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SOLAR ENERGY

SOLAR ENERGY. ► The amount of insolation reaching the earth’s outer atmosphere varies with distance and variations of the earth’s orbit . This causes fluctuations of up to 4% on a time scale of centuries or more.

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SOLAR ENERGY

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  1. SOLAR ENERGY ► The amount of insolation reaching the earth’s outer atmosphere varies with distance and variations of the earth’s orbit. This causes fluctuations of up to 4% on a time scale of centuries or more. ► Solar energy is transmitted to earth in the form of short and long wave (SW and LW) radiation, since the sun is very hot. SW is light (visible and ultraviolet). LW is heat. ► The earth also emits radiation (all bodies at a temperature above -273°C do); this radiation is almost entirely long wave (LW) since it is cooler than the sun. Incoming Solar Radiation, orINSOLATION is the energy which drives the atmospheric system. ►Over the last 4.6 billion years (the age of the Earth), the sun’s output has increased around 30%

  2. Radiation Basics The sun emits all types of radiation. Anything above .0007 mm is Long Wave (LW). Below .0007 mm is Short Wave (SW). SW LW

  3. Insolation received at the earth’s surface varies with latitude. The higher angle of the sun in the sky at the equator conveys more energy per unit area than at higher latitudes. GLOBAL VARIATIONS in INSOLATION • A given amount of radiation covers a smaller area when overhead than when at a low angle; it is more concentrated • Radiation passes through a greater length of atmosphere when at a low angle in the sky than when overhead. Atmospheric gases, dust and vapor absorb more energy before it reaches the earth’s surface. Low sun (as sunset) shows more red due to atmospheric dust and pollution A high sun is more effective than a sun low in the sky

  4. The time of year causes the sun’s angle in the sky and the amount of daylight

  5. Lengths of day and night vary more between the seasons at higher latitudes. This makes climate more seasonal at the poles than the equator Six months daytime (March-Sept), six months night (Sept - March) DAY LENGTH One day with 24 hours daylight (June 21st); one day with 24 hours darkness (Dec 21st) North Pole 90°N Arctic circle 66.5°N Sun is overhead once a year (June 21st). Day length always at least 10 hours. More seasonal Tropic of Cancer 23.5°N Constant day length - 12 hours day and night all year round Equator 0° Tropic of Capricorn 23.5°N More seasonal Sun is overhead once a year (Dec 21st). Day length always at least 10 hours. Antarctic circle 66.5°S One day with 24 hours daylight (March 21st); one day with 24 hours darkness (June 21st) South Pole 90°S Six months daytime (Sept - March), six months night (March-Sept)

  6. Energy must be transferred from Equator to Poles

  7. At 40°N and S, there is a balance of insolation with outgoing LW radiation over the year. Insolation exceeds LW radiation in the daytime, and in summer; the reverse is true at night and in winter. Polewards of 40°N and S, there is an annual heat deficit, despite periods of surplus during some days and in summer. Equatorwards of 40°N and S, there is an annual heat surplus despite periods of deficit at night and in winter. Without movement of heat energy, the poles would become steadily colder and colder, while the equator would get progressively warmer. This clearly does not happen. This heat transfer (flux) occurs by: Global Energy Flux Ocean currents; cold polar water flows towards the equator while warm water flows from equator to pole. Winds which blow warm air to towards the poles and cold to the equator. An excess of evaporation which takes up and stores latent heat in water vapor, releasing it towards the pole where it condenses.

  8. ColdSea temperatureWarm OCEAN CURRENTS Labrador current carries cold water from equator to pole down east coast of N.America Gulf Stream carries warm water from equator towards the pole, and NW Europe Surface currents generally do not cross the Equator. A similar counter-clockwise movement of warm water polewards, and cold water equatorwards can be seen in the Pacific.

  9. POLES - Precipitation exceeds evaporation at high latitudes; condensation releases latent heat stored in water vapor. HEAT TRANSFER by HUMIDITY LATENT HEAT is also transferred from sea to land in this way. Evaporation exceeds condensation over oceans (which uses heat); condensation is greater over land, which is heated up - especially in winter. EQUATOR - Evaporation exceeds condensation (and precipitation); this uses heat energy and stores it in the form of water vapor. Red shows high humidity from high rates of evaporation

  10. Cold, north winds blowing south. Warm south winds blowing north. A typical situation that helps correct the energy imbalance. Winds Winds Winds

  11. Jan 20-22, 2014

  12. Calculating the Earth’s Energy Transfer Generally, this does not require higher mathematics but it does require logic. A good example of logic was used by an ancient Greek mathematician, Eratosthenes

  13. Eratosthenes’ Calculation of the Earth’s circumference This is an ungraded exercise so the class can see what the graded ones will be like. At Noon of the Summer Solstice, the sun angle at Alexandria is 7.1 deg but at the same time, it is directly overhead at Syene (Aswan) at 23.5 N. How did Eratothenes calculate the circumference of the Earth from this data? Hint: How many degrees in a complete circle?

  14. Next challenge (more climatological): How much energy does the Earth receive from the sun? i.e., how do we calculate the Solar Constant which is 1.98 cal cm-2 min-1? E = σT4 where T = 5800K and σ = 8.128 x 10-11 cal cm-2 min-1 K-4. Also the radius of the sun is 6.96 x 1010 cm and the radius of the Earth is 6.37 x 108 cm. The radius of the Earth’s orbit is 150 million km. The surface area of a sphere is 4πr2 (assume both sun and Earth are spheres). (Hint: Calculate the total number of calories leaving the sun each minute, then follow that energy until it reaches the Earth. Draw a picture of the sun emitting energy.) The important part is how you get to the answer. Explain fully, using complete sentences. Grammar and spelling will be graded as part of this. Due MONDAY.

  15. The Solar “Constant” is the energy received at the average Earth-Sun distance. At any particular point on the Earth, calculating the energy received is more complex. Without an atmosphere, energy depends solely on latitude. At any particular time, the energy received is on the curve. For the year, the energy received is the area under the curve. While the poles get more at the summer solstice, overall, they get less energy.

  16. Energy Budget What comes in versus what goes out.

  17. This is the solar constant. (Scattering) Adding an atmosphere introduces clouds, reflection, and scattering.

  18. The Earth’s Energy Budget is Balanced (not always true for people)

  19. ENERGY CASCADE - DAY The budget on the previous slide is an average. Fluxes will change with time of year and time of day

  20. ENERGY CASCADE -NIGHT

  21. Albedo is reflection (%)

  22. Next: How does radiation lead to temperature climatology?

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