Monday, February 12, 2007

Weather FAQs

-34.2 C (-29.6 F)
wind chill: -49.7 C (-57.7 F)
wind: 13.5 knots

The following are frequently asked questions regarding the South Pole weather as answered by the stations meterologists.


Why is Antarctica so cold?

Antarctica is synonymous with cold, thanks to its polar location, its high elevation, its lack of a protective water vapor-filled atmosphere, and its permanent ice cover which reflects 80% of the sun's radiation back into space (re-radiation). Angle of incidence also has a lot to do with the temperature in the Antarctic. The highest sun angle here is only 23.5 degrees, so the South Pole gets a much less concentrated amount of the sun’s rays, versus locations near the equator, which get the highest concentration of the sun’s rays. The South Pole is located within a permanent polar high created by the normal Hadley Circulation. This creates an extremely cold air mass, which descends at the poles of the earth. Unlike the Arctic region, Antarctica is a continent surrounded by an ocean which means that interior areas do not benefit from the moderating influence of water. During the winter, the size of Antarctica doubles as the surrounding seawater freezes, blocking heat from the warmer surrounding ocean water.


Why does High Pressure bring us bad weather when everywhere else in the world gets good weather from High Pressure?

Explanation A: At the South Pole we are at a much higher altitude than normal (9301ft) but the biggest difference is the temperatures. If you go up 1000 feet from the surface here you will experience warmer temperatures than at the surface while everywhere else in the world has colder temperatures above the surface. Under high pressure, the downward vertical motion is actually bringing warmer, moister air down to us causing increasing clouds and worsening weather. Low pressure means upward vertical motion and here at the Pole, so when we have low pressure, it is bringing in surface air from all around us to replace the air that is being forced up. This air is colder and dryer than the air it is moving into which causes a decrease in cloud cover and improving weather. Everywhere else in the world experiences worsening weather in low pressure due to the air cooling as it goes up. When air rises it cools causing condensation and clouds and decreasing weather conditions, but here at the South Pole the air above us is warmer than at the surface so we don’t experience the condensation and the increasing clouds in low pressure.

Explanation B: Explanation A describes what happens when the pressure changes first, leading to other changes in the weather. But there are other times when the pressure change is a response to a change in temperature. When the winds come from grid east here at the South Pole, the skies clear and the temperature falls. This is because the wind is bringing drier, colder air down from the higher parts of the Polar Plateau. As the air gets colder it also becomes more dense and continues to flow downhill off the Plateau. This reduces the total airmass that is above the South Pole. Since the station pressure is just an indication of the total amount of mass above you in the atmosphere, this reduction of airmass causes the pressure to fall. The reverse happens when warmer air comes in from grid west or grid north. The warm air moves upslope toward the Pole, expanding the total air column above us, adding mass, and thus increasing the pressure. This warmer air from grid west through north is usually associated with a disturbance originating in the Weddell Sea, and is accompanied by increased moisture and winds. There is often a strong correlation between pressure and temperature, especially in the winter when the coldest temperatures often coincide with the lowest pressures.




What is the Aurora Australis?

The Aurora Australis or Southern Lights are mesmerizing, dynamic displays of light that appear in the Antarctic night skies. They appear in many forms, pillars, streaks, wisps, haloes, and curtains of vibrating color. Auroras are caused by collisions between electrically charged particles from the sun and gases in Earth's upper atmosphere (the thermosphere). There collisions produce electrical discharges which energize atoms of oxygen and nitrogen, causing the release of various colors of light. In the northern hemisphere this same phenomenon is called Aurora Borealis.




What is a katabatic wind?

Katabatic winds occur where cold, dense air flows down the slopes of the inland mountains and the Polar Plateau. This is a frequent phenomenon in Antarctica, as the continent is dome shaped and the interior is very cold. As a surface flow, these winds may be smooth and low in velocity, but there are many times when they become exceedingly turbulent, sweeping up any loose snow in their path. This fierce, turbulent air may suddenly just appear and produce localized Antarctic blizzards, where the skies are still clear and no snow actually falls to the ground. Here at the South Pole we usually see the steady, low velocity katabatic winds, but these winds pick up momentum as they move off the Polar Plateau and are often very strong by the time they reach the coast.



What is a "mock sun"?"

