Seasons are caused by the tilt of the Earth away or toward the sun

What causes the seasons?

The seasons are caused by the tilt of the Earth's rotational axis away or toward the sun as it travels through its year-long path around the sun.

The Earth has a tilt of 23.5 degrees relative to the "ecliptic plane" (the imaginary surface formed by it's almost-cicular path around the sun). The tilt toward the sun is maximized during Northern Hemisphere summer in late June (the "summer solstice"). At this time, the amount of sunlight reaching the Northern Hemisphere is at a maximum.

In late December, on the date of the "winter solstice", the Earth's tilt away from the sun is maximized, leading to a minimum of sunlight reaching the Northern Hemisphere. The seasons, of course, are reversed in the Southern Hemisphere.

During the winter, cold air masses build up over North America, Europe, and Asia, due to the low intensity of sunlight. The oceanic air masses are much less affected by the seasons because circulations in the upper ocean replenish warm surface water if it has been cooled.

The strong temperature contrast between the cold air masses over land and the relatively warmer air masses over the ocean lead to extratropical (non-tropical) cyclone formation (low pressure). These storms are thus much more frequent and intense in the winter than in the summer.
Interesting facts:
The sun is actually closest to the Earth during Northern Hemisphere winter (not summer). Thus, the amount of sunlight averaged over the whole Earth, is as much as 7% more intense in the winter than the summer. Despite this fact, the global-average surface temperature is warmer in Northern Hemisphere summer, due to the much greater expanse of land there, and since land heats to a higher temperature than the ocean does.

(page last updated 11/26/2010)
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