Temperature, pressure, and Temperature, pressure, and windswinds

Review of last lectureReview of last lecture

EarthEarth’’s energy balance at the top of the s energy balance at the top of the atmosphere and at the surface. What atmosphere and at the surface. What percentage of solar energy is absorbed by the percentage of solar energy is absorbed by the surface?surface?

Atmospheric influences on radiation (3 ways)Atmospheric influences on radiation (3 ways) What cause the greenhouse effect? What are the What cause the greenhouse effect? What are the

major greenhouse gases? Why is methane major greenhouse gases? Why is methane important? important?

The three types of atmospheric scattering. What The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-causes the blue sky? Why causes the reddish-orange sunsets? orange sunsets?

Atmospheric ThicknessAtmospheric Thickness

No defined top to the atmosphereNo defined top to the atmosphere The atmosphere is very shallow—and is The atmosphere is very shallow—and is

less than 2% of the Earth’s thicknessless than 2% of the Earth’s thickness

Over 90% ofatmosphere inthe lowest 16km& is where nearlyall weather occurs

Temperature BasicsTemperature Basics TemperatureTemperature – measure of average kinetic energy – measure of average kinetic energy

(motion) of individual molecules in matter(motion) of individual molecules in matter Three temperature scales (units): Kelvin (K), Celsius (C), Three temperature scales (units): Kelvin (K), Celsius (C),

Fahrenheit (F)Fahrenheit (F) All scales are relativeAll scales are relative degrees F = degrees F = 99⁄⁄55 degrees C + 32 degrees C + 32 degrees K = degrees C + 273.15degrees K = degrees C + 273.15

Temperature LayersTemperature Layers

Due to surface heating (Longwave, Latent heat, Sensible heat)

Due to ozone absorption of sunlight

Due to Solar winds, Cosmic rays

Decreasing rate w/ height (Lapse rate):6.5 oC/km

Space shuttle Endeavour straddles Space shuttle Endeavour straddles mesosphere and stratospheremesosphere and stratosphere

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

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

ΔΔp/p/ΔΔz = z = ρρgg

Pressure decreases non-linearly w/ height (Because air is compressible, so denser near the surface)

Vertical pressure distribution: Hydrostatic Vertical pressure distribution: Hydrostatic equilibriumequilibrium

ρρgg

ΔΔp/p/ΔΔzz

Movie timeMovie time 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

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:

Winds near the surfaceWinds near the surface

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

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

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 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. Four layers of the atmosphere, what separate them?Four layers of the atmosphere, what separate them? 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?