Temperature changes at the surface of the earth are dominated by two cycles: the

Question

Temperature changes at the surface of the earth are dominated by two cycles: the seasonal cycle and the diurnal (daily) cycle.

a. Suppose the tilt of the earth were 0 degrees instead of 23.5. How would theseasonal and diurnal cycle of temperature differ from today? Explain.

b. Suppose the tilt of the earth were 90 degrees instead of 23.5. How would the seasonal and diurnal cycle of temperature differ from today? Explain.

c. For each of the above two situations, what time of year would the length of day be the same as it is in reality? (The answer is the same

for all locations on the earth).

Explanation / Answer

A.The tilt of the earth’s axis is now zero degrees from the vertical. The earth continues to spin about this axis (one revolution per day) as it goes around the sun (one revolution per year). In this case, the daily (diurnal) cycle is always 12 hours of light and 12 hours of dark everywhere on earth, all year. There is no more seasonal cycle (the length of day remains the same all year). Therefore the daily cycle is now the most important factor.

B.Now the axis that the earth spins about is 90 degrees from the vertical (it’s horizontal in this picture). Now the winter each hemisphere is totally dark all day, and the summer is light all day. Therefore, at least in summer and winter, there is no daily cycle. So the seasonal cycle is the most important factor in this case.

C.First of all, the difference in distance from the sun in summer and winter does not affect the earth’s seasonal cycle. The major factor is the tilt of the earth’s axis. Second, the reason the earth is not warmest in June but in July is the same reason that the warmest part of the day is not at noon, but later. The Northern Hemisphere receives most solar radiation on (around) June 21 st, and least on December 21 st. But the net radiative balance continues to be negative after December 21 st, so the temperature continues to decrease. In late January, the energy emitted from the earth becomes equal to the energy received from the sun (minimum temperature). After this, the energy received is greater than the energy lost, so the temperature increases. In late July, the energy coming in and energy going out are equal again (maximum temperature). After this, the energy emitted is greater than the energy received, so the temperature begins to decrease. Refer to the figure showing the diurnal cycle in your notes the concept is very similar.

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