Lecture2 Fall07

Energy in the Atmosphere: 

Energy in the Atmosphere Fall Semester, 2007 Lecture 2 September 4

Forces, Work, and Heat: 

Forces, Work, and Heat Forces cause a body to accelerate Acceleration is a change in speed or direction Name the three accelerators in a car Gas Pedal, Brake, Steering Wheel!

Forces, Work, and Heat: 

Forces, Work, and Heat Energy is needed to do work Wind is air in motion Getting air to move requires work Two principle energy types: kinetic and potential

Energy: 

Energy Kinetic Energy A body can do work by virtue of its motion because motion implies kinetic energy If two different objects are moving at the same speed, the heavier one has more kinetic energy KE = 1/2 m v*v Potential Energy A body can do work by virtue of its position Lift an object to great heights and it has a lot of potential energy -- that can be converted to kinetic energy by dropping it. PE = m*g*h

Heat: 

Heat Heat is energy produced by molecular motions Temperature measures the average kinetic energy of a substance’s molecules A cup of coffee may be warmer than Lake Mendota, but Lake Mendota has more heat because its mass is so much greater Heat is the energy transferred between objects as a result of the temperature difference between them.

Specific Heat: 

Specific Heat Amount of heat needed to raise the temperature Some substances can absorb a lot of energy before the temperature budges (they have high specific heat) -- other substances warm very quickly once they start absorbing energy (they have low specific heat) What common substance has a very high specific heat? WATER

More On:: 

More On: HEAT

Heat can be transferred 3 ways:: 

Heat can be transferred 3 ways: Radiation Convection Conduction Heat transfer is important: the atmosphere is constantly trying to come to an equilibrated temperature…heat is transferred in 3 ways

Radiation: 

Radiation This is how solar energy reaches the Earth All things with a finite temperature (on the Kelvin scale) radiate Sun feels warm on skin because of radiation Ground warms up under the sun because of radiation

Radiation: 

Radiation Sun emits radiation at many different wavelengths Peak energy in visible part of electromagnetic spectrum Amount of radiation emitted is a function of temperature

Radiation: 

Radiation This equation governs how radiation relates to Temperature: R = s T4 [ s = 5.67x10-8 W m-2 K-4 ] If the temperature doubles, the amount of radiation emitted increases by 24, or 16. If the temperature triples, the amount of radiation emitted increases by 34, or 81 If the temperature quadruples, the amount of radiation emitted increases by 44, or 256

Radiation: 

Radiation So, given the Temperature of the Sun, we could compute the radiation being emitted: R = s * 6000K * 6000K * 6000K * 6000K R = 5.67 x 10-8 W m-2 K-4 (1.296x1015 K4 ) R = 73483200 W m-2 Given the temperature of the Earth, we could compute the radiation being emitted R = s * 288K * 288K * 288K * 288K R = 5.67 x 10-8 W m-2 K-4 (6.88x109 K4 ) R = 390 W m-2

Radiation: 

Radiation The amount of radiation emitted changes with temperature The character of the radiation changes as well as the temperature changes. Solar radiation is short-wave (peak around .5 m) Terrestrial radiation is long-wave (peak around 10 m) Body radiation is long-wave (9.3 m) The peak wavelength is determined from Wien’s Law: l = 2897/T

Slide14: 

Microwave Infrared Visible

Conduction: 

Conduction Heat transferred at the molecular level Heat measures the kinetic energy of molecules -- as something warms up, the molecules move faster and faster A cold substance touching a warm substance will absorb momentum from the faster molecules in the warmer substance -- its molecules will then speed up, and its temperature will increase

Conduction: 

Conduction Conduction very inefficient in atmosphere gases aren’t very dense Molecules don’t stay in one place long enough for energy transfer to occur Conduction is most important at the land-air interface. There, air molecules aren’t moving very fast and they’re in contact with the ground long enough to acquire heat from the ground (which has been heated by radiation)

Convection: 

Convection A mass of warm air moves -- it carries its heat with it….Hot air rises Warm air is less dense than cold air, so it will rise in the atmosphere. Radiation warms surface, air next to surface warms by conduction, and that warm blob of air that is then warmer than its surroundings rises by convection. In this way, heat at the surface can move into the atmosphere. Very little direct heating of the atmosphere by the Sun

Convection: 

Convection Air can also move horizontally advection South winds bring warm air (‘warm advection’) Ahead of Low Pressure Behind High Pressure North winds bring cold air (‘cold advection’) Ahead of High Pressure Behind Low Pressure

Convection: 

Convection Convection has another meaning in weather: upward motion that is unstable. Thunderstorms are convective features in the atmosphere. Associated with very strong movement of warm air up into the atmosphere. Upward currents can reach the tropopause.

