iIptIIt

EXERC'SE

Earth-Sun Relations

To life on this planet, the relations between Earth andthe Sun are perhaps the most important of all astro-nomical phenomena. The variations in solar energystriking Earth as it rotates and revolves around theSun cause the seasons and therefore are Em appropriatestarting point for studying weather and climate.

In this exercise you will investigate the reasonswhy the amount of solar radiation intercepted by Earthvaries for different latitudes and changes throughoutthe year at a particular place. The next exercise, Exer-cise Thirteery examines how the atmosphere is warmedby this radiation.

ObjectivesAfter you have completed this exercise, you should beable to

1. Describe the effect that Sun angle has on theamount of solar radiation a place receives.

2 Explain why the intensity and duration of solarradiation varies with latitude.

3. Explain why the intensity and duration of solarradiation varies at any one place throughout theyear.

4. Describe the significance of these special parallelsof latitude: Tropic of Cancer, Tropic of Capricom,Arctic Circle, Antarctic Circle, and equator.

5. Diagram the relation between Earth and the Sunon the dates of the solstices and equinoxes.

6. Determine the latitude where the overhead Sun islocated on any day of the year.

7. Calculate the noon Sun angle for any place onEarth on any day.

8. Calculate the latitude of a place using the noonSun angle.

AntarcticCircle

solsticeequinoxanalemmanoon Sun

angle

lntroductionWeather is the state of the atmosphere at a particularplace for a short period of time. The condition of theatmosphere at any place and time is described by mea-suring the four basic elements of weather: tempera-fure, moisture, air pressure, and wind. Of all the con-trols that are responsible for causing global variationsin the weather elements, the amount of solar radiationreceived at any location is the most important.

Solar Radiationand the SeasonsThe amount of solar energy (radiation) striking theouter edge of the atmosphere is not uniform over theface of Earth at any one time, nor is it constant through-out the year at any particular place. The amount of so-lar radiation received at any particular place on Earth,

12

Materialsmetric ruler colored pencilsprotractor calculatorMaterials Supplied by Your lnstructorglobelarge rubber band or string

Termsweather calorieweather element solar constantweathercontrol equatorsolar intensity Tropic of Cancersolar duration Tropic of Capricomlangley Arctic Circle

186 Part Three/Meteorology

on any given day, is determined by the Sun's intensifyand duration. Intensity is the angle at which the rays ofsunlight strike a surface, whereas duration refers to thelength of daylight.

The standard unit of solar radiation is the langley,equal to one caloriel per square centimeter. The solarconstant, or average intensity of solar radiation fallingon a surface perpendicular to the solar beam at theouter edge of the atmosphere, is about 2 langleys perminute. As the radiation passes through the atmos-phere, it undergoes absorption, reflection, and scatter-ing. Therefore, at any one location, less radiationreaches Earth's surface than was originally interceptedat the upper atmosphere.

Solar Radiation and LatitudeThe amount of radiation striking a square meter at theouter edge of the atmosphere, and eventually Earth'ssurface, varies with latitude because of a changing Sunangle. To illustrate this fact, answer questions L-11 us-ing the appropriate figure.

1. On Figure 12.1, extend the 1 cm wide beam of sun-light from the Sun vertically to point A on the sur-face. Extend the second 1 cm wide beam,beginning at the Sun, to the surface at point B.

Notice in Figure 12.1 that the Sun is directly over-head (vertical) at point A and the beam of sunlightstrikes the surface at a 90o angle above the horizon.

lThe most familiar energy unit used to measure heat is thecalorie, which is the quantity of heat energy needed to raise thetemperature of one gram of water one degree Celsius. Do notconfuse it with the so-called large Calorie (note the capital C), thekind counted by weight watchers. A Calorie is the amount of heatenergy needed to raise the temperature of a kilogram (1000 grams)of water one degree Celsius.

Figura 12.1 Vertical and obliqueSun beams.

Using Figure 12.1,, answer questions 2-5.

2. Using a prohactor, measure the angle between thesurface and the beam of sunlight coming from theSun to point B.

o = angle of the Sun above the surface(horizon) at point B.

