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.
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| 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.
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