlier, but instead of showing pressure variations on a constant height surface, it shows height variations on a constant pressure surface.

• Small changes over large regions: approximate 10% difference across North America

Example of typical 500 mb height chart

Forces Affecting the Speed and Direction of the Wind

Horizontal pressure gradients responsible for wind Horizontal pressure gradients responsible for wind generationgeneration

Three factors affect wind speed and/or direction Three factors affect wind speed and/or direction ((velocityvelocity):):

1.1. Pressure Gradient Force  (PGF)Pressure Gradient Force  (PGF)

2.2. Coriolis Effect  (CE) Coriolis Effect  (CE)

3.3. Friction Force (FF)Friction Force (FF)

1. Pressure 1. Pressure GradientGradient

ForceForce(PGF)(PGF)

• pressure gradient: high pressure low pressure

• pressure differences exits due to unequal heating of Earth’s surface

• spacing between isobars indicates intensity of gradient

• flow is perpendicular to isobars

2. The 2. The CoriolisCoriolis Effect Effect

objects in the atmosphere are influenced by the Earthobjects in the atmosphere are influenced by the Earth’’s rotations rotation Rotation of Earth is counter-clockwise looking down from N. Pole.Rotation of Earth is counter-clockwise looking down from N. Pole.

results in an results in an ‘‘apparentapparent’’ deflection (relative to surface) deflection (relative to surface) deflection to the right in Northern Hemisphere (left in S. Hemisphere)deflection to the right in Northern Hemisphere (left in S. Hemisphere) Greatest at the poles, 0 at the equatorGreatest at the poles, 0 at the equator Increases with speed of moving objectIncreases with speed of moving object and distance and distance CE changes direction not speedCE changes direction not speed

Winds in the upper air:Geostrophic Balance

• Now the wind speed/direction is simply a balance between the PGF and CE. This is called GEOSTROPHIC BALANCE. • Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection• Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force

Friction is very small in the upper air:

Geostrophic Versus Gradient Winds

In reality, PGF is rarely uniform since height contours curve and vary In reality, PGF is rarely uniform since height contours curve and vary in distance in distance Geostrophic flow assumption is too simplistic Geostrophic flow assumption is too simplistic

wind still flows parallel to contours wind still flows parallel to contours HOWEVER it is continuously HOWEVER it is continuously changing direction (and thus experiencing acceleration)changing direction (and thus experiencing acceleration)

for isobar-parallel flow to occur an imbalance must exist between PGF for isobar-parallel flow to occur an imbalance must exist between PGF and CE and CE Gradient FlowGradient Flow

Ideal Reality

Winds near the surfaceWinds near the surface

Friction is important for air within ~1.5 km of the surface (the Friction is important for air within ~1.5 km of the surface (the so-called so-called planetary boundary layer).planetary boundary layer). It varies with surface It varies with surface texture, wind speed, time of day/year and atmospheric texture, wind speed, time of day/year and atmospheric conditions. Friction above 1.5 km is often small (often called conditions. Friction above 1.5 km is often small (often called the the free atmospherefree atmosphere), except over regions with storms and ), except over regions with storms and gravity waves.gravity waves.

Friction slows down wind speed and reduces Coriolis deflectionFriction slows down wind speed and reduces Coriolis deflection

Friction causes air converging into low pressure areas, but Friction causes air converging into low pressure areas, but diverging away from high pressure areasdiverging away from high pressure areas

The third term (friction) must be considered:The third term (friction) must be considered:

Effect of frictional force

Upper airw/out Friction(geostrophic balance)

Near surfacew/ Friction

Counterclockwise in NH (opposite in SH)characterized by ascending/diverging air which cools to form clouds/precipitation

Clockwise airflow in NH (opposite in SH) Characterized by descending/converging air which warms creating clear skies

Upper airUpper air: isobars usually not : isobars usually not closed offclosed offTroughsTroughs (low pressure areas) (low pressure areas)RidgesRidges (high pressure areas) (high pressure areas)

Near surfaceNear surface: isobars usually : isobars usually closed off due to surface frictionclosed off due to surface frictionCyclonesCyclones (Low pressure areas)(Low pressure areas)AnticyclonesAnticyclones (High pressure (High pressure areas)areas)

Cyclones, Anticyclones, Troughs and Ridges

SummarySummary Definition of pressure and its unit. Definition of pressure and its unit. Definition of pressure gradient. Pressure gradient sets Definition of pressure gradient. Pressure gradient sets

the air in motion.the air in motion. Equation of state (Relationship between P, Equation of state (Relationship between P, ρρ, and T), and T) Vertical Pressure Distribution. How does pressure Vertical Pressure Distribution. How does pressure

change with height? What is the hydrostatic change with height? What is the hydrostatic equilibrium?equilibrium?

Summary (cont.)

Know 3 Forces that affect wind speed /directionKnow 3 Forces that affect wind speed /direction Especially work on Coriolis force, as this is the hardest Especially work on Coriolis force, as this is the hardest

to understand. Which direction is air deflected to by to understand. Which direction is air deflected to by Coriolis force?Coriolis force?

What is the geostrophic balance? At which level is it What is the geostrophic balance? At which level is it valid? Difference between upper level and surface valid? Difference between upper level and surface windswinds

Does cyclones correspond to high or low surface Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter-pressure? Is the air moving clockwise or counter-clockwise around them? How about anticyclones?clockwise around them? How about anticyclones?

What are troughs and ridges?What are troughs and ridges?