1. GEU 0047: Meteorology Lecture 3 Seasonal Cycle

2. Reasons of Seasonality and its Changes
Earth’s Tilt (Seasonal)
Tilt of rotational axis with respect to its orbit around sun
Solar Radiation (Diurnal or Daily)
More Direct Sun Light
Longer Duration of Day Light Hours (Length of day)
Orbit Variations (Very Long-Term, Small Effects)
tilt
Eccentricity (偏心率)
Precession (進動)
Eccentricity:怪癖; 偏心率
e = (rmax – rmin)/(rmax + rmin) [ currently, e = for earth ]
Over thousands of years, the eccentricity of the Earth's orbit varies from nearly to almost as a result of gravitational attractions between the planets
Precession:先行, 歲差;進動
the slow backward motion of the equinoctial points along the ecliptic, at the rate of 50.2[sec] annually, caused by the action of the sun, moon, and planets, upon the protuberant matter about the earth's equator, in connection with its diurnal rotation; -- so called because either equinox, owing to its westerly motion, comes to the meridian sooner each day than the point it would have occupied without the motion of precession, and thus precedes that point continually with reference to the time of transit and motion.
由於地球的外型成微扁又上下凹陷的「梨」形。因此當受到太陽傾斜23.5度的引力時，會產生不對稱的引力，這個引力對於地球的運動造成擾動的力矩。我們知道有轉動體受到外加的力矩後，會產生角動量變化。這個現象我們稱為進動。地球的進動速率不會太快，它的運動週期約25600年。換句話說，我們制訂的黃道與赤道交角稱為春分點，這也是赤道座標系統與黃道座標系統的參考點。由於春分點受到進動的影響，會沿著赤道面滑動，其週期為25600年。

4. Tilt
Earth’s rotational axis remains fixed with respect to
the stars.

5. Earth’s Circle of Illumination
50 % of the earth’s
surface is illuminated
by the sun,
every hour
of every “day”.
Illumination:照明
Circle of illumination: boundary between day and night. It always bisects the equator.

6. Incident Angle

7. Changing Aspect Angle (方向角)
Direct Sunlight is more intense and concentrated.
Solar Incidence Angle is Latitude and Time Dependent
Seasonal and Diurnal
Aspect angle:方向角

8. Summer Solstice
Sunlight is most direct at the Tropic of Cancer (23.5 N) on ~ June 21, less direct elsewhere.
Solstice:至點
cancer: crab:螃蟹
Tropic of capricorn (磨羯座)

9. SOLSTICE Summer (June 21st-ish)
＝> Sun Located at the Tropic of Cancer
Winter (December 21st-ish)
＝> Sun Located at the Tropic of Capricorn
Tropic of Cancer
Tropic of Capricorn
Equator
Prime Meridian

10. June Solar Insolation

11. Winter Solstice
Sunlight is most direct at the Tropic of Capricorn (23.5 S) on ~ January 21.

12. Solar Insolation
Energy received by the earth at various latitudes for December 21 (solstice)

13. December Solar Insolation

14. Equinox Equinoxes (Equal Day and Equal Night, 12 hours)
Spring (Vernal Equinox) March 21st
Fall (Autumnal Equinox) September 21st
Sun Located on Celestial Equator (Earth’s Equator)
Tropic of Cancer
Tropic of Capricorn
Equator
Prime Meridian
Vernal:春天的,青春的
Vernal bloom; vernal day of life
L. vernalis; F. vernus

15. Seasonal Orientations

16. Day Light Hours (June Solstice)

17. Day Light Hours (Equinox)

18. Day Light Hours (c.f., Table 3.1)

19. Combined Annual Average Solar Insolation

20. Fictitious Extreme Seasons Tilt

21. Solar Altitude Angle (Solstice)

22. Seasonal Solar Latitude

23. Solar Altitude Angle Solar Altitude at Local Noon of Taipei
What is your Latitude? 25N
What is the Date? Oct 25

24. Solar Altitude Angle Solar Altitude at Local Noon of Taipei
What is your Latitude? 25N
What is the Date? Oct 25
What is the apparent latitude (declination) of the Sun today? ~10oS

25. Solar Altitude Angle Solar Altitude at Local Noon of Taipei
What is your Latitude? 25N
What is the Date? Oct 25
What is the apparent latitude (declination) of the Sun? ~10S
Calculate the distance in degrees between your latitude and the declination of the sun N + 10 S = 35 degrees

26. Solar Altitude Angle Solar Altitude at Local Noon of Taipei
What is your Latitude? 25N
What is the Date? Oct 25
What is the apparent latitude (declination) of the Sun? ~10S
Calculate the distance in degrees between your latitude and the declination of the sun. 25 N + 10 S = 35 degrees
90 degrees minus the latitude declination difference will be the solar altitude at local noon for your location on that date = 55

27. Altitude-Azimuth Taipei 2011 Oct 25 Zenith Path of the Sun 55 degreesW

28. An environmental issue Solar Homes Application
Position Angle of Solar Panels

29. Zenith
There is much less atmosphere to travel through directly overhead than on the horizon. The sun is more direct and intense.

30. Diurnal Solar Intensity
By the time the sun gets on the horizon, it is at a grazing angle and heavily scattered, refracted and absorbed.

31. Diurnal Temperature

32. Thermal Inertia Noon June 21 For Any Given Northern Latitude
Maximum Daily Solar Insolation Occurs At
Maximum Yearly Solar Insolation Occurs On
Hottest Hour of the Day (on average)
Hottest Month of the Year (on average)
Noon
June 21

33. Thermal Inertia Noon June 21 4pm August
For Any Given Northern Latitude
Maximum Daily Solar Insolation Occurs At
Maximum Yearly Solar Insolation Occurs On
Hottest Hour of the Day (on average)
Hottest Month of the Year (on average)
It takes a while to heat up and cool down!
Noon
June 21
4pm
August

34. Local Noon Shadow

35. Local Noon Shadow

36. Equinox Shadow
Sun rises
due East,
Sun sets
due West

37. sundial 8a.m. Made by Egyptian 1500 BC 4p.m.
Sundial bridge (Redding California)

38. Summary Earth-Sun Geometry Seasons Solar Energy
Seasonal (Latitude) Changes
Diurnal (Daily) Changes

39. HW#2 (due date: TBA, because it depends on weather)
Perform the field experiment (you may need a partner)
Calculate local (e.x. Taipei) solar altitude angle someday in recent days
Obtain the solar declination angle in the day you perform the measurement