Re: Why are the North/South Poles the coldest areas on earth?

Many factors affect temperatures on the earth. Some of the most important are: location, nearness to oceans, land cover, and weather patterns. I’ll speak to the extreme cold at the South Pole, but most of the arguments hold for the North Pole as well.

Location: Over one year, every point on earth gets the same amount of daylight. Based on your location, it changes with the seasons. In the tropics, the day length doesn’t change very much from summer to winter. For example, at Christmas Island in the Pacific Ocean (only about 1.5 degrees north of the Equator), the longest day of the year is June 15 with a sunrise of 5:53 AM and sunset at 6:08 PM. The shortest day of the year is January 2 with a sunrise at 6:04 AM and a sunset at 6:04 PM – only 15 minutes difference between the longest and shortest day of the year. Thus, this area receives nearly constant sunshine…no great differences in day length to allow great temperature swings.

At the exact South Pole, the sun rises on March 21, and sets on Sep 21. Only one ‘day’ per year, with day and night each lasting 6 months. Thus, when it’s dark for so long, it can get very cold. (Source : http://www.usno.navy.mil/

Another factor of location is called solar path length. The sunlight that shines on the equator shines nearly straight down, while the sunlight that shines on the South Pole has to go through relatively more atmosphere, which filters out more of the energy.

Nearness to oceans: Generally, oceans act to prevent big swings in temperatures. For example, Seattle Washington is about as far north of the equator as Grand Forks, North Dakota. The average low temperature in January in Seattle is 34 degrees, while in Grand Forks, the average low in January is 4 below zero. Most of this difference is due to the Pacific Ocean providing relatively warm water to keep Seattle warm. At the South Pole, the ocean is very far away, so there is no relatively ‘warm’ water to keep temperatures from falling.

Land cover: The nature of the land’s surface affects how much of the sun’s energy gets absorbed (making it warm) or reflected (which keeps it from being warm). If the ground id dirt, water, trees, cities, etc., then plenty of solar energy is absorbed. At the South Pole, the snow and ice reflect nearly 100% of all the sun’s energy back into space. Thus, when the sun does shine, it doesn’t warm up anything, and it keeps getting colder.

The following figure shows the net solar radiation received across the earth in January. This is the sum of sunlight in minus heat out. Notice the South Pole has a net value around zero which means no net warming by the sun. This is in the middle of summer! In the winter, the sun doesn’t shine, so the net is negative, and it just gets colder.

Finally, weather patterns affect extreme temperatures. Weather patterns are somewhat dependant on the above factors. Very cloudy places tend to be warmer in the winter as the clouds act like a blanket to keep heat close to the ground. At the South Pole, clouds actually increase the temperature. However, the atmosphere is so cold, and dry, that there are often not many clouds, and it stays very cold.

The coldest temperature ever recorded on the earth’s surface was at Vostok, Antarctica in August 1960, with a reading of -129 degrees Fahrenheit!

Here are some other web links that contain good information about the earth’s energy budget and how it affects temperature: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/outline.html

http://cimss.ssec.wisc.edu/wxwise/homerbe.html

Ken Harding
National Weather Service