Page 130 - SST Class 06

WHAT CAUSES THE SEASONS? extratropical (non-tropical) cyclone formation

The seasons are caused by the tilt of the earth’s (low pressure). These storms are thus much more
rotational axis away or toward the sun as it frequent and intense in the winter than in the
travels through its year-long path around the sun. summer.
The earth has a tilt of 23.5 degrees relative to the
“ecliptic plane” (the imaginary surface formed by
its almost-circular 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 maxi-
mum.
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 Winter Solstice
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 sun light. 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
Summer Solstice
warmer air masses over the ocean lead to

Terminology

Inclination : degree of sloping
Equinoxes : one of the two days in the year when day and night are of equal length
Replenish : fill up again

Inside Story
Inside Story
g The earth rotates on its axis everyday and orbits the sun every year.
g The tilting of the earth on its axis is known as the inclination of the earth’s axis.
g It takes the earth just under 24 hours to make one complete rotation.

g Being spherical in shape, the earth completes one motion in 24 hours and moves from
west to east.
g The circle which divides the earth into lit-half is known as circle of illumination.

Social Science-6 130