Energy: Warming the Earth & the Atmosphere

This chapter discusses:

1. The role of solar energy (e.g. short wave radiation) in generating temperature & heat

2. Earth's processes for heat transfer in the atmosphere, including long wave radiation, to maintain an energy balance

What is Energy?

• Energy is the ability or capacity to do work on some form of matter.

• Work is done by pushing, pulling, or lifting of matter.

• Since the size of the atmosphere is undefined, the size of the atmosphere is handled on a case-to-case basis.

• Two types of energy – Potential Energy (PE) = mass × gravity × height– Kinetic Energy (KE) = ½ × mass × velocity2

PE = mass × gravity × height

• Defines as the total amount of energy stored in any object determines how much work that object is capable of doing.

• Can be thought of as an internal energy.• Also called gravitational potential energy.• Since this course only deals with the earth's

atmosphere and gravity changes in the troposphere are very minimal, gravity is held as a constant.

• Example: a volume of air aloft has more PE than the same size volume of air just above the surface. Why?

KE = ½ × mass × velocity2

• Once an object is set in motion then it is said to acquire “energy of motion” or KE

• KE of an object is equal to ½ of its mass times the velocity squared

• So mass and velocity of an object plays an critical role in determining the KE

• Does an identical volume of air and water have the same KE?

• Commonly refers to as heat energy • A 75 miles per hour (mph) wind (minimal hurricane-force)

is likely to knock someone over than a strong breeze of 20 mph due to the minimal hurricane-force wind having more energy. Now let's take this example further, a 75 mph wind at Boulder, CO versus a 75 mph over Miami, FL have different KE values. Why?

Energy

• Energy comes in many forms and can change form from one to another.

• Energy cannot be created or destroyed.• Energy is conserved during a change in

form.• First law of thermodynamics

• When solar radiation collides with atmospheric gas molecules, the gas molecules move.

This produces:a) temperature, defined as the moving molecules average speedb) kinetic energy • Total energy increases with greater molecule volumes.

• The figure shows that how the average speed of the molecules can be the same but the internal energy can be different.

Temperature

Heat• Defines as the process of energy being

transferred from one object to another because of the temperature difference between them.

• After the transfer, the heat is return to a state of internal energy

• Heat can be transferred by:– conduction– convection– radiation

Temperature Scales

• Thermometers detect the movement of molecules to register temperature.

• Fahrenheit and Celsius scales are calibrated to freezing and boiling water at sea-level, but the Celsius range is about 1.8 times more compact.

• While the Kelvin scale does not go below 0K.

• The image below show how these scales compare with one another with respect to specific points and events.

Specific Heat• Defines as how much heat is needed to raise the

temperature of a substance that weights one gram by one degree Celsius.– Heat capacity is the ratio of the amount of heat

absorbed by a substance to the corresponding temperature change

• Example: – One gram of water would take one calorie to raise its

temperature by 1°C– One calorie = 4.186 Joules– Calorie (Cal) is the amount of heat required to raise the

temperature of 1 g of water from 14.5°C to 15°C. Therefore, a kilocalorie is 1000 calories and is the heat required to raise 1 kg of water 1°C.

• Water has special properties such as :

• Heat slowly

• High capacity of storing heat

• Why is this important?

Latent Heat

• Defines as the heat energy requires to change a substance from state to another.

• This heat source is sometimes referred to as a hidden warmth.

• Latent heat is released from or absorbed within the water molecules when phase change occurs.

Latent & Sensible Heat

• Heat energy, which is a measure of molecular motion, moves between water's vapor, liquid, and ice phases.

• As water moves toward vapor it absorbs latent heat to keep the molecules in rapid motion and vice versa.

