What Causes Seasons?
The Earth's axis, the imaginary line around which the earth turns in its daily rotation, is slightly tilted. The seasons are caused by this tilt and by the fact that the Earth's axis always holds the same location in space. If the axis were not tilted, the sun would always be directly over the Equator and the northern and southern hemispheres would both have constant, similar weather the year round.
As the earth travels around the sun, the North Pole is sometimes pointed toward the sun and sometimes pointed away from it. When the North Pole is pointed toward the sun, the sun's rays strike most directly on the northern hemisphere. Vertical rays of sunlight are more effective in producing light and heat than are slanting rays. There are two reasons why: 1) slanting rays must pass through a greater thickness of atmosphere than vertical rays and thus lose more of their heat; and 2) slanting rays are spread out over a greater area than vertical rays and are therefore less concentrated. When the North Pole is pointed toward the sun, summer is produced in the northern hemisphere; the southern hemisphere has winter. When the South Pole is pointed toward the sun, the southern hemisphere receives the most heat and has summer.
The effectiveness of the sun's rays is one reason summer is warmer than winter. A second reason is that the days are longer. While the sun is shining, the land and air heat up. When the sun goes down, they begin to lose heat. Each summer day in the northern hemisphere is longer than the night and more heat is gained from the sun during the day than is lost at night. The opposite is true as winter approaches and the northern half of the Earth begins to cool. Not only does the sun move lower in the sky and become less effective in providing heat, but it shines for a shorter time each day, and heat is lost for a longer time than it is gained.