Chapter 4, Problem 1E

At 5° N latitude there are two peaks of incident solar radiation (that is, solar radiation striking Earth’s surface) during the year. Why does this location differ from other latitudes that have only one peak?

To determine

The reason why the $5^{\circ }$ N latitude differs from other latitudes by having two peaks of incident solar radiation in a year.

Answer to Problem 1E

Every latitude to the north of ${23.5}^{\circ }$ in the Northern Hemisphere obtains the highest solar intensity and the longest daytime on one day, which is on June 21st. The Sun would be directly above the $5^{\circ }$ N two times in a year, which can be on April 4th and September 9th. Therefore, when all other latitudes have one single peak of incident solar radiation during a year, the $5^{\circ }$ N latitude has two peaks.

Explanation of Solution

The region of the Earth receiving direct insolation from the Sun fluctuates seasonally during a year owing to the axial tilt, rotation, and revolution of the Earth. The daylight hours will be more in the region that is inclined more toward the Sun.

On June 21st, the day of June Solstice or Summer Solstice, the Sun will be directly above the Tropic of Cancer (${23.5}^{\circ }$), and the North Pole will also be slanted toward the Sun. This causes the Northern Hemisphere to receive more solar insolation on this day and the adjacent days with more sunlit hours. Hence, there will be one peak of incident solar radiation for the ${23.5}^{\circ }$ N latitude and the latitudes toward its north, which may be in June.

On December 22nd, the day of December Solstice or Winter Solstice, the Sun will be directly above the Tropic of Capricorn (${23.5}^{\circ }$), and the South Pole also will be slanted toward the Sun. This causes the Southern Hemisphere to receive more solar insolation on this day and the adjacent days with more sunlit hours. Hence, there will be one peak of incident solar radiation for the ${23.5}^{\circ }$ S latitude and the latitudes toward its south, which may be in December.

Unlike these latitudes, the Sun would be above the $5^{\circ }$ N latitude twice in a year, which may be in April and September. Therefore, this latitude will exhibit two peaks of incident solar radiation for a year.