Mock sun" (also "sun dogs") is a colloquial term for a phenomenon called "parhelion" which occurs fairly commonly in polar atmospheres. It is a false image of the sun, created by the bending of rays of sunlight within crystals of ice in the atmosphere. Parhelia are usually observed in pairs, one on each side of the sun and at the same elevation. They tend to be red-colored on the side nearest the sun. Parhelia are quite close to the sun when the sun is close to the horizon, but move further away as the angle above the horizon increases.

Why is the air so dry in Antarctica? And what is the typical relative humidity at the South Pole?
The important thing to know here is that cold air can hold less moisture than warm air. Therefore, the low temperatures of the South Pole account for the fact that not very much moisture is present in the air. This is enhanced by the great distance from the ocean, which is the atmosphere’s source for water vapor. However, many people are surprised to learn that the outdoor relative humidity at the South Pole is typically around 60 to 80%. But the key word here is RELATIVE. What it’s relative to is temperature. Relative humidity is just an indication of how close to saturation the air is for the current temperature. So a relative humidity of 80% with a temperature of -40C/F means that the air is getting close to saturation, but there’s still not much moisture there because the air can’t hold much moisture at -40. So you can think of it as 80% of not much moisture whereas an 80% humidity reading in Georgia during the summer is 80% of a whole lot of moisture.

There are some measurements that indicate the actual amount of moisture in the air, not just a relative degree of saturation. These include absolute humidity, mixing ratio, and dew point temperature. Of these, the dew point temperature is most familiar and relates pretty well to how the air actually feels to a person. During the northern summer, an 80F day with a dew point of 45F tends to be quite pleasant, whereas and 80 degree day with a dew point of 70F is pretty uncomfortable. The dew point temperature is always lower than or equal to the air temperature. So here at the South Pole, if it’s -40 then the dew point is -40 or lower. Obviously a dew point temperature of around -40 represents a lot less moisture than a dew point of +70, even though both can occur with the same relative humidity reading.

So what about the indoor relative humidity?

The numbers here are very different from the outdoor relative humidity, and are typically somewhere around 5% or lower. That’s because you have about the same amount of moisture as outside (unless there’s a humidifier or some other manmade source of moisture) but the temperature is much higher. So you have a tiny amount of water vapor in a temperature that could support a lot of water vapor. That situation gives you a very low REALTIVE humidity.

Does it snow at the pole?

Actual snowflakes (branched crystals) are pretty rare at the South Pole, and generally are only seen during the warmest periods of the summer. But according to data from snow stake measurements, the annual snow accumulation averages about 9 inches/23cm. (3.4 inches/86mm of water equivalent). Most of this precipitation falls as ice crystals. Ice crystals are very common at the South Pole, often falling out of a clear sky when the air becomes saturated. The precipitation intensity is normally very light, but over the course of a year it adds up.

What is a temperature inversion?

A common feature of the Polar Plateau is a temperature inversion. A temperature inversion occurs when the coldest temperatures occur at the Earth's surface, and warmer temperatures are some distance above the surface (normally, temperature decreases with height). The temperature inversion may less than 1,000 feet (300 meters) thick, but the temperature difference can be 54 degrees F (30C) in that short distance! The intensity of inversions is related to altitude and latitude, and is greater in the winter. Strong winds, cloud cover, or precipitation can destroy inversions.


What is the Antarctic Convergence?

This is the region of the Southern Ocean encircling Antarctica, roughly around latitude 55 degrees South but deviating from this in places, where the cold waters of the Antarctic Circumpolar Current meet and mingle with warmer waters to the north. This mingling creates local variations in weather, such as fogs, and also a concentration of marine plants and animals because of its higher than average amount of nutrients.


Is it windy at the South Pole?


Many people think of Antarctica as a windy place. That is true, but only toward the edges of the continent. High on the plateau at the South Pole, the annual wind speed average is 12 mph and the highest recorded peak wind is 55 mph. Cold, dense air tends to settle at the Pole making for relatively calm and clear, yet frigid conditions. (See also Question 4 concerning katabatic winds.)


What's the difference between whiteouts & blizzards?

Blizzards:Winds greater than 35 miles per hour, with sustained or frequent gusts, and visibility less than or equal to 1/4 statute mile due to snow or blowing snow for at least 5 hours. Whiteouts:Whiteouts are another peculiar Antarctica condition, in which there are no shadows or contrasts between objects. A uniformly gray or white sky over a snow covered surface can yield these whiteouts, which cause a loss of depth perception and surface definition.

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This is enhanced by the great distance from the ocean, which is the atmosphere’s source for water vapor.
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