Why is Antarctica cold?

From space the earth appears as a smooth round blue ball with white caps at the poles. We are so used to this picture that we rarely question why the poles are cold enough to maintain permanent regions of ice while the tropics are always hot. After all, the energy we receive from the sun is sufficient to heat the whole world to 14oC and the source of our energy, the sun, is 150 million kilometres away so why should it heat one part of the earth more than another?

 

The main reason why there is not equal distribution of heat over the surface of the earth lies in the fact that the earth is a sphere. The equator points directly at the sun but the poles are angled away so sunlight strikes the surface at an oblique angle. At 30° north or south of the equator (roughly the latitude of Florida or Sydney, Australia) this tilting reduces the amount of sunlight received to about 86 per cent of that falling on the equator. At 60° (say Oslo, Norway) the intensity of sunlight is reduced to 50 per cent and by 80° it has fallen to 17.4 per cent. From an polar earthbound vantage point the sun appears lower in the sky and its rays less warming. If the earth wasn’t tilted, the poles themselves would receive no sunlight at all – the sun would appear to merely travel around the horizon but never rising above it.

 

As if part of a vast cosmic plot, two other factors contribute to depriving the poles of the sun’s warmth. The first is that the greater angle means that sun’s rays must pass through more of the earth’s atmosphere before reaching the polar surface. But more importantly, because snow is white, nearly all  (at least 80-85 per cent) of the sun’s radiation that does reach the surface of Antarctica is reflected back into the atmosphere. See diagram ??

 

If the world didn’t spin then the side in the sun would become very hot and the area in shadow would be very cold – life on earth would be impossible. Diurnal heating and cooling moderate the effects of the blast or energy we receive from the sun. The poles are the points of rotation so, effectively, they move very little.  That’s at great variance to points on the equator that, to complete a rotation in 24 hours, move at 1670 km/h.

 

While the earth spins around the poles, it also rotates around the sun. If the poles were perpendicular to the sun’s rays everywhere on earth would have 12 hours of daylight and 12 hours of darkness every day of the year. In fact it would be hard to mark the passing of a year because there would be no seasons. The changing pattern of night and day and the seasons all come about because the earth’s axis of rotation is tilted about 23.4° off perpendicular to our orbital plane around the sun. So, for six months of the year the north pole is pointing towards the sun and experiencing summer while for the other six months the sun lies in the southern hemisphere. The two annual (December and June) solstices are the times when the earth is at the point in its orbit that the poles are most tilted towards the sun. Because one pole is bathed in 24 hour daylight at this time, it receives more sunlight than anywhere on the equator ever does – but most is reflected straight back into the atmosphere.

 

Midway between the solstices are the March and September equinoxes – when, from our earthbound perspective, the centre of the sun crosses the equator. As the name implies, everywhere on earth should have 12 hours of daylight and 12 hours of darkness on that day. However, because we judge sunrise and sunset not from the centre of the sun but from the first and last rays the time of equal night and day for different locations can fall several days either side of the equinox. The equinox also marks the time when the polar day shifts from six months of sunshine to six months of darkness and vice versa. At the South Pole the sun rises on September 21 and sets on March 23. At 70°S the sun returns on July 19 and stays above the horizon from November 19 until January 26 and finally sets on May 25. The months that the temperate zones of the Southern Hemisphere call “spring” and “autumn” are more aptly considered “twilight” in Antarctica – the temperature rises rapidly after the sun reappears and plummets when it drops below the horizon.

 

The earth’s orbit around the sun is not a perfect circle: at the time when the south pole is pointing towards the sun the earth is about three per cent closer to the sun. Even so, Antarctica is much colder than the Arctic. That’s because it is a continent with land blocking the moderating influence of the ocean but above all through the dominant effect of the high polar ice cap.