1. Physics of Planetary Climate Cors221: Physics in Everyday Life Fall 2010 Module 3 Lecture 3: Seasons

2. From Last Time In equilibrium, a body in space emits the same amount of energy as it absorbs Equilibrium Temperature Earth's equilibrium T = 255K (-18 o C!). Teq is good for airless worlds, not for ones with air Transmission + Reflection + Absorption =1 Gases in Earth's atmosphere absorb at different wavelengths The greenhouse effect comes about because atmospheric gases transmit sunlight at visible wavelengths, but absorb Earth's blackbody emission in the infrared #1: H 2 O #2: CO 2 #3: CH 4 T = (AF / (4  a 2 )).25 This has been true throughout the past 500 million years

3. Equator to Pole Temperatures

4. Incidence Angle

5. Incidence Angles on a Planet Saturn's moon Titan Graphic of incidence angle

6. Incidence Angle and Sunlight

7. Sunlight Falloff with Incidence

8. Temperature Falloff with Incidence F cos(i) =  T 4

9. Moon Thermal Image

10. Earth Thermal Image NN

11. Temperature on Earth (a rapidly-rotating planet) at Equinox F cos(i) /  =  T 4

12. Obliquity Earth's obliquity = 23.44 degrees

13. Obliquity Changes Incidence Earth's obliquity = 23.44 degrees Moscow, Idaho’s latitude: 46.5 degrees

14. Obliquity Changes Day Length

15. Earth’s obliquity (axis tilt) changes incidence & day length changing the daily solar heating creating the seasons http://www.gaisma.com/en/location/fairbanks-alaska.htmlhttp://www.gaisma.com/en/location/fairbanks-alaska.html 65 N http://www.gaisma.com/en/location/moscow-idaho.htmlhttp://www.gaisma.com/en/location/moscow-idaho.html 46.5 N http://www.gaisma.com/en/location/mexico-city.htmlhttp://www.gaisma.com/en/location/mexico-city.html 20N http://www.gaisma.com/en/location/manta.htmlhttp://www.gaisma.com/en/location/manta.html 1 S http://www.gaisma.com/en/location/darwin.htmlhttp://www.gaisma.com/en/location/darwin.html 12 S http://www.gaisma.com/en/location/puerto-montt.htmlhttp://www.gaisma.com/en/location/puerto-montt.html 42 S http://www.gaisma.com/en/location/amundsen-scott--permanent-station-of-the-us.html

16. Note that seasons are NOT a function of DISTANCE from the Sun!

17. Key Points Incidence angle is how far the sun is from straight up. Solar heating is highest at 0 incidence; is lower at higher incidence as cos(i) Due to incidence heating effect, poles are cold, and morning and evening are cold(er). Without an atmosphere, the Moon’s surface temperature follows its heating very closely with incidence angle. Earth rotates fast enough to even things out some. Earth has an OBLIQUITY, or axis tilt, of 23.44 degrees Obliquity drives changes in incidence angle and day length throughout the year, creating the seasons. (seasons don’t have anything to do with distance from Sun)