Q. Annual average position of the Thermal equator is 5° North latitude. What could be possible reason that thermal equator is in Northern hemisphere?
- The distribution of continents is uneven
- Insolation from sun is more in northern hemisphere on an average
Which among the above is / are correct?
Answer:
Only 1
Notes: The first statement is correct and an obvious answer. The second statement needs some explanation. The first thing we should know that Sun, in its seasonal apparent movement across the sky, passes directly over the Equator twice each year, at the March and September equinoxes. This means that during a year, the Insolation equator is always NOT identical. At the Equator, the rays of the sun are perpendicular to the surface of the earth on these dates. We have read in our modules that the highest mean annual temperature shifts towards northwards during the summer solstice to a much greater extent than it does towards south at the time of winter solstice. The thermal equator is defined as the latitude at which Insolation is identical throughout the year; this is not the same as the astronomical equator because the Earth reaches perihelion (the minimum distance from the Sun in its orbit) in early January and is at aphelion (maximum distance) in early July. So Insolation is somewhat higher at 0° latitude in January than in July even though the height of the Sun (at noon) and the length of day (from sunrise to sunset) is essentially the same. At a few degrees north of the equator the perihelion/aphelion factor is balanced by the fact that the angle of the Sun is slightly more direct, and the days are slightly longer, at the time of the summer solstice for the Northern Hemisphere (most commonly on June 21), making the level of Insolation virtually the same in both “summer” and “winter. So, the second statement does not explain the phenomena. There is one more reason, that glaciated Antarctic continent maintains colder summers in the southern hemisphere than does the Arctic, with a much smaller land area, in the northern hemisphere.