Temperature Patterns

Latitude and Temperature

The average surface temperature of Earth as a whole is 15°C (59°F), and the most obvious spatial pattern is a latitudinal gradient: Low-lying tropical regions near the equator typically experience annual average temperatures around 27°C (81°F), while high-latitude ice caps in Antarctica and Greenland have annual average temperatures below −40°C (–40°F). The seasonal variation in temperature will likewise differ significantly across the planet—low-latitude coasts will have seasonal temperature swings of less than 3°C (37°F), while high-latitude continental interiors will experience swings in excess of 40°C (104°F).

The latitudinal temperature gradient is due to the variation in the sun angle (the elevation of the sun above the horizon) between the equator and the poles. Earth is an oblate spheroid orbiting the sun at an average distance of 150 million kilometers (93 million miles). Due to the curve of the Earth, the noontime sun will pass directly overhead (the sun angle will be 90°) at a specific latitude, known as the solar declination, with lower sun angles at latitudes away from the solar declination. Due to the constant tilt of Earth's axis relative to the plane of the orbit, the solar declination cycles between the Tropic of Cancer (23.5° N) on the summer solstice to the Tropic of Capricorn (23.5° S) on the winter solstice, passes over the equator twice a year—once on each equinox.

Lower sun angles mainly result in cooler temperatures because the solar radiation is spread over a larger surface area when the sunlight strikes the surface obliquely. When the sunlight is thus diffused, less energy is absorbed per unit of surface and less warming occurs. Two additional [Page 2794]factors also contribute to cooler temperatures at higher latitudes: First, polar surfaces such as ice tend to have higher albedos, so a larger amount of insolation is reflected rather than absorbed. Second, radiation arriving with a low sun angle travels a longer, slanting path through the atmosphere than when the sun is directly overhead, and therefore, more energy is absorbed or scattered before reaching the surface.

Seasonal variability in temperature results from seasonal variations in the sun angle. As Earth orbits the sun, the solar declination latitude moves north and south, producing higher sun angles and warmer temperatures during the summer season. Outside the tropics, there is a 47° swing in noontime sun angle from summer to winter. In addition to sun angle, day length (hours of sunlight) varies from summer to winter, with larger variations at greater distances from the equator. At high latitudes, winters are characterized by short days and low sun angles, while in the summer the days are longer and the sun is higher. As a result, high-latitude locations experience large seasonal variations in temperature, with extremely cold winters and moderate summers. In contrast, at low-latitude locations, the sun remains relatively high above the horizon throughout the year, and seasonal changes in day length are small, resulting in reduced seasonal variation in temperature. Therefore, during winter, the extremely low temperatures at high latitudes result in a significant temperature gradient from the equator to the poles, while high-latitude warming during summer produces a reduced latitudinal gradient.

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