#5: FEWER DAYLIGHT HOURS. Fewer daylight hours starts the list off. This made the list but it is only relevant in the cool season (especially winter). In the winter, above the Arctic Circle, there is very little daylight in the winter. Without the sun's energy to warm the surface, surface temperatures chill significantly. In the warm season (especially summer), there is actually many more daylight hours in the polar areas. Above the Arctic Circle, there is near 24-hours of daylight in the summer. Temperatures in the polar areas are much warmer in the summer as compared to the frigid winter. #4: LESS TROPOSPHERIC WATER VAPOR. The greenhouse gas water vapor makes the list at number 4. Water vapor is the most significant greenhouse gas in the lower troposphere because of its large quantity. In equal amounts, carbon dioxide is a much more powerful greenhouse gas than water vapor but generally there is much less carbon dioxide in the air as compared to water vapor. The amount of water vapor in the air is controlled by temperature and the potential for water vapor to be evaporated or transported into a region. The polar regions are cold. Because of the chill, polar air has a low capacity to evaporate water vapor in the air. The result is air with a low amount of water vapor. With less of this greenhouse gas in the air, less longwave energy is absorbed in the lower troposphere. More longwave energy escapes to space, thus helping to lower surface temperature on the whole. Only in the summer does it really get warm enough for significant water vapor to exist. #3: HIGH ALBEDO. Albedo is the ratio which the light reflected from an unpolished surface bears to the total light falling upon that surface. When the albedo is high, reflection is high. Ice, water and snow are good reflectors of solar radiation when the sun angle is low (sun near the horizon). The sun angle is fairly low throughout the year in polar areas. A significant amount of solar radiation that does reach the polar surface is reflected back into space. #2: TROPOSPHERIC LENGTH. The tropospheric length is the length a beam of solar radiation must travel to make it from the upper troposphere to the earth's surface. This distance is minimized when the sun is directly overhead and is maximized when the sun is near the horizon. When the sun is near the horizon as it is in polar areas, more solar radiation is scattered and reflected by the atmosphere than when the sun is more directly overhead. This results in some solar radiation not reaching the surface that otherwise would have in lower latitudes. #1: SUN ANGLE. Sun angle ties together all of the other factors that made the list and is thus number 1. It is low sun angle that makes the tropospheric length longer, reflection higher, the air colder (thus less water vapor) and the daylight hours fewer in winter. You will notice when looking at the shadow from a tree that it becomes longer as the sun moves closer to the horizon. When the sun is near the horizon, solar radiation is spread over a larger area. Thus, the intensity of solar radiation is far less at the polar areas than in the low latitudes. When the solar intensity is less the earth's surface can not warm as rapidly. |