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The Seasons
Earth undergoes a yearly seasonal cycle because it has a tilted axis which does not rotate as the Earth revolves around the sun. As a result, each hemisphere receives different levels of solar radiation during different points in the Earth's orbit. The amount of solar energy absorbed by the Earth's surface is known as insolation. In July, the Northern Hemisphere is positioned to absorb a great deal of solar energy, resulting in higher levels of insolation. At the same time, the Southern Hemisphere is receiving solar radiation at a more indirect angle, and receives lower levels of insolation. This variance in solar radiation produces the seasons.
Land and Water
Dry land and large bodies of water respond to insolation in different ways. Water has a large amount of thermal inertia, which means that it takes a comparatively large amount of energy to raise water's temperature, and it cools more slowly than many materials when energy levels are reduced. On average, it takes about three times more energy to heat water to a specific temperature than it does to heat dry land to the same degree. Likewise, water cools about three times more slowly than dry land when ambient temperatures fall.
Geography of the Hemispheres
Most of the Earth's land mass is concentrated in the Northern Hemisphere. By comparison, the Southern Hemisphere is mostly water, with only a few land masses and a much lower proportion of land far from a coastline. The continents of North America and Eurasia have extensive interiors, while South America, sub-equatorial Africa and Australia present much narrower profiles. Therefore, the oceans play a much larger role in temperature regulation in the Southern Hemisphere than in the Northern Hemisphere.
Oceans and the Seasons
Due to the thermal inertia of water, coastal areas tend to have milder seasons than areas in the interiors of large land masses. The oceans are slow to warm up in the summer and retain much of their warmth in the winter, which helps to even out the seasonal changes in coastal areas. Locations far inland lack the mitigating influence of the ocean and tend to heat up and cool down more quickly than coastal locations. This means that the Northern Hemisphere, with its extensive land masses, undergoes greater temperature changes than the Southern Hemisphere when exposed to the same changes in solar energy.