Geography 204

Fall 2020

Lab #2: Sun Angles, Daylength, Insolation, and Temperature Patterns

(due by 11:59 pm, Monday, 10/12)

25 points


The sun is the single most important source of energy on the surface of the earth as well as the atmosphere. The distribution of the Earth’s atmospheric phenomena and climate patterns, as well as the distribution of its ecosystems, are significantly influenced by the distribution of incoming solar radiation. In heating the earth’s atmosphere, visible light is the most important part of the sun’s electromagnetic spectrum. This exercise examines sun angle and intensity of insolation, daylength and temperature patterns on the earth’s surface. These variables are examined as they interrelate on the earth’s surface over the course of a year.

Sun Angle

Because solar energy received by the earth follows essentially parallel pathways, and because the earth is spherical, at only one place on the earth’s surface can the sun’s rays strike vertically (this is known as the subsolar point). In other words, at only one place at any one time can the sun appear directly overhead. This occurs at solar noon when the sun reaches the highest position in the sky for that day. Because of the earth’s limited axial tilt, the sun can appear directly overhead at the subsolar point at a relatively narrow range of latitudes over the course of a year (between 23.5° N and 23.5° S).
An important relationship exists between latitude and the angle of the noon sun. On the equinoxes (on March 21 or 22 and September 21 or 22) the sun’s rays are perpendicular to the earth at the equator. Those same rays would also be tangent at both of the poles, so that the sun would appear only on the horizon at those locations. On the same dates an observer at 30° N would record a sun angle of 60° above the southern horizon. Remember, the sun is 90° to the observer at the equator, minus the latitude of 30° (30° of arc) which equals 60°. This is called the angle of incidence, or sun angle. The angle of incidence decreases by 1° for every degree of arc of latitude between the observer’s position and the location where the sun’s rays are vertical. This rule is the same for the other times of the year but is complicated by the earth’s declination–the shift in angle when the sun’s rays are not perpendicular to the equator. If the declination is 10° S, this means that the sun’s rays are vertical at 10° S and an observer at 30° N would see the sun at 50° above the horizon 90-40 or 90-(30+10).
Use the formula:

angle of incidence = 90° – (latitude in degrees + declination in degrees*)

* If the declination is in the same hemisphere as the observer, subtract this from latitude.


Seattle (47° N) on December 21 (23.5° S) would be:

90° – (47 + 23.5)

90° – (70.5) = 19.5°

Thus the angle of incidence for Seattle on December 21 is 19.5°

Note: Keep in mind that solar noon is not the same as noon on our cloc

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