3. What are the lengths of the line segments on thesurface covered by the Sun beam at point A andpoint B?

PointA:

-

mm point B:

-

mm

4. Of the two beams, beam (A, B) is more spread outat the surface and covers a larger area. Circle youranswer.

5. More langleys per minute would be received bya square centimeter on the surface at point (A, B).Circle your answer.

Use Figure 12.2to answer questions 6-11 concern-ing the total amount of solar radiation intercepted byeach 30o segment of latitude on Earth.

6. With a metric ruler, measure the total width of in-coming rays from point x to point y tnFigarcI2.2.The total width is

-

centimeters (-millimeters). Fill in your answers.

7. Assume the total width of the incoming rays frompoint x to point y equals 100% of the solar radia-tion that is intercepted by Earth. Each centimeterwould eoual % andeach millimeter wouldeoual %. FilI in vour answers.

g--C_Gqq@G=G=C=++g=gg=tGt}=++ggsg*&+C-g=g=tseg=F-g-F

Exercise Twelve/Earth-Sun Relations 187

Figure 12.2 Distribution ofsolar radiation per 30o segmentof latitude on Earth.

0o-30o =

-

30'-60' =

-60o-90o =

Angle a:

Angleb:

\A/hat percentage of the total incoming radiationis concentrated in each of the following zones?

9.

10.

Use a protractor to measure the angle between thesurface and Sun ray at each of the following loca-tions. (Angle b is already done as an example.)

mm=-7omm=

-%mm=

-%angle c:

angle d:

What is the general relation between the amountof radiation received in each 30o segment and theangle of the Sun's rays?

11. Explain in your own words what fact about Earthcreates the unequal distribution of solar energy/even though each zone rePresents an equal 30'segment of latitude.

Yearly Variation in Solar EnergYThe amount of solar radiation received at a particularplace would remain constant throughout the year if itwere not for these facts:

o Earth rotates on its axis and revolves aroundthe Sun.

o The axis of Earth is inclined 23'5' from theperpendicular to the plane of its orbit-

. Throughout the year, the axis of Earth pointsto the same place in the sky, which causes theoverhead (vertical or 90') noon Sun to crossover the equator twice as it migrates from theTropic of Cancer (23.5'N latitude) to theTropic of Capricorn (23.5'5 latitude) and backto the Tropic of Cancer.

As a consequence, *te position of the vertical oroverhead noon Sun shifts between the hemispheres,causing variations in the intensity of solar radiation andchanges in the length of daylight and darkness.The sea-

sons are the result of this changing intensity and duration ofsolar energy and subsequent huting of the atmosphere.

To help understand how the intensity and dura-tion of solar radiation varies throughout the year, an-swer questions 12-31 after you have thoroughly re-viewed the appropriate chapter of your text and haveexamined the location of the Tropic of Cancer, Tropicof Capricorn, Arctic Circle, and Antarctic Circle on aglobe or world map.

o

oo60

188 Part Three/MeteorologY

12. List some of thespecial parallels

Tropic of Cancer:

Tiopic of Capricom:

Arctic Circle:

1,3. Write the date represented by each position ofEarth at the appropriate place in Figure 12'3' Thenlabel the following on Earth at an equinox and asolstice position.

North Pole and South PoleAxis of EarthEquator, Tropic of Cancer, Tiopic of CapricornArctic Circle and Antarctic CircleCircle of illumination (day-night line)

Questions 1'4-!9 rcIer to the June solstice positionof Earth in Figure L2.3.

14. What term is used to describe the June 21'-22 datein each hemisphere?

countries each of the followingof latitude passes through.

Northem HemisPhere:Southem Hemisphere:

L5. OnJune 21--22theSun's rays are perpendicular toEarth's surface at noon at the (Tropic of Cancer,equator, Tropic of Capricorn)' Circle your answer'

15. \A/hat latitude is receiving the most intense solarenergy on |une 21--22?

Latitude:

17. Toward what direction, north or south, would youlook to see the Sun at noon on hne 21-22 if youlived at the following latitudes?