• Evaporation has a cooling effect• Condensation is a warming effect• Latent heat can manifest itself as sensible heat• Sensible heat is the heat that we can feel and measure with a

thermometer• Latent heat of (at room temperature):

– Condensation (heat is added into the environment)• Vapor to liquid• 2.5 × 106 J kg−1

– Evaporation (heat is needed from the environment)• Liquid to vapor• 2.5 × 106 J kg−1

– Fusion (heat is needed from the environment)• Ice to liquid• 3.35 × 105 J kg−1

– Sublimation (heat is needed from the environment)• Ice to vapor • 2.83 × 106 J kg−1

– Deposition (heat is added into the environment)• Vapor to ice• 2.85 × 106 J kg−1

Heat Energy for Storms

• Latent heat released from the billions of vapor droplets during condensation and cloud formation fuels storm energy needs, warms the air, and encourages taller cloud growth.

• An average thunderstorm contains several thousand metric tons of water.

• Condensing 1 kg of water releases ~ 2.5 x 106 J of latent heat energy .

• An average thunderstorm containing around 1500 tons of water will release 3.45 billion Joules of energy.

Heat is transferred by:• Conduction - transfer of heat from molecule to

molecule with in a substance

• Convection - transfer of heat by the mass movement of a fluid (in the vertical)– Advection - transfer of heat or some atmospheric

properties from one area to another area (in the horizontal).

• Radiation - transfer of energy from one object to another without spaces in between heated

Conduction - Heat Transfer

• Conduction of heat energy occurs as warmer molecules transmit vibration, and hence heat, to the adjacent cooler molecules.

• Warm ground surfaces heat overlying air by conduction.

• Air is a extremely poor conductor of heat.

• Heat transferred by conduction always flows from warmer to colder regions.

• Typically, the greater the temperature difference, the faster the heat transfer.

Heat Conductivity

Poor

Good

Convection - Heat Transfer

• Convection is heat energy moving as a fluid from hotter to cooler areas.

• Warm air at the ground surface rises as a thermal bubble, expends energy to expand, and hence cools. This is called convective circulation or a thermal cell.

• Any rising air “bubble” will expand and cool, and any sinking air “bubble” will compress and warm.

•As the air parcel rises to a a lower pressure region, in order to equalize the pressure on the inside, the parcel molecules inside push the parcel wall outward and expanding it.

How does radiation transfer energy from one object to another without spaces in between

heated?

• Radiant energy or radiation travels in the form of waves.

• Energy is released when they are absorbed by an object.

• These waves are called electromagnetic (EM) waves because it has magnetic and electrical properties.

• In space, void of air molecules, EM waves travel at 3 x 105 km s-1 or 186,000 miles per second.

Radiation - Heat Transfer

• Waves can come in different sizes.

• Wavelength (λ) is a measurement of these wave sizes.

• All objects above 0 K release radiation, and its heat energy value increases to the 4th power of its temperature.

• Stefan-Boltzmann Law: E = σT4 where σ is the Stefan-Boltzmann Constant.• σ = 5.67 x 10-8 W m-2 K-4

3 important facts about radiation• All things emit radiation

– size does not matter.

• The wavelengths of radiation that an object emits are mainly dependent on the object’s temperature.– Temperature is inversely proportional to the wavelength -- the

higher the temperature of an object, the shorter the wavelength.

• Objects that have high temperatures emit radiation at a greater rate, stronger intensity, and wider range than objects with lower temperatures.– Sun’s surface temperature is greater than earth’s surface

temperature therefore the sun emits more radiation and at more wavelengths than the earth’s surface.

Longwave & Shortwave Radiation

• The hot sun radiates at shorter wavelengths that carry more energy.

• The sun maximum radiative wavelength is about 0.5 μm. (visible light)

• The cooler earth only absorbs a small fraction of the sun’s radiation which is then re-radiated at longer wavelengths, as predicted by Wein's law.

• The earth maximum radiative wavelength is about 10 μm. (infrared)

How does Wien’s law work?

