Lecture 3: The global energy budget

Question 1

What is the solar constant (whose value is 1365 W/m^2)?

Answer 1

The rate at which the suns electromagnetic energy passes through an imaginary surface perpendicular to the suns rays at the top of the atmosphere.

Question 2

How can this be used to determine the total energy received by the atmosphere?

Answer 2

Solar constant x earth’s projected circular area on an imaginary surface. (-341W/m^2)

Question 3

What are the strongest reflectors of incoming solar radiation?

Answer 3

Snow and high cloud (cumulus stratus)

Question 4

What is earths planetary albedo?

Answer 4

0.3

Question 5

Once a fraction of the incoming solar radiation has been reflected, 240W/m^2 is left. Of this, how much is absorbed by atm and how much by the earth?

Answer 5

80 by the atmosphere, 160 by the surface. (Ratio determined by earths planetary emissivity).

Question 6

What characterises a ‘blackbody’ - the sun is almost one.

Answer 6

Absorbas all incoming light, and when heated, emits electromagnetic radiation in a way that only depends on temperature.

Question 7

In what part of the spectrum is the suns electromagnetic energy most intense and why?

Answer 7

In the visible range, as when temperature increases, energy emitted moves from IR to visible.

Question 8

The total energy emitted by a blackbody is given by what law?

Answer 8

The stefan boltzman law - E= σT^4
Where σ = the Stefan Boltzman constant
T = Temperature in Kelvin

Question 9

As the earths surface behaves approximately like a blackbody emitter, what can the stefan boltzman equation be used to find?

Answer 9

Earths brightness temp (-18 degrees) - the temperature earth would be if atm didnt exist.

Question 10

Almost all energy in which range reaches the earths surface?

Answer 10

Visible

Question 11

What is responsible for most of the 80 W m-2 of

solar radiation absorbed by the atmosphere?

Answer 11

Water vapour

Question 12

What prevents earths surface acting as a true blackbody?

Answer 12

The atmosphere

Question 13

Earths planetary emissivity (1 for blackbodies) can be determined by what equation?

Answer 13

Outgoing LW radiation at top of atm / LW radiation emitted by earths surface

Question 14

Why is more energy lost to space at higher levels in the atmosphere?

Answer 14

There is less water vapour, so adsorption is less.

Question 15

What part of the spectrum does solar radiation arrive in, and what does radiation leave the atmosphere as. What interferes with this, creating the greenhouse effect?

Answer 15

Incoming short wave radiation in visible part of the spectrum - therefore passes through the atmosphere relatively easily. Outgoing radiation is in the IR range and much of this is then absorbed by CO2 and water vapour.

Question 16

Which two factors make up the earths energy balance?

Answer 16

Net emitted solar = net emitted longwave

Question 17

As the earth receives 160W/m^2 from the sun, and reflects back 390 W/m^2, what supplies the extra energy that prevents the earths temperature falling?

Answer 17

Atm supplies 333W/m^2

Question 18

Therefore the net radiative energy balance for the earths surface is 103W/m^2 - what can this do?

Answer 18

Drive atmospheric circulation

Question 19

Why do the atmosphere and ocean transport heat?

Answer 19

The energy received by the equator is 2.5 times the amount received by the poles, and the equator receives more than it radiates, whilst the poles receive less. Therefore, the oceans and atmosphere transport heat between equator and poles.

Question 20

Where is maximum heat transport greatest (at 6 PetaWatts)?

Answer 20

30-40 degrees S/N

Question 21

Why do large seasonal variations in insolation occur?

Answer 21

Due to earths elliptical orbit - precession of equinoxes means current situation will reverse.

Question 22

What are the 3 factors of milankovitch cycles, and on what time scale do they occur?

Answer 22

Precession - 21,000 years
Obliquity - 41,000 years
Eccentricity - 100,000-400, 000 years

Question 23

What provides the earths surface with the most energy?

Answer 23

The atmosphere (as opposed to the sun!)

Question 24

When is the earth closest to the sun, and what does precession of equinoxes mean for seasonal release of solar energy?

Answer 24

In the present Northern hemisphere winter. 6% more energy from sun in january than june.

Question 25

How does precession of equinoxes affect the ice sheets?

Answer 25

Prevents ice sheet formation when Northern Hemisphere summer occurs at closest point.

Question 26

Distance from the sun; Albedo; Actual average T (in K and C) 
Venus 1.1 * 1011m ; 0.78 ; 700 
Earth 1.5 * 1011m ; 0.30 ; 288 
Mars 2.3 * 1011m ; 0.17 ; 210 
Jupiter 7.8 * 1011m ; 0.45 ; 160 

• Given that σ = 5.67 x 10-8W /m2/deg4, Earth’s solar constant is 1370 W/m2, and that the solar constant is inversely proportional to the square of the distance from
the sun, determine the brightness temperature of the planets.

• Which planet has the largest greenhouse effect?

Answer 26

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