40'N latitude:10oN latitude:

1,9. On ]une 2l-22,latitudes north of the Arctic Circleare receiving (6,12,24) hours of daylight, whilelatitudes south of the Antarctic Circle are experi-encing (6,12,24) hours of darkness. Circle youranswers.

Questions 2014tefer to the December solstice po-sition of Earth in Figure 12.3.

20. What name is used to describe the December2l-22 date in each hemisPhere?

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25. For those livingwhat terms aredates?

26. For those living in the Southem Hemisphere, whatterms are used to describe the following dates?

March 21: equnoxequinox

27. On March 21 and September 22the Sun's rays areperpendicular to Earth's surface at noon at the(ftopi" of Cancer, equator, Tropic of Capricorn)'Circle your answer.

March2l:September 22:

September 22:

solsticesolstice

NorthemHemisphere:Southem HemisPhere:

21. On December 21'-22 the Sun's rays are perpen-dicular to Earth's surface at noon on the (Tropicof Cancer, equator, Tropic of Capricorn)' Circleyour answer.

22. On December 21'-22 the (Northern, Southern)Hemisphere is receiving the most intense solarenergy.

23. If youlived at the equator, on December2l-22youwould look (north, south) to see the Sun at noon'

24. Refer to Table 12.1, "Length of daylight'" Howmany hours of daylight would there be at each ofthe following latitudes on December 21-22?

90'N latitude: hrs

40'S latitude:

-

hrs

40'N latitude:

-

hrs

90'S latitude:

-

hrs

0'latitude:

Questions 25-31 refer to the March and Septemberequinox positions of Earth in Figure 12.3.

solstice

solstice

Northern Hemisphere,describe the following

L8. Position a rubber band or string on a globe corre-sponding to the circle of illumination on |une21.-22. Then determine the length of daylight atthe following latitudes by examining the propor-tionate number of degrees of longitude a place lo-cated at each latitude spends in daylight as Earthrotates. (Nofe: Earth rotates a total of 360' of long-itude per day. Therefore, each 15o of longitude isequivalent to one hour.)

70oN latitude: hrs

in theused to

equlnoxequinox

40"S latitude:

-40'N latitude:

-90"S latitude: --0'latitude:

Exercise Twelve/Earth-5un Relations 189

Figure 12.3 Earth-Sun relations.

EQUINOX

EOUINOX

WinterSolstice Equinoxes

Latitude(degrees)

SummerSolstice

0102030405060708090

28. \A/hat latitude is receiving the most intense solarenergy on March 2L andSeptember 22?

Latitude:If you lived at 20'S latitude, you would look(north, south) to see the Sun at noon on March 21and September 22. Circle your answer.

\zVhat is the relation between the North and SouthPoles and the circle of illumination on March 21and September 22?

31. Write a brief statement describing the length ofdaylight everywhere on Earth on March 21. andSeptember 22.

As you have seery the latitude where the noon sunis directly overhead (vertical, or 90o above the horizon)is easily determined for the solstices and equinoxes.

Figure 12.4 is a graph,-called an analemma, thatcan be used to determine the latitude where the over-head noon Sun is located for any date. To determinethe latitude of the overhead noon Sun from theanalemma, find the desired date on the graph and readthe coinciding latitude along the left axis. Don't forgetto indicate North or South when writing latitude.

Using a colored pencil, draw lines on Figure 12.4that correspond to the equator, Tropic of Cancer,and Tropic of Capricorn. Label each of these spe-cial parallels of latitude on the figure.Using the analemma, Figure 12.4, determinewhere the Sun is overhead at noon on the follow-ing dates.

December 10:

March 21:

May 5:

June22:August 10:

October 15:

The position of the overhead noon Sun is alwayslocated on or between which two parallels oflatitude?

oN (named the Tropic of ) and

12h12 h 35 min13 1213 5614 5216 18t8 2724h (for 2mo)24h(for 4mo)24h(for6mo)

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Figure 12.4 The analemma, a graph illustrating the latitude of the overhead (vertical) noon Sunthroughout the year.

35. The overhead noon Sun is located at the equatoron September _and March

-

To-gether, these two days are called the

-

Fillin your answers.

36. Write a brief paragraph summarizing the yearlymovement of the overhead noon Sun and how theintensity and duration of solar radiation variesover Earth's surface throughout the year.

