Vertical structure of the Vertical structure of the atmosphereatmosphere

Review of last lectureReview of last lecture EarthEarth’’s energy balance at the top of the atmosphere s energy balance at the top of the atmosphere

and at the surface. What percentage of solar energy is and at the surface. What percentage of solar energy is absorbed by the surface?absorbed by the surface?

Atmospheric influences on radiation (3 ways)Atmospheric influences on radiation (3 ways)

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

What cause the greenhouse effect? What are the What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? major greenhouse gases? Why is methane important?

Sensible heat flux (dry flux from warm to cold regions) Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)and latent heat flux (wet flux from wet to dry regions)

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 Different temperature scales: Kelvin, Celsius, Different temperature scales: Kelvin, Celsius,

Fahrenheit Fahrenheit 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

Longwave, Latent heat, Sensible heat

Ultra-violet sunlight

Solar winds, Cosmic rays

An artistAn artist’’s views view

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

Video: Video: Ionosphere Ionosphere Troposphere and Troposphere and

MesosphereMesosphere http://www.youtube.com/watch?v=E

ZuX1L1MD24

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.)

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

L: low pressure

• Simply determined by temperature: Simply determined by temperature: Warmer air Warmer air has higher pressure in the upper levelhas higher pressure in the upper level

• 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 for the same amount of air, its This means that for the same amount of air, its thickness is determined by its density.thickness 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 has a larger thickneswarmer air has a larger thickness (thermal s (thermal expansion)expansion)

Horizontal pressure distribution: Horizontal pressure distribution: Upper AirUpper Air

Horizontal Pressure distribution: Upper Air

Warm temperature leads to high pressure in the upper level

Cold temperature leads to low pressure in the upper level

Pressure difference drives wind to flow from high pressure to low pressure

Low pressure at the surface

High pressure at the surface

The most common The most common atmospheric circulation atmospheric circulation

structurestructure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

SummarySummary Thickness of the atmosphere: less than 2% of EarthThickness of the atmosphere: less than 2% of Earth’’s s

thicknessthickness Definition of temperature and its unit. Definition of temperature and its unit. 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. What is the hydrostatic Vertical Pressure Distribution. What is the hydrostatic

equilibrium? How does pressure change with height? equilibrium? How does pressure change with height? Horizontal Pressure Distribution. Most common Horizontal Pressure Distribution. Most common

atmospheric circulation structureatmospheric circulation structure