The entire year is divided into several parts according to the difference of temperature. Each part is called a Season. The whole year is divided into 4 parts according to the difference of temperature. They are : Summer, Autumn, Winter and Spring. We know, the entire world is divided into two poles. The northern part of the equator is north pole and the southern part is south pole. When there is summer in the north pole, winter prevails in the south pole. Again, when there is winter in the north pole, there is summer in south pole. In the same way, when there is Spring in the north pole, there is Autumn in the south pole. And Autumn comes to the North pole and spring is in the South pole. The geographical location of Bangladesh is in the north pole. Here extreme heat is felt around the month of June. During this period cold winter prevails in the south pole.
Causes of changing season : There are causes
for changing the season on earth :
(1) Increase and decrease of heat because of the difference of
temperature in day and night in different
parts of the world :The sun comes near the pole which sees the longest day and
shortest night because of the revolution of earth. Its opposite pole
experiences longest night and smallest day. The earth receives temperature at
day time and the land surface becomes heated. And at night, it becomes cool
after radiating the heat. During this time the amount of temperature it absorbs
in the longer day, the same amount of temperature cannot radiate in a shorter night.
That place experiences hot weather because of the heat stored there summer
is felt there. In the opposite pole the day is small and the heat it absorbs cannot
radiate at night and winter is felt there.
(2) The round shape of the earth : As
the earth is round, the sun rays fall on somewhere vertically and somewhat
obliquely. As a result, there is difference of temperature and the change of
season.
(3) Parabolic orbiting path of the earth :
The orbiting path of the earth is parabolic. So, the distance between the earth
and the sun increases and decreases at different times of the year. Temperature
changes because of it and so the season changes.
(4) Angular location on the orbit of the earth : At
the time of revolution, earth's polar region lies towards the same direction at
66.5° angle without making a right angle. For this reason the north pole and
the south pole come near the sun due to it once a year.
The pole which bends towards the sun, experiences vertical
sunshine. At that time it experiences the highest temperature. When the sun
does not shine vertically, the temperature falls and the season changes.
(5) Due to revolution : The sunshine falls on
different places due to revolution and the difference of temperature of air
occurs. As a result, the difference is seen in climate. It is called Changing
of season.
The process of changing season : We
know there are four seasons on earth - Summer, Autumn, Winter and Spring. Now
we will see how the season changes. The causes of changing season can be learnt
from the four locations of earth during its moving round the sun.
Summer in the North Pole and Winter in the South Pole : After
21st March the earth bends towards the sun gradually when going forward on its
axis, in this situation the greater is the lighted up days in the north pole.
Following this system on 21st June the sun starts shining vertically on the
Tropic of cancer. So, 21st June has the longest day and the shortest night in
the North Pole. That day is the last day of the to come towards the north, from
next day it again starts coming back to the South Pole. In the North Pole
before the beginning of summer it starts one and a half months earlier because
of the day being bigger and the next one and half months
Summer lasts. During Summer time in South Pole just the opposite situation is
seen and winter is set. The sun slimes for a lesser period because of its bending
position. So, the day becomes shorter and night is longer. At day time the surface
becomes heated, at night it becomes cooler because of the radiation of heat. Then
winter weather prevails there. In the South Pole this time is called Winter
Season (fig. 2.19).
Autumn in the North Pole and Spring in the South Pole :
From June 21 the South Pole starts bending towards the Sun. The parts of South
Poles experiences less sunshine and the parts of south see more sunshine. So,
on September 23 the sun shines vertically on the equator. Therefore, day and
night become equal everywhere at that time. The heat absorbed at day time gets
radiated at night. So, the amount of heat and cold in the weather remains same.
During this time Autumn and Spring prevail in North and South Poles
respectively. One and a half months before the 23rd September Autumn
dawns in the North Pole and it lasts for one and a half months.
Winter in the North Pole and Summer in the South Pole :
After September 23 the earth starts bending towards the sun, during this time
the South Pole comes closer to the sun. The North Pole starts going further
away. So, the sun shines vertically in the south and angularly in the north
pole. As a result, the day becomes shorter in the North and longer day and
shorter night in the South Pole. On the 22nd December the sun shines vertically
on the Tropic of capricom. On that day, day becomes
shorter and night becomes longer in North
Pole, and so winter season dawns. That is the last day of the sun to towards
south and from the next day the sun again starts coming towards the north.
Winter season begins in the North Pole before one and a half months of December
22 and continues up to one and half months. During this period summer prevails
in the South Pole.
Spring in the North Pole and Autumn in the South Pole : On
moving in its own orbit the earth comes back to such a place after December 22
up to such a place between December 22 and March 21 that it starts shining
vertically on the equator. So, day and night become equal all over the world on
March 21. The air space remains heated at day time because of sunshine and at
night it gets cooled after radiating heat. During this period spring is in the
North pole and Autumn in the South Pole. On March 21 day and night become equal
all over the world and that day is called Vernal Equinox (fig. 2.20).
Impact of Changing Season
• The change of season controls the daily life of human beings.
• It influences the economic activities as well as the
professions of human beings.
• Many places in the world appear to be the centres of
recreation due to season change.
• The change of season controls the growth and distribution of
animal kingdom.
• Different kinds of natural disaster are related with the change of season.
✔ Learn about The Universe and Our Earth
✔ Learn about Size and Shape of the World
✔ Learn about Important Lines Described in Geography
✔ Learn about The Movement of the Earth
✔ Learn about The Change of Season on Earth