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Chapter 2 Notes(MEA)

# Chapter 2 Notes(MEA) - Chapter 2 Heating Earth's Surface...

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Chapter 2: Heating Earth’s Surface and Atmosphere Earth-Sun Relationships - Solar radiation accounts for more than 99.9% of the energy that heats the earth and its atmosphere - This energy is not distributed evenly, however, as it varies by: - latitude; time of day; time of year. - It is this uneven distribution of energy that creates most of what we consider to be weather. Earth’s Motions - Rotation: earth rotates on its axis once per 24 hours - Revolution: earth orbits around the sun once per 365.25 days The Seasons - Regulated by the amount of solar radiation received by the surface of the Earth; which is dictated by: 1) angle at which the radiation strikes the surface, - radiation striking the earth perpendicularly is: - much more intense than that striking at an angle (Fig 2.3)

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- Also traverses through less atmosphere (Fig 2.4, Table 2.1) 2) length of the “day” (# of daylight hours) - longer days allow for more radiation to be absorbed by the earth’s surface (Table 2.2) Earth’s Orientation - These changes are due to the Earth being tilted by 23.5 o from the plane of its elliptical orbit around the sun (Fig 2.5) The Earth’s axis (center of rotation) points in the same direction into space all year long; as a result: The NH: is tilted toward the sun in the summer
is tilted away from the sun in the winter Solar Elevation Angle (Noontime) Figure 2.6 and Box 2.2 (The Analemma) provide ways to calculate the solar elevation angle ( 5 ) at local noon for any latitude and time of year. This angle can also be calculated using the following equation: ( 29 + × × - Φ - = 365 ) 10 ( 360 cos 5 . 23 90 N o o β where: Φ is the latitude N is the Julian day of the year(1…365.25) Once we know the solar elevation angle ( 5 ), we can then calculate the: Solar Radiation (Noontime ) β sin 1367 2 × = meter Watts J which provides an instantaneous measure of the intensity of the solar radiation (Watts) per square meter (ignoring effects of atmosphere).

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Solstices and Equinoxes (Northern Hemisphere) Summer Solstice “Astronomical start of summer” ( ~June 21 : Julian Day 173) Sun is at its highest position in the sky, as a result, radiation shines down on the surface: - more directly than at any other time of the year. - Longest “day” of the year - each latitude (in NH) will have: More than 12 hours of sunlight - the farther north you go, the greater the # of daylight hours (Table 2.2) - from Arctic Circle (66.5 o N) north, 24 hours of sunlight - The noon sun is directly overhead at 23.5 o N, the: Fig. 2.7a Tropic of Cancer
Exercise : Calculate the noon solar elevation angle for Raleigh (35 o N) at the summer solstice (Day 173). + × × - - = 365 ) 10 173 ( 360 cos 5 . 23 35 90 o o o β = β 78.5° Modified (for Raleigh) Fig. 2.2a Exercise: Calculate the noon solar radiation for Raleigh 5 . 78 sin 1367 2 × = meter Watts J

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= J 1339.6 Watts/meter 2 - The sun is never directly overhead Raleigh (or any latitude > 23.5°N).
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Chapter 2 Notes(MEA) - Chapter 2 Heating Earth's Surface...

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