Chapter 3: Energy Balance and Temperature

Planetary Energy Balance

Solar Energy Absorbed = Terrestrial Energy Emitted

Solar Flux Density Reaching Earth

Solar Constant (S)

    The solar energy density at the mean distance of Earth from the sun (1.5 x 10¹¹ m)

        S = L / (4 p d²)

           = (3.9 x 10²⁶ W) / [4 x 3.14 x (1.5 x 10¹¹ m)²]

           = 1370 W/m²

Solar Energy Incident On the Earth

Solar energy incident on the Earth

      = total amount of solar energy can be absorbed by Earth

      = (Solar constant) x (Shadow Area)

      = S x p R²_Earth

Solar Energy Absorbed by Earth

Albedo = [Reflected] / [Incoming] Sunlight

Albedo and Surface Type

What Happens After the Earth Absorbs Solar Energy?

The Earth warms up and has to emit radiative energy back to the space to reach a equilibrium condition.

The radiation emitted by the Earth is called “terrestrial radiation” which is assumed to be like blackbody radiation.

Energy Emitted from Earth

Planetary Energy Balance

Greenhouse Effect

Greenhouse Gases

Factors Determine Planet Temperature

Distance from the Sun

Albedo

Greenhouse effect

Mars, Earth, and Venus

Global Temperature

Greenhouse Effects

On Venus è 510°K (very large!!)

On Earth è 33°K

On Mars è 6°K (very small)

Why Large Greenhouse Effect On Venus?

Venus is very close to the Sun

Venus temperature is very high

Very difficult for Venus’s atmosphere to get saturated in water vapor

Evaporation keep on bringing water vapor into Venus’s atmosphere

Greenhouse effect is very large

A “run away” greenhouse happened on Venus

Water vapor is dissociated into hydrogen and oxygen

Hydrogen then escaped to space and oxygen reacted with carbon to form carbon dioxide

No liquid water left on Venus

Why Small Greenhouse Effect on Mars?

Mars is too small in size

Mars had no large internal heat

Mars lost all the internal heat quickly

No tectonic activity on Mars

Carbon can not be injected back to the atmosphere

Little greenhouse effect

A very cold Mars!!

Selective Absorption and Emission

The atmosphere is not a perfect blackbody, it absorbs some wavelength of radiation and is transparent to others (such as solar radiation). è Greenhouse effect.

Objective that selectively absorbs radiation usually selectively emit radiation at the same wavelength.

For example, water vapor and CO2 are strong absorbers of infrared radiation and poor absorbers of visible solar radiation.

"A portion of the longwave..."

A portion of the longwave spectrum can pass through the atmosphere unimpeded.

This range of wavelengths, 8-15μm, match those radiated with greatest intensity by the Earth’s surface.

This range of wavelengths not absorbed is called the atmospheric window.

Why Selective Absorption/Emission?

Radiation energy is absorbed or emitted to change the energy levels of atoms or molecular.

The energy levels of atoms and molecular are discrete but not continuous.

Therefore, atoms and molecular can absorb or emit certain amounts of energy that correspond to the differences between the differences of their energy levels.

è Absorb or emit at selective frequencies.

Different Forms of Energy Levels

The energy of a molecule can be stored in (1) translational (the gross movement of molecules or atoms through space), (2) vibrational, (3) rotational, and (4) electronic (energy related to the orbit) forms.

Energy Required to Change the Levels

The most energetic photons (with shortest wavelength) are at the top of the figure, toward the bottom, energy level decreases, and wavelengths increase.

Atmospheric Influences on Insolation

Absorption

    - convert insolation to heat the atmosphere

Reflection / Scattering

    - change the direction and intensity of insolation

Transmission

    - no change on the direction and intensity of insolation

Reflection and Scattering

Reflection: light bounces back from an objective at the same angle at which it encounters a surface and with the same intensity.

Scattering: light is split into a larger number of rays, traveling in different directions.

Although scattering disperses light both forward and backward (backscattering), more energy is dispersed in the forward direction.

Scattering

Rayleigh Scattering (Gas Molecules)

Involves gases, or other scattering agents that are smaller than the energy wavelengths.

Scatter energy forward and backward.

Partial to shorter wavelength energy, such as those which inhabit the shorter portion of the visible spectrum.

Responsible for (1) blue sky in clear days, (2) blue tint of the atmosphere when viewed from space, and the redness of sunsets and sunrises.

Scattering and Colors

Short wavelengths (blue and violet) of visible light are scattered more effectively than are longer wavelengths (red, orange). Therefore, when the Sun is overhead, an observer can look in any direction and see predominantly blue light that was selectively scattered by the gases in the atmosphere.

At sunset, the path of light must take through the atmosphere is much longer. Most of the blue light is scattered before it reaches an observer. Thus the Sun appears reddish in color.

Slide 29

Spectrum of Radiation

Radiation energy comes in an infinite number of wavelengths.

We can divide these wavelengths into a few bands.

Mie Scattering (Aerosols)

Larger scattering agents, such as suspended aerosols, scatter energy only in a forward manner.

Larger particles interact with wavelengths across the visible spectrum.

Produces hazy or grayish skies.

Enhances longer wavelengths during sunrises and sunsets, indicative of a rather aerosol laden atmosphere.

Nonselective Scattering (Clouds)

Water droplets in clouds, typically larger than energy wavelengths, equally scatter wavelengths along the visible portion of the spectrum.

