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Office Hours Tue: 12:30 PM to 2:30 PM Wed: 9:00 AM to 10:30 AM & 12:00 PM to 2:00 PM Thr: 9:00 AM to 10:30 AM Course Syllabus can be found at: This lecture will be posted AFTER class at:
Lesson 11 Solar Angle Hess, McKnight’s Physical Geography, 10 ed pp.
Sun Declination Recall from last week, the vertical rays (direct rays) are those from the sun that strike the Earth at a 90 ° angle. The location of these vertical rays changes throughout the year ◦ Located at the Tropic of Cancer during the summer solstice ◦ Located at the equator during the equinoxes. The latitude at any given time of the year where the sun’s vertical rays strike the Earth is known as the sun declination.
Sun Declination, cont. The sun’s declination can be plotted on a graph, known as an analemma (next slide) ◦ The sun’s declination is plotted along the vertical axis ◦ The days of the year are along the analemma itself See page 52 of your lab manual for an example
Solar Altitude Example Suppose it’s noon and we’re outside on campus during a snowstorm on January 12 th. The clouds and snow showers are blocking the sun from being visible. Even though we can’t see the sun, what is the solar altitude (sun’s elevation above the horizon)?
Solar Altitude Example, cont.
Tangent Rays and Daylight/Darkness
Tangent Rays and Daylight/Darkness, cont.
For the Northern Hemisphere, remember this: ◦ If it (the day) is between March 20 and September 22, then those areas north of the latitude of the tangent rays are experiencing 24 hours of daylight. ◦ If the day is between September 23 and March 21, then those areas north of the latitude of the tangent rays are experiencing 24 hours of darkness.
Lesson 12 Insolation Hess, McKnight’s Physical Geography, 10 ed. pp. 70, 80-84, and Fig on p. 78
Insolation From lesson 11 we now know that the sun’s direct rays strike the earth at different locations throughout the year. These differences give us our seasons and influence the amount of average daily insolation (incoming solar radiation) ◦ Average daily insolation is the rate or intensity of the sun’s radiation that strikes the surface over a 24-hour period Measured in watts per square meter (W· m -2 ) The average insolation hitting the Earth’s upper atmosphere is ~ 1372 W· m -2. This is known as the solar constant
Insolation, cont. However, the amount of insolation hitting the surface of the Earth varies widely due to three factors: 1.The angle of incidence 2.Day length 3.Atmospheric obstructions We will discuss each of these next…
Angle of Incidence Angle of incidence: the angle at which the Sun’s rays strike the surface of the Earth (solar altitude) ◦ This can be directly related to the intensity of radiation that reaches the surface. Areas that have a high angle of incidence have a given amount of radiation concentrated on a small area ◦ Therefore, radiation is higher in intensity While areas with a low angle of incidence have that same amount of radiation concentrated on a larger area ◦ Lower intensity radiation
Angle of Incidence, cont.
Length of Day We all know that the length of daylight influences how much solar radiation is received (e.g. longer days generally mean warmer days) Even if it is cloudy, longer days generally mean a significant increase in solar radiation received Take a look at Fig. 3 and Fig. 4 on pages 58 and 59. These provide the hours of daylight and daily insolation, respectively, for location at the equator, 45 ° N and 90 N °.
Atmospheric Obstruction The amount of atmosphere that radiation has to travel through affects the total amount received. ◦ e.g. If the angle of incidence is low, then solar radiation has to travel through more atmosphere, thereby reducing the amount received when it finally reaches the surface Water droplets (clouds) and other atmospheric particulates also affect the amount received. The percentage of solar radiation reaching Earth’s surface through the atmosphere is listed in Fig. 5