It should come as no surprise that the lowest temperatures in the Southern Hemisphere, and indeed on Earth, have been recorded in Antarctica. The Antarctic polar plateau, which is perched up to 12,000 feet above sea level, is the source region of much of the cold Antarctic air and is where the lowest temperatures have been recorded. The Admundsen-Scott South Pole Station (US) and Vostok Station (Russian) are the two locations where the world's lowest temperatures are usually recorded during the months from March to October.

Since the South Pole is at the bottom of the Earth, and receives no sunlight at all for more than 5 months of the year, it would seem that it should be the coldest spot on Earth. It's not - that distinction goes to Vostok. The South Pole is more than 9,000 feet above seas level, but Vostok is more than 2,000 feet higher, and this difference is elevation is enough to make Vostok colder. The lapse rate for dry air is about 5.5 degrees F for every 1,000 feet change in elevation. This means that, providing no heat is added to or subtracted from a parcel of air, when air rises 2000 feet in elevation, it'll be nearly 11 degrees F colder. By the way, for this discussion, I'm using the Fahrenheit Scale instead of the Celsius Scale, if only because the temperatures seem more impressive.

Darkness is one of the requirements needed for extremely cold temperatures, at least on our planet, but like the South pole, Vostok is dark for most of the Southern Hemisphere winter too, and it really doesn't matter how dark it is, just that no sunlight is available to heat up the air. Both Vostok and the South Pole are far enough inland so that incursions of relatively warmer air rarely ever make it this far south. Vostok is at a latitude of 78 degrees (south) and is approximately 800 miles from the South Pole. In about 2 months or so, when the Sun disappears from the Antarctic sky, temperatures will drop faster than Enron stock.

On July 21, 1983, a temperature of -129 F (officially 128.6 F) was recorded at Vostok! This is not a wind chill temperature, but the actual temperature of the air. In fact, if the wind is blowing, the temperatures will often rise on the polar plateau. As mentioned last week, when the wind blows, while it makes us feel colder, it also breaks up the deep inversions that occur during the winter months, and the turbulent motion of the air keeps the temperature from falling.

Of course, another thing that keeps Antarctic so cold is that it's snow covered all year long, with the exception of some coastal areas. Snow is one of the brightest natural surfaces (it has a high albedo), and it's thus very efficient at reflecting shortwave solar radiation back to space. The reflected energy is unavailable to heat the air and melt the snow. Snow is also more efficient at emitting longwave (infrared) radiation than is grass or soil, so temperatures cool more quickly over a snow covered meadow than over a grassy field. Furthermore, since a covering of snow usually is much smoother than is the underlying surface, there's less turbulence at the microscale - this enables temperatures to fall more quickly.

In the Arctic, both the minimum and maximum temperatures are higher than in Antarctica because the continentality is not as great (the Arctic Ocean is not ice covered throughout the year), and the elevations are, in general, not as high. The coldest temperatures in the Northern Hemisphere occur in northeastern Siberia. Temperatures in the negative triple digits have never been recorded, but on several occasions, the thermometer has plummeted to the -90 F range. Perhaps the two coldest locations in the Northern Hemisphere, at least in terms of minimum temperatures, are the towns of Verkhoyansk and Omyakon. Both of these towns have hit the -90 F mark! Today, at 10 p.m. in Verkhoyansk (their time), the temperature is -55 F and it's calm and clear.

What makes these two location so darned cold? Verkhoyansk is about 400 miles south of the Arctic Ocean, on the Yanu River. With a latitude of 67.5 degrees (north), it's barely above the Arctic Circle. Omyakon is about 700 miles south of the Arctic Ocean, on a tributary of the Indigurka River. It's below the Arctic Circle - its latitude is approximately 63 degrees (north). What they have in common is that they're both surrounded by mountains over 7,000 feet high, and they both lie in river valleys. Verkhoyansk is a little more than 400 feet above sea level, and Omyakon is about 2,600 feet above sea level. The river bottom locations ensure that cold air from above will drain into the valleys, and the flanking mountains act to trap it there. This refrigerated air can become amazingly cold if it's undisturbed by wind.

Lack of wind is often a feature of winter weather in Siberia. The "Siberian High" may sit over eastern Siberia for weeks at a time. This stable high pressure system provides an ideal environment for inversions. In valley bottoms, there's very little air movement for days at a time during the winter. The Sun is up for a few hours during the day now, but it's not strong enough to bust the inversion, and the snow cover reflects most all of the solar energy back to space.

How cold can it get in Antarctica? Unlike, the Northern Hemisphere where temperatures are lowest in the core winter months, from about mid December until early March, in Antarctica, the yearly minimum low temperature may occur anytime from April until September - winters are said to be coreless there. Temperatures have been -100 F in 5 different months. Although, the polar plateau near Vostok is the coldest area on Earth, It would be a stretch to think that the location of the temperature sensor at Vostok happened to be in the very site that the coldest air ever found on Earth was in place. It's more likely that somewhere near Vostok, the temperature may have been slightly colder, perhaps before Vostok first became operational in the late 1950s. A temperature record of only 45 years or so is not really long enough to establish extreme temperatures at a given site. A big factor is measuring temperature when inversions are present, is the height of the temperature sensor above the surface. A standard height is 5 feet. A sensor located a foot higher or a foot lower than this would likely be several degrees warmer or colder, respectively. Wind blown snow can periodically alter the height of temperature shelters. All-in-all, it seems possible that a temperature of -135 F could occur at some location on the polar plateau. In 1997, an unconfirmed reading of -132 F was observed at Vostok.

How cold can it get in the Northern Hemisphere? Once in the 1890s, Verkhoyansk reported an unofficial temperature of -96 F. It's possible that temperatures may have plunged close to -100 F in the Omyakon and Verkhoyansk areas, but due to the relatively low elevations of these towns, it's hard to imagine that they could drop much below this. Whereas Vostok rests on more than 2 miles of ice, Verkhoyansk, in winter, is usually covered by less than a foot or so of snow. If northeastern Siberia were atop a couple of miles of ice, Verkhoyank's temperature might surpass those on the polar plateau (providing it was still in a river valley surrounded by mountains). Using the 5.5 degree lapse rate, and if Verkhoyansk was 2 miles higher than it is now, an air mass that would permit a temperature of -90 F at sea level could result in a temperature of -145 at 10,500 feet.

The official record minimum temperature in Greenland is -87 F, but the unofficial record is -94 F. Why hasn't the lowest temperature in the Northern Hemisphere been recorded on Greenland, where the ice cap is as much as 2 miles thick? For one reason, there are few long-term meteorological stations on the ice cap (the polar plateau of Antarctic has a longer temperature record than does Greenland's ice cap), and for another, even in winter, storm systems from the north Atlantic can effect Greenland's weather. Greenland's a big island, but it's an island nonetheless, and it's continentality isn't as great as Antarctica's. As compared to Antarctica's interior, there are more clouds and a greater chance that warmer (relatively) air from sub polar areas will reach the Greenland Ice Cap. This helps deter the temperatures on Greenland's ice cap from rivaling those on the polar plateau in Antarctica.