Convection: 

Convection

How to cool the atmosphere?: 

How to cool the atmosphere? Radiation : emit energy, cool down [as at night] Convection : [move cooler air in by winds] Conduction : [Touch colder surface] You can also cool by LIFTING

Slide22: 

As a parcel rises, the pressure pushing in on a parcel drops and the parcel will expand. That expansion requires work, and energy, and the energy comes from the parcel’s internal energy (heat).

Parcels in the atmosphere: 

Parcels in the atmosphere Theoretical, isolated blob of air that moves without mixing with its environment As parcels move in the vertical, they warm or cool at specific rates: Rising, unsaturated parcels cool at 9.8 oC/km Sinking parcels warm at 9.8 oC/km Rising, saturated parcels cool at about 6 oC/km Cooling is slower than for unsaturated parcels Cooling is slower for parcels that have more moisture Cooling is faster (but still slower than dry parcels) for parcels with less moisture

Parcels in the atmosphere: 

Parcels in the atmosphere Dry Adiabatic Lapse Rate: Rising, unsaturated parcels cool at 9.8 oC/km Moist Adiabatic Lapse Rate: Rising, saturated parcels cool at about 6 oC/km What causes the difference? LATENT HEAT IN PARCEL -- this is the heat associated with a phase change.

Latent Heat: 

Latent Heat Energy associated with phase changes Energy needed to evaporate water stays with the water vapor in the atmosphere -- it is liberated when the vapor subsequently condenses Wind moving humid air moves much latent heat

Phase Changes: 

Phase Changes Leave a glass of water sitting out for many days. What happens to the water level in the glass? ….does it go up? …..does it go down? …..does it remain constant?

Phase Changes: 

Phase Changes Cover a glass of water with plastic wrap and let it sit out for many days. What happens? …….does the water level go up? …….does it go down? …….does it stay the same? When evaporation balances condensation…….. the air is said to be saturated with water vapor. Each molecule that becomes vapor replaces one that becomes liquid

What determines the evaporation rate?: 

What determines the evaporation rate? Wind Water temperature Air temperature What determines the condensation rate? Temperature measures speed of molecules : Higher temperatures means Faster molecules

Slide29: 

Cool air: Vapor can more easily “stick” together because it’s not moving as fast -- Condensation will increase Remember: Temperature measures the kinetic energy (speed) of the molecules!

Phase Changes that Release Latent Heat [warming]: 

Phase Changes that Release Latent Heat [warming] Condensation Vapor => Liquid [about 600 calories/g] Freezing Liquid => Solid (Ice) [about 80 calories/g] Deposition Vapor => Solid (Ice) [about 680 calories/g]

Phase Changes that Absorb Latent Heat [cooling]: 

Phase Changes that Absorb Latent Heat [cooling] Evaporation Liquid => Vapor Melting Solid (Ice) => Liquid Sublimation Solid (Ice) => Vapor Snow on ground: Much harder to get warmer temperatures

Slide32: 

Incoming vs. Outgoing Radiation Poles are cooling, equator is warming Weather moves surplus energy to Poles

Where does all energy come from?: 

Where does all energy come from? The Sun! This is a winter shot -- why does the Sun not warm the Earth so much in the winter?

The Sun’s Path is a function of latitude: 

The Sun’s Path is a function of latitude Radiation hitting the ground is stronger when the sun is higher in the sky. Why? Radiation at a slanted angle must traverse more atmosphere than radiation from straight above Slanted angle means more chances of absorption, scattering, reflection Slanted beam irradiates a larger surface of the Earth, too

Effect of a slanted sunbeam: 

Effect of a slanted sunbeam Energy per unit area is smaller as you move poleward, so it takes longer to heat up the ground Yet another reason the tropics are warmer than the Poles! Again, light beam near Pole goes through more atmosphere

Reason for seasons: 

Note how the same size beam spreads over different areas on the Earth Reason for seasons

Slide37: 

Intensity of solar radiation decreases as you move poleward [Where can you tan more easily? Caribbean or Alaska?]

Slide38: 

Solar intensity also changes with season because the angle of the sun changes. Sun is highest in sky on Summer Solstice [June 21] Sun is lowest in sky on Winter Solstice [December 21]

Slide39: 

This is important for Architecture!

