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The aurora borealis, which forms as energetic particles from the sun interact with the earth’s atmosphere, is seen here over Edmonton, Alberta, Canada.

Energy: Warming the Earth and the Atmosphere

Energy• By Definition, Energy is the ability or capacity to

do work on some form of matter. • Matter is anything that has mass and occupies

space.• Work is done on matter when matter is either

pushed, pulled, or lifted over some distance.• The main intrinsic forms of energy in the

atmosphere are – internal energy – Gravitational-potential energy– Kinetic energy – Latent energy associate with phase transition of water

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Gravitational Potential Energy• Potential to do work

– PE = mghwherem = mass of objectg = acceleration due to gravity = 10 m s-2

h = height of the object above ground

• For a volume of air, the higher it gets, more PE energy it owns.

Energy of motionKinetic Energy

• KE = 1/2 mv2

where m = mass of the object v = the velocity

• Kinetic Energy of air molecules– for molecules of air, the temperature of the air is directly

proportional to the mean kinetic energy of the air molecules.– air temperature is high if mean KE of molecules is large– air temperature is low if mean KE of molecules is small

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Internal Energy• The sum of kinetic and potential energy

stored in molecules• For air parcel the total internal energy is

determined by number of molecules and air temperature.

Examples: which has larger internal energy:1. Parcel A: mass = 5 grams, T = 30°C

Parcel B: mass = 5 grams, T = 10°C

2. Parcel A: mass = 100 grams, T = 0°CParcel B: mass = 1000 grams, T = 0°C

Air temperature is a measure of the average speed of the molecules. In the cold volume of air the molecules move more slowly and crowd closer together. In the warm volume, they move faster and farther apart.

Warm, less-dense air or as cold, more dense air

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Temperature

Temperature ScalesKelvin scale (K): absolute zeroFahrenheit scale (°F):Celsius scale (°C):

°C= 5/9 (°F -32)

K= °C + 273.15

Heat Capacity and Specific Heat• Very important concepts in meteorology....

• Heat Capacity: ratio of the amount of energy absorbed to the associated temperature rise.

• Example: if it takes 10 calories to raise the temperature of a glass of water by 2 °C, then the heat capacity of the glass of water is 10 calories/2°C = 5 calories per °C.

• Specific Heat: the heat capacity of a substance per unit mass

• Example: for water, it takes 1 calorie to raise the temperature of 1 gram of water by 1°C. So the specific heat for water is 1cal/gram °C

• The difference in specific heats between land and water creates the sea breeze circulation

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Q: Which has the highest heat capacity:2 grams of pure water4 grams of ice at 0 °C10 grams of quartz sand

Q: Which will become warmer:adding 10 calories to 1 gram of wateradding 10 calories to 1 gram of quartz sand

Latent energy -- Latent HeatsThe Hidden Warmth

• When a substance changes from one state to another, latent heat is added or released in the process.

• Latent heat: the energy required to change from one state to another at constant temperature.

• Consider the water substance: all three phases of water (vapor, ice, and liquid) can be present at the same time.

• In a thunderstorm, for example, water is changing phases on a continual basis, therefore, latent heat is added or released on a continual basis.

• Sensible heat and latent heat

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Latent Heats in water phase transition

liquid --> vapor, latent heat of evaporation is added (about 600 cal per gram)vapor --> liquid, latent heat of condensation is released

liquid --> ice, latent heat of freezing is released (about 80 cal per gram)ice --> liquid, latent heat of fusion (melting) is added

• Q: Is latent heat a big deal in the atmosphere?

• QUESTION FOR THOUGHT: a. In a hurricane, if 100 grams of water vapor

condense into water every second, how many calories of heat are released into the atmosphere in one day? How much additional heat per day would be released if the water then froze into ice particles?

b. If the mass of a dry air parcel in a thunderstorm is 50 kg, how much cooler will the parcel be if 40 grams of ice sublimates into water vapor within the parcel?

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The law of Conservation Energy

• It is also called the first law of thermodynamics

• Energy cannot be created nor can it be destroyed. It merely changes from one form to another in any ordinary physical or chemical process.

Heat TransferThere are three mechanisms by which heat

(energy in the process of being transferred from one object to another) is transferred in the atmosphere:

• Conduction

• Convection

• Radiation

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Conduction• Heat transfer through molecular motions• From warm to cold• Other examples of conduction?• What is an atmospheric example of

conduction?

What is the difference for holding same length wood and metal?

T high

T low

Conductivity:materials ability to transfer heat by conduction.

QUESTION FOR THOUGHT:In the northern latitudes, the

oceans are warmer in summer than they are in winter. In which season do the oceans lose heat most rapidly to the atmosphere

by conduction? Explain.

