Lecture #03. January 20, 2010, Wednesday

Transcription

1 Lecture #03 January 20, 2010, Wednesday Causes of Earth s Seasons Earth-Sun geometry Day length Solar angle (beam spread) Atmospheric beam depletion

2 Shape and Size of the Earth North Pole E Geoid: not perfectly spherical. ~ 8,000 mile diameter. W ~ 25,000 mile circumference. Rotates on an tilted axis running between the north and south poles. South Pole Completes a full rotation on its axis in ~ 24 hours.

3 Latitude Curves (circles) parallel to the equator. 90 o N Axis of Rotation 60 o N 30 o N Only the equator is a great circle. Equator 30 o 30 o 0 o Latitude is in degrees (0-90) north (N, positive) and south (S, negative) of the equator. 90 o S 60 o S 30 o S

4 Longitude Meridians. Each meridian runs through the two poles. All great circles. 90 o N Prime meridian 0 degree Longitude is in degrees (0-180) east (E, positive) or west (W, negative) of Greenwich, England (prime meridian). 90 o S

5 Earth s Revolution Around the Sun or 4 or 4 91 million mi 94 million mi Average distance from Sun = 93 million mi 3% difference in distances between Perihelion & Aphelion Inverse square law radiation intensity difference of 7% BUT, ecliptic orbit NOT the cause for seasons!!!

6 Constant tilt (23.5 o ) of Earth s axis of rotation points to the North Star (Polaris) 3 3 Earth is at aphelion 13 days after NH summer solstice

7 Solstices & Equinoxes

8 Hypothetical 90 o tilt from the plane normal to the ecliptic plane Changing circle of illumination relative to hemispheric latitudes through revolution cause of season

9 NH Summer Solstice 24 hour daylight Solar Angle Solar Declination = the latitude of subsolar point Antarctic Circle 24 hour night Circle of Illumination

10 Solar Declination Migrating between 23.5 o N (+23.5 o ) and 23.5 o S ( 23.5 o )

11 N Sun N 23.5 o S 23.5 o N Dec. solstice N S Sun June solstice N S 0 o 0 o Equinoxes

12 Day length varies with latitude & season: equinoxes Solar Angle at noon = 90 o latitude difference between the location of concern and the subsolar point Note: solar angle should be between 0 o (horizon) and 90 o (overhead).

13 Day length varies with latitude & season: summer solstice Solar Angle at noon = 90 o latitude difference between the location of concern and the subsolar point

14 Day length varies with latitude & season: winter solstice Solar Angle at noon = 90 o latitude difference between the location of concern and the subsolar point

15 Beam Spreading Same amount of energy over greater area reduced intensity

16 Beam Spreading The smaller the solar angle, the greater area of beam spreading A C B BC AB = sin ( = solar angle)

17 Solar Energy Distribution: Equinoxes 100 = 90 o 35 o + 0 o = 55 o 80 82% Greenville 20 Subsolar point Latitude

18 Solar Energy Distribution: Summer Solstice % = 90 o 35 o o = 78.5 o 60 Greenville Subsolar point Latitude

19 Solar Energy Distribution: Winter Solstice 100 = 90 o 35 o 23.5 o = 31.5 o % 20 Subsolar point Greenville Latitude

20 Solar Angle & Radiation Intensity The maximum incoming solar radiation intensity occurs at the subsolar point at noon Of that maximum, Greenville receives: 98 % at the Summer Solstice 82 % at both Spring & Autumnal Equinoxes 52 % at the Winter Solstice Building on a south-facing slope, you could increase radiation, and the increased % depends on the season, more potential in winter than summer.

21 Atmospheric Beam Depletion Shorter optical path Absorption Back scattering (reflection) Longer optical path

22 Summary Earth s seasons are manifested by the amount of insolation (incoming solar radiation) received during the course of a year. The constant tilt of the Earth s rotating axis relative to the ecliptic plane it revolves around the Sun over a course of a year is the reason of the Earth s seasons. The seasonal changes in Earth-Sun geometric relationship are demonstrated by the fact that, day length, solar angle, depth of atmosphere over which insolation must penetrates, are all functions of latitude and time of the year.

23 Which of the following is true? a) The Equator always gets 12 hours of sunlight b) The Sun is always directly overhead at noon on the Equator c) For a moment the Sun is directly overhead at Greenville, NC on the summer solstice d) Northern Hemisphere summers receive more incoming solar radiation than Southern Hemisphere summers