Along the same lines . . . : 

Along the same lines . . . Why does the Earth have seasons? Earth-Sun distance varies during the year Solar output varies during the year The Earth axis is tilted

Does the Earth-Sun distance vary?: 

Does the Earth-Sun distance vary? YES -- orbit around sun is not circular, but elliptic MEAN distance: 150,000,000 km Perihelion: 147,500,000 km [January 5] Aphelion: 152,600,000 km [July 5] Think: Aphelion and afar or Aphelion and Away to keep the meanings of perihelion and aphelion straight Radiation difference between perihelion and aphelion because of the change in distance

Does the Earth-Sun distance vary?: 

Does the Earth-Sun distance vary? Earth orbit is more circular than Mercury’s or Mars’ -- but more eccentric than Neptune’s or Venus’ SUN

Earth revolves around Sun on an elliptical path: 

Earth revolves around Sun on an elliptical path Perihelion Aphelion The warm months last longer in WI because Earth is closer during Winter and therefore moving more quickly around the Sun

Does the Earth-Sun distance vary?: 

Does the Earth-Sun distance vary? The catch: Perihelion is January 5th Winter is warmer, and the globe is warmer, than it would be if Aphelion were in January -- closer to the Sun in winter, getting more energy, staying warmer. But Why is the GLOBE warmer?

Hemispheric Differences: 

Hemispheric Differences Northern Hemisphere: 61% water Southern Hemisphere: 81% water

Water has high heat capacity: 

Water has high heat capacity Absorbs a lot of energy before it warms up, especially compared to land Stores energy in summer and releases it in Winter: warmer near lakes and oceans Northern Hemisphere has more land: it can cool off more rapidly than the southern hemisphere in winter Difference in temperatures in NH winter with winter at aphelion and winter at perihelion is much bigger than difference in SH winter -- so the Earth on a whole is colder if NH winter occurs at aphelion

Does the solar output vary?: 

Does the solar output vary? NO!! It’s called a solar constant for a reason!!

The Earth has seasons because its axis is tilted 23.5o: 

The Earth has seasons because its axis is tilted 23.5o Sun is higher in the sky in the Summer Sun is lower in the sky in winter Days longer in summer, shorter in winter

Slide50: 

What is the global annual mean day length? 12 hours!

Two things to remind you about: 

Two things to remind you about Kinetic Energy: Energy of motion Temperature is a kind of KE: As an object warms, the molecules within it speed up Atmospheric Window: That part in the electromagnetic spectrum where radiation can escape from the Earth without being absorbed (around 11 m)

What happens to incoming solar energy?: 

What happens to incoming solar energy? “Solar Constant” at top of atmosphere: 1367 W m-2 30% reflected immediately to space The Earth has an ALBEDO of 30% 4% from ground 20% from clouds 6% from atmosphere Venus has an albedo of 76% [Very Bright!] Moon has an albedo of about 7% Reflected energy has the same radiative characteristics as sunlight. Don’t confuse it with emitted radiation that has different characteristics A cloudier Earth has a higher albedo and will reflect more energy (and be cooler)

What happens to incoming solar energy?: 

What happens to incoming solar energy? “Solar Constant” at top of atmosphere: 1367 W m-2 19% absorbed by clouds/atmosphere 51% absorbed by the ground About half of that is used to evaporate water! Absorbed at ground: Warms ground through radiation Conduction/convection then redistribute that heat Once it’s absorbed, it can be re-emitted Re-emitted energy has different radiative characteristics from sunlight. Don’t confuse it with reflected radiation that has the same characteristics

What happens to incoming solar energy?: 

What happens to incoming solar energy? Things to ponder: The solar temperature doubles: how does the solar constant change? The solar temperature is cut by 10%: how does the solar constant change? If the Earth’s oceans warm, evaporation increases, which means there will be more vapor in the air, which means cloudiness might increase. What will the clouds do to the global temperature?

Slide55: 

This is the spectrum on incoming and outgoing radiation Blue means radiation is absorbed …. Note how all radiation longer than 20m and shorter than .3m is absorbed Intensity here is not to scale…. Incoming intensity much larger than outgoing! Visible light not really absorbed by atmosphere (What if it were?)

Selective Absorbers: 

Selective Absorbers The atmosphere absorbs some radiation wavelengths, but is transparent to others.

Can you get sunburned through a window?: 

Can you get sunburned through a window? Glass is a selective absorber. Glass absorbs some infrared and ultraviolet radiation. Glass does not absorb visible light wavelengths

Why can you get sunburned on cloudy days?: 

Why can you get sunburned on cloudy days? Clouds are also selective absorbers!

Slide60: 

Infrared radiation is absorbed by greenhouse gases. Clouds can absorb in the “atmospheric window”, where CO2 and H2O don’t.

Kirchhoff’s Law: 

Kirchhoff’s Law Objects that selectively absorb radiation also selectively emit radiation at the same wavelength. Definition: Good absorbers are good emitters at a particular wavelength, and poor absorbers are poor emitters at the same wavelength [Like snow]

How do absorption and emittingrelate to color?: 

How do absorption and emitting relate to color? Can you tell in the dark what color your shirt is? NO Color is NOT the wavelength at which a cold object is emitting. You do not want to wear a shirt that is red because that’s the wavelength the shirt emits at! Burn City! Color represents the wavelengths that an object is reflecting or scattering.