1. Lecture 4 Absorption Emission Equilibrium & SEASONS

2. Learning Goals for Part 2 of Chapter 2
Know the difference between ABSORPTION, EMISSION, and REFLECTION (ALBEDO) and what they have to do with the atmosphere and weather (e.g. clouds).
Be able to describe what is meant by EQUILIBRIUM with regard to the Earth and why we care.
Have an understanding of why the Earth has SEASONS.
You can just read this slide.

3. Blackbodies Sun and Earth are blackbodies.
Earth’s radiative equilibrium temperature is about 255 K
This is equal to -18°C or 0°F
Why isn’t this this average surface temperature?
The atmosphere is NOT a black body!!!
Atmospheric Gases are selective absorbers and absorb and emit IR Radiation.

4. Heating the Atmosphere
Gases are the most effective absorbers of radiation and play the primary role in heating the atmosphere:
Water vapor, oxygen and ozone absorb most of the energy in the atmosphere
CO2 is important at long wavelengths (like IR)
“Openings” are Atmospheric windows

5. Atmosphere Absorbs Radiation
The Atmosphere is nearly transparent for shortwave but absorbs strongly longwave radiation.
% absorbed
FIGURE 2.9 Absorption of radiation by gases in the atmosphere. The shaded area represents the percent of radiation absorbed. The strongest absorbers of infrared radiation are water vapor and carbon dioxide.

6. Greenhouse Effect FACT: The atmosphere warms the planet
Water Vapor, CO2 and other trace gases absorb the outgoing IR radiation
The absorbed energy heats the air.
The average temperature of the earth is 59°F (33°C)

7. Greenhouse Effect
Greenhouse Gases (GHGs) are the “villain” in the Global Warming Debate.
The Greenhouse Effect and Global Warming are NOT the same thing
Without the Greenhouse Effect Earth would be uninhabitable!!!!!
Human activity IS making the atmosphere more efficient at retaining long wave emissions from the Earth.

8. What Happens to Incoming Solar Radiation?
Absorbed
Transmitted
Reflected
Scattered
DEPENDS ON THE WAVELENGTH OF THE ENERGY

9. Reflection and Scattering
Bounces off at the same angle
Scattering
Produces a larger number of weaker rays
And at the same intensity
Less is “Backscattered”
More is scattered forward

10. Reflection and the Earth’s Albedo
Energy is returned to space via reflection and emission
ALBEDO
The percentage reflected
About 30% is reflected for Earth
5% from land and ocean
25% from clouds and ice

11. Role of Clouds in Heating the Earth
FACT: Clouds are good absorbers of IR radiation emitted from the Earth
Thick cloud cover absorbs most outgoing IR radiation, re-radiating it back to the surface
Warm cloudy nights
The effect of heating really depends on the type of cloud…

12. Role of Clouds in Heating the Earth
High Thin Clouds transmit incoming SW solar radiation
Absorb some of the outgoing LW radiation, and re-emit back down
Tend to WARM the surface

13. Role of Clouds in Heating the Earth
Low Thick Clouds block incoming SW solar radiation
They have a high albedo and reflect most of the energy back to space..
Tend to COOL the surface

14. Role of Clouds in Heating the Earth
Whether a specific cloud will warm or cool the surface depends on:
The time of day
Cloud’s thickness
Cloud’s height above the surface
Liquid, ice or both
On average clouds Cool the Earth

15. Summary: Heating the Atmosphere
The atmosphere is largely transparent to incoming shortwave (high energy) radiation
The atmosphere is more absorptive of the longwave (lower energy) emitted by the Earth
The atmosphere is HEATED from the GROUND UP
Explains why temperature decreases in the troposphere

16. Up NEXT……Energy Balance + Seasons
Earth’s average temperature remains fairly constant
There is a balance of incoming and outgoing radiation
Otherwise we’d keep heating up or keep getting colder.
Seasons - Regulated by the amount of solar energy received by the surface.

17. Heat transfer and balance in the atmosphere
FIGURE 2.14 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.

18. Latitudinal Heat Balance
Why don’t the tropics keep getting hotter and the poles keep getting colder?
The atmosphere and oceans MOVE and transfer energy from the equator towards the poles!
The energy imbalance drives ocean currents and winds!

19. Net surface solar radiation

20. Earth-Sun Relationships
FACT: Unequal heating is responsible for winds and ocean currents (Weather).
WINDS & PRESSURE
OCEAN CURRENTS

21. Earth-Sun Relationships
FACT: The Earth intercepts only a tiny percentage of the energy given off by the sun!
NOT TO SCALE!

22. Earth’s Motions Earth’s two principle motions are: Rotation Revolution
The spinning of Earth about its axis
Why we have day and night
Movement around the Sun
Travels at nearly 113,000 km/hr (70,000 miles/hr)
Elliptical orbit – not perfectly circular
(exaggerated!)