• Start with:

• λmax is the maximum radiation emission (μm)

• T is the temperature of the object• Constant = 2897 μm K

mK

K 5.06000

3000max

T

Constantmax

mK

K 10288

3000max

Sun Earth

• Sun emits shortwave radiation (solar radiation)

• Earth emits longwave radiation (terrestrial radiation)

Electromagnetic Spectrum

• Solar radiation is largely found in the shorter wavelengths such as ultraviolet, visible, and near infrared portions of the EM spectrum.

• Solar radiation also extends at low intensity into longwave regions such as far infrared, microwaves, and radio waves.

• 7% UV + 44% VIS + 37% near IR+ 11% far IR + 1% others = 100%

• Earth’s surface and the sun are considered blackbody objects. ~ nearly 100% absorption and emission.

• When the rate of absorption equals the rate of emission by radiation transfer only, this is called the radiative equilibrium state.

• So, the temperature at which this state occurs is known as radiative equilibrium temperature (RET) ~ the earth’s RET is about 255 K.

What is a Blackbody?Any object that is a perfect absorber (to all radiation that strikes it) and a perfect

emitter (where the maximum radiation possible is emitted at its given temperature).

50 % absorption

50 % emission

• Does not have to be black to be considered a blackbody.

• Wein’s law and Stefan-Boltzmann law works well with blackbody objects.

Therefore, the object is a blackbody because its absorption/emission efficiency is at 100%.

So if the earth’s radiative equilibrium temperature is about 255 K (0°F) why is this value much lower than the earth’s observed temperature 288K (59°F)?

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

• The atmosphere does not behave like the earth’s surface (blackbody).

• The atmosphere is a “selective” absorber and emitter of radiation.

Atmospheric Greenhouse Effect

• Earth's energy balance requires that absorbed solar radiation is emitted to maintain a constant temperature.

• Without natural levels of greenhouse gases absorbing and emitting, this surface temperature would be 33°C cooler than the observed temperature.

Atmospheric Absorption

• Solar radiation passes rather freely through earth's atmosphere, but earth's re-emitted longwave energy either fits through a narrow window or is absorbed by greenhouse gases and re-radiated toward earth.

• As these gases absorb infrared radiation from the earth’s surface, they acquire kinetic energy (energy of motion).

• The different gas molecules share this energy by collision with adjacent air molecules, such as O2 and N2 (poor absorbers of IR). These collisions increase the overall kinetic energy of the air which results in increase in air temperature

Absorption of Nitrous Oxide

|------UV--------|--VIS---|----------------------------------IR-------------------------------------|

Absorption of Methane

|------UV--------|--VIS---|----------------------------------IR-------------------------------------|

Absorption of Oxygen and Ozone

|------UV--------|--VIS---|----------------------------------IR-------------------------------------|

Absorption of Water Vapor

|------UV--------|--VIS---|----------------------------------IR-------------------------------------|

Absorption of Carbon Dioxide

|------UV--------|--VIS---|----------------------------------IR-------------------------------------|

Total Absorption of the Atmosphere

• Atmospheric greenhouse effect is associated with the role of water vapor, CO2, and other greenhouse gases in maintaining the earth’s averaged surface temperature higher than the predicted value without an atmosphere.

• Atmospheric Window is the region where IR radiation (8 – 11μm) is neither absorbed or emitted by water vapor and CO2 and is freely to pass through the atmosphere.

• Clouds (good absorber of IR but poor absorber of visible light) can enhance the atmospheric gashouse effect as well by absorbing radiation between 8 – 11μm, thereby closing the atmospheric window.– Calm, cloudy night = warmer temperature– Calm, clear night = cooler temperature– Cloudy day = cooler temperature– Sunny day = warmer temperature

Warming Earth's Atmosphere from Below

1. Solar radiation passes first through the upper atmosphere, but only after absorption by earth's surface does it generate sensible heat (heat that we can feel and measure) to warm the ground and generate longwave energy.

2. This heat and energy at the surface then warms the atmosphere from below.

3. Since water vapor decreases with rapidly above the earth, most of the absorption occurs in a layer near the surface. Therefore, the lower atmosphere is mainly heat from below.

