GEU 0047: Meteorology Lecture 3 Seasonal Cycle

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 = 0.0167 for earth ] Over thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 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。

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

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.

Incident Angle

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

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)

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

June Solar Insolation

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

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

December Solar Insolation

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

Seasonal Orientations

Day Light Hours (June Solstice)

Day Light Hours (Equinox)

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

Combined Annual Average Solar Insolation

Fictitious Extreme Seasons Tilt

Solar Altitude Angle (Solstice)

Seasonal Solar Latitude

Solar Altitude Angle Solar Altitude at Local Noon of Taipei What is your Latitude? 25N What is the Date? 2011 Oct 25

Solar Altitude Angle Solar Altitude at Local Noon of Taipei What is your Latitude? 25N What is the Date? 2011 Oct 25 What is the apparent latitude (declination) of the Sun today? ~10oS

Solar Altitude Angle Solar Altitude at Local Noon of Taipei What is your Latitude? 25N What is the Date? 2011 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

Solar Altitude Angle Solar Altitude at Local Noon of Taipei What is your Latitude? 25N What is the Date? 2011 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. 90 - 35 = 55

Altitude-Azimuth Taipei 2011 Oct 25 Zenith Path of the Sun 55 degrees W

An environmental issue Solar Homes Application Position Angle of Solar Panels

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

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

Diurnal Temperature

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

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

Local Noon Shadow

Local Noon Shadow

Equinox Shadow Sun rises due East, Sun sets due West

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

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

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