23. *****NOT TO SCALE!!*****
Earth’s Motions
Elliptical Orbit – Eccentricity is ~0.8
NH Winter January 3rd
NH Summer July 4th
152.1 million km
147.3 million km
Aphelion
Perihelion
FARTHEST from the SUN
CLOSEST to the SUN
*****NOT TO SCALE!!*****

24. The Seasons Primarily due to:
FACT: Variations in the distance between the Sun and Earth DO NOT cause the seasonal temperature change!
Primarily due to:
Change in the length of day accounts for some.
Gradual change in the angle of the sun at noon.
Affects the amount of energy received at Earth’s surface
When overhead  strongest
Lower angle  less intense

25. Length of day
Angle of incoming sunlight: path length through atmosphere ; intensity per unit surface area
FIGURE 2.19 During the Northern Hemisphere summer, sunlight that reaches the earth’s surface in far northern latitudes has passed through a thicker layer of absorbing, scattering, and reflecting atmosphere than sunlight that reaches the earth’s surface farther south. Sunlight is lost through both the thickness of the pure atmosphere and by impurities in the atmosphere. As the sun’s rays become more oblique, these effects become more pronounced.

26. Angle of incoming sunlight  intensity per unit surface area
Same flash light
FIGURE 2.16 Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays.
Larger lighted area
Smaller radiation per unit area

27. Alaska… Land of the Midnight Sun
Latitude (degrees)
NH Summer Solstice
(Jun 21)
NH Winter Solstice
(Dec 21)
Equinoxes
(Mar. 20, Sep. 22)
12 hr 10
12 hr 35 min
11 hr 25 min 20
13 hr 12 min
10 hr 48 min 30
13 hr 56 min
10 hr 04 min 40
14 hr 52 min
9 hr 8 min 50
16 hr 18 min
7 hr 42 min 60
18 hr 27 min
5 hr 33 min 70
2 months
0 hr 00 min 80
4 months 90
6 months
Length of Daylight
Oahu, Hawaii
° N, ° W
Based on Table 2-3 from your text!
Circle of Illumination
Boundary separating the dark and light halves of the Earth
Alaska… Land of the Midnight Sun

28. The Seasons
SUMMARY: Most important reasons for variations in the amount of solar energy:
Seasonal changes in the angle at which the Sun’s rays strike the surface
Changes in the length of daylight

29. Earth’s Orientation Why does length of day and sun angle change?
Earth’s orientation to the sun constantly changes!!!

30. WITHOUT THE TILT WE WOULD NOT HAVE SEASONS!
Earth’s Orientation
Earth’s Axis
Imaginary lines through the poles!
Inclination of the axis:
23.5° from the perpendicular
WITHOUT THE TILT WE WOULD NOT HAVE SEASONS!
23.5°

31. Earth’s Orientation Solstices Summer Solstice Winter Solstice
Tropic of Cancer
Northern limit of the Sun’s rays
23.5 N Latitude
June 21-22
First day of NH summer
NH tilted toward Sun
Winter Solstice
Tropic of Capricorn
Southern limit of the Sun’s rays
23.5 S Latitude
December 21-22
First day of NH winter
NH tilted away from Sun

32. Earth’s Orientation Equinoxes Autumnal Equinox Spring Equinox
Occur mid way between the Solstices
Vertical rays strike along the equator (0 deg latitude)
Earth not tilted toward or away
Autumnal Equinox
September 22 or 23
Spring Equinox
March 21 or 22
Also called Vernal Equinox

33. Less Tilt (<23.5) More Tilt (> 23.5) More extreme seasons
More mild seasons
Milder winters
Cooler summers
No Seasons if Tilt = 0
More Tilt (> 23.5)
More extreme seasons
Colder winters
Warmer summers
For Both Cases:
The location of Tropic of Cancer and Capricorn shift
For 30 Tilt: Tropic of Cancer = 30N; Tropic of Capricorn = 30S
For 20 Tilt: Tropic of Cancer = 20N; Tropic of Capricorn = 20S
The Arctic and Antarctic circles would shift too
For 30 Tilt: the Arctic = 60N; Antarctic circle = 60S (90 minus tilt)
For 20 Tilt: the Arctic = 70N; Antarctic circle = 70S (90 minus tilt)

34. Key Information 1
Know the difference between ABSORPTION, EMISSION, and REFLECTION (ALBEDO) and what they have to do with the atmosphere and weather (e.g. clouds).
Atmospheric Gases are selective absorbers and absorb and emit IR Radiation and play a role in heading the atmosphere.
Energy is returned to space via reflection and emission
Albedo: is the percentage of radiation reflected back to space.
Clouds can both warm and cool the depending on the time of day, the cloud’s thickness, the cloud’s height above the surface, and if it is made of liquid, ice or both
You can just read this slide.

35. Key Information 2
Be able to describe what is meant by EQUILIBRIUM with regard to the Earth and why we care.
Earth’s average temperature remains fairly CONSTANT!
There is a balance of incoming and outgoing radiation, otherwise we’d keep heating up or keep getting colder.
The tropics don’t keep getting too hot because the atmosphere and oceans MOVE and transfer energy from the equator towards the poles!
You can just read this slide.

36. Key Information 3
Have an understanding of why the Earth has SEASONS.
The Earth has 4 seasons
Summer Solstice (shortest day) ~ June 21-22
Winter Solstice (longest day) ~ Dec 21-22
Autumnal Equinox (12 day/12 night) ~ Sept 21-22
Vernal Equinox (12 day/12 night) ~ Mar 21-22
The Earth has SEASONS primarily because:
Gradual change in the angle of the sun at noon.
Change in the length of day accounts for some.
Both of these affects the amount of energy received at Earth’s surface
When overhead  strongest
Lower angle  less intense
The Length of Day and Sun Angle change due to the TILT OF THE EARTH
You can just read this slide.