Calculating Noon Sun AngleKnowing where the noon Sun is overhead on anygiven date (the analemma), you can determine the an-gle above the horizon of the noon Sun at any other lat-itude on that same day. The relation between latitudeand noon Sun angle is

For each degree of latitude the place is away from thelatitude where the noon Sun is overhead, the angle ofthe noon Sun becomes one degree lower fuombeing ver-tical (or 90') above the horizon.

For example, a place which is 30' of latitude awayfrom the latitude where the overhead noon Sun is 1o-cated would have a noon Sun angle of 60o (90o- 30o =60'). A place 60' of latitude away would have a 30onoon Sun angle (see Figarcl22).

37. Complete Table 12.2by calculating the noon Sunangle for each of the indicated latitudes on thedates given. Some of the calculations have alreadybeen done.

38. From Table '1.2.2, the highest average noon Sunangle occurs at (40'N, 0', 20'S). Circle youranswer.

Exercise Twelve/Earth-Sun Relations 191

39. Calculate the noon Sun angle for your latitude ontoday's date.

Date:

Latitude of overhead noon Sun:

Your latitude:

Your noon Sun angle:

Calculate the maximum and minimum noon Sunangles for your latitude.

40.

Maximum NoonSun Angle

Date:

Minimum NoonSun Angle

Date:Angle: Angle:

41. Calculate the average noon Sun angle (maximumplus minimum, divided by 2) andthe range of thenoon Sun angle (maximum minus minimum) foryour location.

Average noon Sun angle =Range of the noon Sun angle =Describe some situations in which knowing thenoon Sun angle mightbe useful.

Using Noon Sun AngleOne very practical use of noon Sun angle is in naviga-tion. You, like a navigator, can determine your latitudeif the date and angle of the noon Sun at your locationare known. As you answer questions 43 and 44, keepin mind the relation between latitude and noon Sunangle.

43. \A/hat is your latitude if, on March2l.,youobservethe noon Sun to the north at 1Bo above the hori-zon?

Latitude:

What is your latitude if, on October L6, you ob-serve the noon Sun to the south at 39o above thehorizon?Latitude:

Solar Radiation at the Outer Edgeof the AtmosphereTable 12.3 shows the average daily radiation receivedat the outer edge of the atmosphere at select latitudesfor different months.

42.

ETTE={*{,

44.Mar21 Apr 11Latitude ofoverhead = (___l L__lnoon Sun

Jun 21 Dee22

L*-l ( )

Noon Sun Angle90'N40'N

00

20"s

26tt"500

620

66%

0325840

50 1050 50700 950 720890 780 880

90'N40"N

0o

192 Part Three/Meteorology

Latitude March June September December

To help visualize the pattern, plot the data fromTable 12.3 on the graph in Figure L2.5. Using a dif-ferent color for each latitude, draw lines through themonthly values to obtain yearly curves. Then answerquestions 4548.

45. \A/hy do two periods of maximum solar radiationoccur at the equator?

In |une, why does the outer edge of the atmos-phere at the equator receive less solar radiationthan both the North Pole and 40"N latitude?

Why does the outer edge of the atmosphere atthe North Pole receive no solar radiation in De-cember?

Figure 12.5 Graph of solarradiation received at the outer edgeof the atmosphere.

48. \A/hat would be the approximate monthly valuesfor solar radiation at the outer edge of the atmos-phere at 40oS latitude? Explain how you arrived atthe values.

March:

fune:

September:

December:

Explanation:

Earth-Sun Relationson the lnternetUnderstanding weather and climate begins with asolid familiarity of the scientific principles involved inatmospheric heating and cooling.Apply the conceptsfrom this exercise to an examination of solar and ter-restrial radiation by completing the corresponding on-line activity onthe Applications & Inaestigations in EarthScience w ebsite at http://prenhall.com/earthsciencelab.hr this web-based exercise you will:

Explore an online Earth-Sun geometry interactivelab.Use an online Sun angle calculator to determinethe noon Sun angle at any latitude, on any date.Investigate the surface radiation budget for a sta-tion near your location.

46.

47.

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