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image adopted from The Atmosphere by F. Lutgens and E. Tarbuck, © Prentice-Hall, Inc

Convection• Transfer of heat through mass movement

of a substance• the "substance" could be air or water• Atmospheric Convection

– What are examples of convection in the atmosphere?

– thermals -->>

Fig. 2-6, p. 34

Atmospheric Convection• consider a hot parcel of air near the ground• Q: what is the parcel going to do?

A: it's going to rise - a thermal is formed•Thermals generate turbulent motions near the ground -noticeable during takeoff/landing in airplanes

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Fig. 2-6, p. 34

Q: when is the best time for thermals to form?

If the thermal is vigorous enough, it will often form a cloud and sometimes grow into a thunderstorm.Hence, thunderstorms are often referred to as "convection"

Fig. 2, p. 34

Rising air expands and cools; sinking air is compressed and warms.

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Radiation

• Radiant energy - the transfer of energy via electromagnetic waves.

• Examples:– sun warms your face– apparent heat of a fire– others?

Heat Transfer• Conduction

– Heat transfer through molecular motions– from warm to cold

• Convection – Transfer of heat through mass movement of a substance

• Radiation– via electromagnetic waves.

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Radiation - Waves• What is radiation -

electromagnetic waves?

• Characteristics of a wave: Wavelength and amplitude

• units of micrometers are often used to characterize the wavelength of radiation– 1 micrometer = 10-6 meters– notebook paper is about 100

micrometers thick

Fig. 2-7, p. 35

Q: What are typical wavelengths of radiation?

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Radiation emitted by objects

• All objects that have a temperature greater than 0 K emit radiation

• hot objects emit more radiation that colder objects

• Q: How much radiation is being emitted by an object, and at what wavelengths?

• Black Body - any object that is a perfect emitter and a perfect absorber of radiation -->>– object does not have to appear "black"– sun and earth's surface behave approximately as black bodies

• So, let's define some basic black-body radiation laws

Stefan-Boltzman Law• The Sefan-Boltzman law relates the total amount of

radiation emitted by an object to its temperature:• E=σT4

where:– E = total amount of radiation emitted by an object per

square meter (Watts m-2)– σ is a constant called the Stefan-Boltzman constant =

5.67 x 10-8 Watts m-2 K-4

– T is the temperature of the object in K• As the temperature of an object increases, more

total radiation is emitted each second.

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The Sun and Earth• Consider the earth and sun: Sun: T = 6000 K

so E = 5.67 x 10-8 Watts m-2 K-4 (6000 K)4 = 7.3 x 107

Watts m-2

• Q: is this a lot of radiation??? Compare to a 100 Watt light bulb.....

• Earth: T = 288Kso E = 5.67 x 10-8 Watts m-2 K-4 (288 K)4 = 390 Watts m-2

• Q: If you double the temperature of an object, how much more radiation will it emit?

Weins Law• Most objects emit radiation at many wavelengths• However, there is one wavelength where an object emits the largest amount of

radiation• This wavelength is found with Weins Law:

λmax = 2897 μm K / T(K)• Q: At what wavelength does the sun emit most of its radiation?

• Q: At what wavelength does the earth emit most of its radiation?

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QUESTION FOR THOUGHT• Suppose your surface body temperature

averages 90°F. How much radiant energy in W m-2 would be emitted from your body? What is the total radiant energy emitted by your body in Watts? At what wavelength is this radiant energy emitted?

• How night-vision camera works?

The sun’s electromagnetic spectrum and some of the descriptive names of each region. The numbers underneath the curve approximate the percent of energy the sun radiates in various regions.

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Balancing Act—Absorption, Emission, and Equilibrium

• All objects not only radiate energy, they also absorb it as well.

• Emission = absorption• Black body:

– perfect absorber: absorbs all the radiation that strikes it

– Perfect emitter: emits the maximum radiation possible at its given temperature.

• The earth’s atmosphere absorbs and emits infrared radiation.

Radiative Equilibrium

• If the temperature of an object is constant with time, the object is in radiative equilibrium at its radiative equilibrium temperature (Te)

• Q: What happens if energy input > energy output?

• Q: What happens if energy input < energy output?

• Q: Is the earth in radiative equilibrium?

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Conservation of Energy• Radiation incident upon a medium can be:

– absorbed– reflected– transmitted

• Ei = Ea + Er + Et

• Define– reflectance r = Er/Ei

– absorptance a = Ea/Ei

– transmittance t = Et/Ei

• Conservation: r + a + t = 1

Ei

Ea

Er

Et

Table 2-3, p. 44

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Fig. 3, p. 38

The intensity, or amount, of radiant energy transported by electromagnetic waves decreases as we move away from a radiating object because the same amount of energy is spread over a larger area.

