In Baltimore-Washington area yesterday, the high or maximum temperature was in the lower 40s F, and the low or minimum temperature was in the upper 20s. The maximum temperature occurred in the early afternoon, and the minimum temperature occurred in the early morning. At the South Pole (Admundsen/Scott Station) in Antarctica, the Sun's up all day now. The maximum temperature was - 8 degrees F at 7:00 p.m., and the minimum was -14 F at 7:00 a.m.. Despite the fact that Old Santa lives at the North Pole and has a workshop there too, there's no meteorological station at the North Pole. One of the coldest weather stations the Northern Hemisphere is Verkhoyansk, Russia, in northeastern Siberia (67.5 degrees N). In Verkhoyansk, the Sun rises now at about 11:40 a.m., and it sets about 3 hours later.Yesterday's high was -54F at 4 a.m, and the low was and the low was -60 at 4 p.m. It's kind of hard to gripe about our weather when it's well below zero near the poles.

It's not really hard to understand why it's cold in the Arctic and Antarctic. The poles simply lose more energy than they receive - they have a negative radiation balance. The net radiation is the balance between incoming energy absorbed by a surface and outgoing radiation that's emitted from the surface. During the midday hours, the net shortwave radiation (energy emitted in the visible and near-infrared wavelengths) is typically greater than the net longwave radiation (energy emitted in the infrared wavelengths), so the net radiation is positive, and the surface gains heat. At night, net radiation is negative, and the surface loses heat by radiating it to space. In a nutshell, this results in the daily cycle of rising and falling air temperatures.

Because the earth rotates on its axis, incoming solar energy can change considerably during the course of the 24 day. Shortwave solar radiation is greatest in the middle of the daylight period, when the Sun is highest in the sky, and it falls to zero at night. While the surface only absorbs shortwave radiation when the Sun is up, it's always emitting longwave radiation. At night, longwave radiation is lost to space, and since there's no shortwave radiation to offset these loses, the surface cools, as long as it's not cloudy.

With or without the Sun, the temperature at the surface can vary during the course of the day. At night, as the air cools, it becomes more dense. By early in the morning, the pool of cold air at the surface can be considerably colder than it was early in the evening, and the temperatures are actually warmer above the surface than they are at the surface itself. In extreme cases, the temperature may be more than 20 degrees F warmer 100 feet above the surface than at ground level. These inversions are prevalent in the Antarctic nearly all year. If there's no heating, from the Sun or other sources, the temperatures may continue to fall at the surface. However, even light winds can disturb the inversion so that the warmer air aloft will mix with the cold surface air, which results in a warming of the surface temperature. Renewed cooling will follow a slackening of the winds. What's interesting is that inversions can form whether or not the Sun is present.

At all latitudes, except at the poles, the Sun reaches a point where it's higher in the sky than it is at other times of the day (local noon). This is when the solar radiation is greatest. At the poles, where the Sun is always about the same elevation above the horizon, noon is more or less an arbitrary designation. Places near the poles keep nearly the same Sun angle all day, and so diurnal heating is negligible when compared to the tropics, for example, where the Sun is up for about 12 hours, it's dark for about 12 hours.

At the North and South Pole, there's no diurnal variation of the solar radiation incoming at the top of the atmosphere, and there's also no significant predisposition for clouds or winds to occur at one time of day as opposed to another (in polar regions, during the winter, clouds and wind act to increase the air temperature). Thus, the maximum or minimum temperature can occur at any hour of the day. The minimum temperature will most likely occur when the inversion is most well developed, and the maximum temperature will typically occur whenever the inversion breaks down.

However, away from the poles, at Byrd Station (80 degrees south latitude) or at Verkhoyansk, for instance, the maximum and minimum temperature often occur at the times when they occur most everywhere else on the globe - in the mid to late afternoon and early in the morning, respectively. At these times, the largest diurnal variations between the high and low temperatures occur during the short summer period. In the winter, there's virtually no difference in temperature at the times the maximum and minimum temperatures occurred in the summer season. The high and low can occur any hour of the day.