The spherical earth is symmetrical about any plane through its center to within a fraction of a percent. But there are surprising differences geographically, astronomically, and meteorologically between the northern and southern hemispheres.
Which Way Does The Wind Blow?
Most people are familiar with the fact that, due to the Coriolis effect (a result of the rotation of the earth), the wind blows with low pressure on its left. But that’s true only in the northern hemisphere.
Once you cross the equator, everything is backwards: Storms spin clockwise and high pressure systems turn counterclockwise.
We can get used to that; it’s just a mirror image. But in important ways, the southern hemisphere is very different from the northern.
Land, Ocean, And Albedo
Overall, 71% of the earth is covered by water and only 29% by land. But the land mass is not shared equally between hemispheres. The northern hemisphere is 39% land and the southern hemisphere only 19%. It makes a difference.
The Albedo of a surface is defined as the fraction of radiation it reflects. Zero albedo means all radiation is absorbed, and one albedo means all radiation is reflected.
Ocean has an albedo of 0.1 and fresh snow has an albedo of 0.9. Other types of surfaces such as trees, grass, and sand have albedos ranging from 0.15 to 0.4. So we would expect the southern hemisphere, with its abundance of ocean, to absorb more solar radiation than the northern hemisphere.
Water is a good conductor of heat, and most of the radiation absorbed is stored in the deeper waters. Land surfaces, on the other hand, conduct poorly, so most of the absorbed radiation simply heats the surface. If you’ve ever walked barefoot on pavement or sand on a sunny day, you understand this.
Some of the heat stored in the surface of the earth is transferred to the lower atmosphere by the process of conduction.
Since the radiation striking the ocean is conducted away rapidly, the surface temperatures vary slowly and have less range than those on land. So we would expect a narrower range of temperatures between winter and summer in the southern hemisphere than in the northern hemisphere. However, there is a compensating factor.
The Orbit Of The Earth Is Not Round
The amount by which the orbit of a planet deviates from a circle is called its eccentricity. A circle has an eccentricity of zero; a parabola has an eccentricity of one. Planets, with eccentricities between zero and one, travel in ellipses — the higher the eccentricity, the more elongated the ellipse. The earth’s eccentricity is 0.017. Mercury has the greatest planetary eccentricity at 0.206 and Venus has the smallest, 0.007.
The earth reaches perihelion (the point closest to the sun) on January 7, and aphelion (farthest from the sun) on July 7. Thus, the sun is closer to the earth during the southern hemisphere’s summer and farthest away during its winter. There is more of a difference in solar radiation between summer and winter in the southern hemisphere than the northern. The difference in the earth-sun distance between January 7 and July 7 is 4% of the mean distance.
Which Has A Greater Effect On Temperature – Eccentricity, Or Land And Water?
Comparison of the temperature between the hemispheres. All cities are near sea level.
These temperatures show that there is a rather stark contrast between the hemispheres. Every city in the northern hemisphere has a greater difference between summer and winter than any city in the southern hemisphere. In Beijing, the difference between summer and winter is 54 degrees. The greatest difference of the 5 southern hemisphere cities sampled is 19 degrees.
The moderating effect of water on southern hemisphere temperatures is obvious. The increased differential heating between summer and winter because of the earth’s orbital eccentricity is overwhelmed.
Moderate Temperature in Australia
If the search for MH370 drags on for years (not so unlikely) with Perth, Australia as the staging area, searchers can at least look forward to moderate temperatures throughout the year, with summer highs in the 80s and winter lows in the 40s.