Produces a white or gray appearance.

No wavelength is especially affected.

Fate of Solar Radiation

Atmospheric reflection averages 25 units, 19 of which are reflected to space by clouds and 6 units which are back-scattered to space from atmospheric gases.

The atmosphere absorbs another 25 units.

Remaining 50 units are available for surface absorption and reflection.

Slide 34

Slide 35

Slide 36

Latitudinal Variations of Net Energy

Polarward heat flux is needed to transport radiation energy from the tropics to higher latitudes.

How Do Atmosphere and Ocean Transport Heat?

Slide 39


	Solar Constant (S)
	The solar energy density at the mean distance of Earth from the sun (1.5 x 10¹¹ m)

	S = L / (4 p d²)
	= (3.9 x 10²⁶ W) / [4 x 3.14 x (1.5 x 10¹¹ m)²]
	= 1370 W/m²


	Solar energy incident on the Earth
	= total amount of solar energy can be absorbed by Earth
	= (Solar constant) x (Shadow Area)
	= S x p R²_Earth


	The Earth warms up and has to emit radiative energy back to the space to reach a equilibrium condition.

	The radiation emitted by the Earth is called “terrestrial radiation” which is assumed to be like blackbody radiation.


	On Venus è 510°K (very large!!)
	On Earth è 33°K
	On Mars è 6°K (very small)


	Venus is very close to the Sun
	Venus temperature is very high
	Very difficult for Venus’s atmosphere to get saturated in water vapor
	Evaporation keep on bringing water vapor into Venus’s atmosphere
	Greenhouse effect is very large
	A “run away” greenhouse happened on Venus
	Water vapor is dissociated into hydrogen and oxygen
	Hydrogen then escaped to space and oxygen reacted with carbon to form carbon dioxide
	No liquid water left on Venus


	Mars is too small in size
	Mars had no large internal heat
	Mars lost all the internal heat quickly
	No tectonic activity on Mars
	Carbon can not be injected back to the atmosphere
	Little greenhouse effect
	A very cold Mars!!


	The atmosphere is not a perfect blackbody, it absorbs some wavelength of radiation and is transparent to others (such as solar radiation). è Greenhouse effect.
	Objective that selectively absorbs radiation usually selectively emit radiation at the same wavelength.
	For example, water vapor and CO2 are strong absorbers of infrared radiation and poor absorbers of visible solar radiation.


		A portion of the longwave spectrum can pass through the atmosphere unimpeded.
		This range of wavelengths, 8-15μm, match those radiated with greatest intensity by the Earth’s surface.
		This range of wavelengths not absorbed is called the atmospheric window.


	Radiation energy is absorbed or emitted to change the energy levels of atoms or molecular.
	The energy levels of atoms and molecular are discrete but not continuous.
	Therefore, atoms and molecular can absorb or emit certain amounts of energy that correspond to the differences between the differences of their energy levels.
	è Absorb or emit at selective frequencies.


	The energy of a molecule can be stored in (1) translational (the gross movement of molecules or atoms through space), (2) vibrational, (3) rotational, and (4) electronic (energy related to the orbit) forms.


	The most energetic photons (with shortest wavelength) are at the top of the figure, toward the bottom, energy level decreases, and wavelengths increase.


	Absorption
	- convert insolation to heat the atmosphere
	Reflection / Scattering
	- change the direction and intensity of insolation
	Transmission
	- no change on the direction and intensity of insolation


	Reflection: light bounces back from an objective at the same angle at which it encounters a surface and with the same intensity.
	Scattering: light is split into a larger number of rays, traveling in different directions.
	Although scattering disperses light both forward and backward (backscattering), more energy is dispersed in the forward direction.


	Involves gases, or other scattering agents that are smaller than the energy wavelengths.
	Scatter energy forward and backward.
	Partial to shorter wavelength energy, such as those which inhabit the shorter portion of the visible spectrum.
	Responsible for (1) blue sky in clear days, (2) blue tint of the atmosphere when viewed from space, and the redness of sunsets and sunrises.


	Short wavelengths (blue and violet) of visible light are scattered more effectively than are longer wavelengths (red, orange). Therefore, when the Sun is overhead, an observer can look in any direction and see predominantly blue light that was selectively scattered by the gases in the atmosphere.
	At sunset, the path of light must take through the atmosphere is much longer. Most of the blue light is scattered before it reaches an observer. Thus the Sun appears reddish in color.


	Radiation energy comes in an infinite number of wavelengths.
	We can divide these wavelengths into a few bands.


		Larger scattering agents, such as suspended aerosols, scatter energy only in a forward manner.
		Larger particles interact with wavelengths across the visible spectrum.
		Produces hazy or grayish skies.
		Enhances longer wavelengths during sunrises and sunsets, indicative of a rather aerosol laden atmosphere.


		Water droplets in clouds, typically larger than energy wavelengths, equally scatter wavelengths along the visible portion of the spectrum.
		Produces a white or gray appearance.
		No wavelength is especially affected.


		Atmospheric reflection averages 25 units, 19 of which are reflected to space by clouds and 6 units which are back-scattered to space from atmospheric gases.
		The atmosphere absorbs another 25 units.
		Remaining 50 units are available for surface absorption and reflection.


	Polarward heat flux is needed to transport radiation energy from the tropics to higher latitudes.