Radiative Equilibrium of the EarthEnergy gained through absorption of short wave radiation is

equal to the emitted long wave radiation

Energy from SunFs=1372 Wm-2

A Fs

Earth albedo A

(Fs- AFs) π a2 4π a2 σ Te4= Te =255 K

The global-mean surface temperature: Ts = 288 K

(288/255)4=1.63What is wrong?

a

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Selective Absorption in the Atmosphere

Q: Is the atmosphere a blackbody?A: No!The atmosphere selectively absorbs

long-wave radiation, does not absorb much of the shortwave

Kirchoffs Law: Any object that selectively absorbs radiation at a particular wavelength - must re-emit radiation at that same wavelength

Snow is another great example of a selective absorber......

Answer: The role of Atmosphere – greenhouse gases• without greenhouse gases, all longwave energy emitted by the earth surface go to space• with greenhouse gases, only a fraction of longwave energy emitted by the earth surface go to space and the rest are sent by to earth to warm the surface. This is known as the "greenhouse effect"

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QUESTIONS FOR THOUGHT:

1. Why does it get colder on clear nights versus cloudy nights?

2. Is the atmosphere a blackbody? Why or why not?

3. Which would have the greatest effect on the earth's greenhouse effect: Removing all of the CO2 from the atmosphere or removing all of the water vapor? Explain.

4. Explain why an increase in cloud cover surrounding the earth would increase the earth's albedo, yet not necessarily lead to a lower earth surface temperature.

Fig. 2-10, p. 40

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Air in the lower atmosphere is heated from below. Sunlight warmsthe ground, and the air above is warmed by conduction, convection, and infrared radiation. Further warming occurs during condensation as latent heat is given up to the air inside the cloud.

Hot

Fig. 2-14, p. 45

The scattering of air by molecules (Rayleigh Scattering). Air molecules tend to selectively scatter the shorter (violet, green, and blue) wavelengths of visible white light more effectively than the longer (orange, yellow, and red) wavelengths.

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Cloud droplets scatter all wavelengths of visible white light about equally (Mie Scattering). This type of scattering by millions of tiny cloud droplets makes clouds appear white.

At noon, the sun usually appears a bright white. At sunrise and at sunset, sunlight must pass through a thick portion of the atmosphere. Much of the blue light is scattered out of the beam, causing the sun to appear more red.

A brilliant red sunset produced by the process of scattering

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70

Where the incoming solar radiation go?

The earth-atmosphere energy balance. Numbers represent approximations based on surface observations and satellite data.While the actual value of each process may vary by several percent, it is the relative size of the numbers that is important.

70

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Fig. 2-17, p. 47

Earth's energy balanceWe have argued that since Te is approximately constant, then the earth is in radiative equilibriumIn other words, radiation from the sun = radiation emitted by the earth

Q: How are:radiation from the sunradiation emitted by the earthdistributed over the globe from the SP to the NP??????

•A deficit of energy exists in the northern latitudes

this implies warming or cooling?•A surplus of energy exists in the equatorial region

this implies warming or cooling?

Heat is therefore transported from the equatorial region to the polar regions

How is the heat transported???? There are three primary mechanisms:

1. Ocean circulation (~40%)2. Weather transporting sensible

heat (~30%)3. Weather transports and

generates latent heat (~30%)

Radiation and the Aurora• Interaction of radiation (more precisely, ions - charged particles) from the sun and the earth's magnetic field creates the aurora -- referred to as the aurora borealis, or northern lights in the northern hemisphere.• Earth's magnetic field is much like that of a bar magnet -->>believed to be created by fluid motion of molten matter in the earth's interior.

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Fig. 2-19, p. 49

The stream of charged particles from the sun—called the solar wind—distorts the

earth’s magnetic field into a teardrop shape known as the magnetosphere.

Various regions of the sun.

Sun Distorts the earth’s Magnetic field

MagnetotailSolar wind

Magnetosphere

Fig. 2-20, p. 50

When an excited atom, ion, or molecule de-excites, it can emit visible light. (a) The electron in its normal orbit becomes excited by a charged particle and (b) jumps into a higher energy level. When the electron returns to its normal orbit, it (c) emits a photon of light.

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The aurora belt

The aurora borealis is a phenomenon that forms as energetic particles from the sun interact with the earth’s atmosphere.

The aurora belt (solid red line) represents the region where you would most likely observe the aurora on a clear night. (The numbers represent the average number of nights per year on which you might see an aurora if the sky were clear.) The flag MN denotes the magnetic north pole, whereas the flag NP denotes the geographic north pole.

http://www.vtphoto.com

Question for Thought #1, p. 52

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Solar energy striking a large body of water goes through many transformations.

Fig. 4, p. 39