Scattered Light• Sunlight passing through earth's

atmosphere is deflected by gases, aerosols, and dusts in all directions. This distribution of light is called scattering.

• Air molecules are smaller than visible light wavelengths, therefore they are better scatterers of shorter (blue) wavelengths than longer (red) wavelengths.

• At the horizon sunlight passes through more scatterers, leaving longer wavelengths and redder colors revealed.

• The midday sun looks white due to less scattering by the air molecules.

• At noon, the sun usually appears a bright white due to less scattering of the blue lights.

• At sunrise and sunset, sunlight must pass through a thicker portion of the atmosphere.

• As the sunlight passes through more of the atmosphere, much of the blue light is scattered out of the beam, causing the sun to appear more red.

• Cloud droplets scatter all wavelengths of visible white light about equally.

• This type of scattering by millions of tiny cloud droplets makes clouds appear white.

• Sunlight can be reflected from objects.

• Albedo – is the percent of radiation returning from a given surface compared to the amount of radiation initially striking the surface. (reflectivity of a surface)

• The earth on the average reflects about 30% of the sun’s incoming radiation back into space.

• The colors of the objects do not play a huge role in controlling the albedo.

Incoming Solar Radiation

• Solar radiation is scattered and reflected by the atmosphere, clouds, and earth's surface, creating an average albedo of 30 (30 units).

• Atmospheric gases and clouds absorb another 19 units, leaving 51 units of shortwave absorbed by the earth's surface.

Solar Constant – 1367 W/m2

Earth-Atmosphere Energy Balance

• The earth's surface absorbs the 51 units of shortwave and 96 more of longwave energy units from atmospheric gases and clouds.

• These 147 units gained by earth are due to shortwave (sun) and longwave (atmosphere) greenhouse gas absorption and emittance.

• Earth's surface loses 117 units through emission of IR, therefore producing 30 units of surplus from the earth’s surface.

• Meanwhile, the atmosphere generates 30 units of deficit at the surface through conduction, convection and evaporation.

• This 117 units lost from IR emission added with the 30 units lost from other processes equal 147 units.

• Basically, annually the earth is gaining as much energy as it is losing on the surface and atmosphere. The balance is created from heat transfer processes such as absorption, conduction, convection, and latent heat release.

• If this balance is shifted where the earth is gaining more energy then this will lead to a warming trend and vice versa with the cooling trend.

Earth-Atmosphere Energy Balance (cont.)

Average annual incoming solar radiation absorbed and outgoing infrared radiation from the earth and the

atmosphere

Surplus heat is transported from the equator to the pole regions.

Solar Particles and the Aurora

• Solar wind is made up of charged particles from the Sun’s atmosphere.

• These charged particles are formed as a results of high temperature stripping electrons away from gases in a violent collisions.

• Aurora are formed as a result of the solar wind interacting with the earth’s magnetic field.

Earth's Magnetic Field

• Earth's molten metal core in motion creates a magnetic field that covers earth from the south to north pole.

• This magnetic field forms the magnetosphere which protects the earth from some of the solar wind bombardments.

Solar Wind• High energy plasma is

blown from the sun in a dangerous solar wind, and the magnetosphere deflects this wind to shield the earth.

• This interaction deforms the magnetosphere into teardrop shape.

• Solar wind normally travels at a velocity of 400 km s-1 but can travel faster during high solar activities.

Ions

• Solar winds entering the magnetosphere excite atmospheric gas electrons.

• The electron jumps into a higher energy orbit when excited by a charged particle.

• When the electron de-excites it emits visible radiation.

• The aurora is created by these solar winds and de-exciting ions, and has belts of expected occurrence at both poles.

• Aurora Borealis (northern lights)

• Aurora Australis (southern lights)

• Solid red light indicates where the aurora would be best seen on a clear night.

• The number of aurora events decreases as you go north and south of the main belt.

• The NP flag depicts the geographic north pole and MN flag denotes the magnetic north pole.

Aurora Belts