Why Do the Four Seasons Occur on Earth?

One of our planet Earth’s wonders is the different weather periods we experience every year, commonly known as seasons. What is even more astounding is the numerous things that happen because of these weather changes. It is not only our daily activities that vary but also the environment around us changes shape and colour.

For instance, days are usually the shortest in winter. The limited sunlight that trees get causes them to go dormant and stop growing leaves or flowers. But days get longer in spring and summer, trees blossom and the environment looks so beautiful.

Animals‘ behaviours also change a lot throughout the year. In winter, many animals hibernate. They get into a state of deep sleep for the entire season to conserve their energy and survive the harsh weather. Most birds migrate from their native cold habitat to other warmer places until the end of winter and then return home in the spring.

Recently, the world has been witnessing new changes associated with the seasons. For instance, summers are becoming hotter and more prolonged. In some regions, the temperature in mid-summer exceeds 50°C. Some scientists expect summer to eventually take over spring and extend for half the year. Winters are also getting shorter and colder.

Such changes in the seasons also cause other severe problems for our dear planet. Boiling summers bring about drought, which leads to wildfires. Australia suffered from disastrous bushfires for almost a year, from May 2019 to June 2020, that were caused by extreme drought.

Sadly, this catastrophe took its toll on nature. It burnt 58,000 square km, destroyed thousands of buildings, and killed millions of animals.

So what is it about seasons that make them so complicated? What causes seasons in the first place? And why are they changing in a way that threatens our ecosystem?

Well, that is precisely what we are discussing in today’s lesson.

So let’s hop right into it.

What are the four seasons?

We are all familiar with seasons thanks to the different weather associated with them.

When it gets too cold, cloudy, or snowy, it is winter, in which days are grey, and daylight is short. Many people like to go skiing, build snowmen, enjoy indoor time with family and friends, or drink lots and lots of hot chocolate in winter.

Then after that, the weather warms up for three months, in what is known as spring. Spring is usually sunny so trees bloom and beautiful flowers grow everywhere. It is lovely to go outside, have a walk, or picnic with friends.

Then, it is the beach season, also known as summer. In summer, the weather gets hot, too hot in some places. It can also be either dry or humid and rainy in other areas. Most people love to escape the heat by going to the beach.

After three hot months, the weather gets mild in autumn. It is neither hot like summer nor cold like winter. Then, the temperature drops gradually until they get to the lowest points. This is when the cycle repeats, and winter starts again.

We can pretty much consider summer and winter as extreme seasons. This is when temperatures are at their highest or their lowest, marking the hottest and coldest times of the year. On the other hand, spring and autumn are intermediate seasons during which the temperature gradually rises or drops.

When do seasons occur?

In one region, seasons occur in succession. When one season ends, another starts. But considering the entire Earth, seasons do happen simultaneously. To be more accurate, winter and summer happen at the same time. Then, they are followed by spring and autumn, which also occur at the same time.

To understand how this is possible, we must first learn where seasons happen.

The Earth is horizontally divided by the equator. The region that lies above the equator is called the northern hemisphere, and that below it is the southern hemisphere.

If you live somewhere in the northern hemisphere, for instance, in Europe, Asia, North Africa, or North America, you usually experience winter from December till February. Spring starts in March and ends in May. Summer then follows for three months, from June to August. And the year ends with autumn lasting from September to November.

On the other hand, for anyone based in the southern hemisphere, like New Zealand, Australia, or Argentina, their seasons are typically six months apart from the northern hemisphere. In other words, summer lasts from December to February, followed by autumn from March to May. Winter starts in June and ends in August, and spring extends from September to November.

That means July is summer in Canada but winter in Australia. In that way, two opposite seasons happen simultaneously but in two different regions.

But what about the north and south poles?

Well, both poles experience two seasons only (we will know why in a bit), summer and winter. It means that each season lasts for six whole months. The two poles are also opposite to one another. When it is summer at the north pole, it is winter at the south pole.

Why do seasons occur?

We know that the Earth spins around its axis once every 24 hours, which is how we experience day and night. The Earth also rotates around the Sun in an elliptical orbit once every 365 days. This elliptical orbit causes the Earth to be closer to the Sun in some periods of the year. Likewise, the Earth gets further away from the Sun in other periods.

Some people mistakenly think that it is the distance from the Sun that causes seasons. If the Earth is the closest, it is summer. When it is the farthest, winter occurs. And anywhere between these two points means it is either spring or autumn.

But this theory is not true. The Earth is usually the closest to the Sun in early January and farthest in early July. According to that theory, it should be summer in January and winter in July in the northern hemisphere. Since this is not true, it is not the distance from the Sun that causes the seasons.

Moreover, that theory does not explain why both hemispheres have different seasons.

Then what is it that causes the seasons? Well, it is the Earth’s tilt.

The Earth‘s axis is the imaginary straight line around which the Earth spins. It runs from the south pole, through the Earth’s core, and to the north pole.

But the Earth is not perfectly vertical; its axis is not perpendicular to its orbital plane but tilted by 23.5°. So at any time of the year, the tilted axis will either point toward or away from the Sun, exposing one hemisphere to the Sun more strongly than the other.

So when the northern axis is pointing toward the Sun, the northern hemisphere is more exposed to the Sun than the southern hemisphere. So, it is summer above the equator and winter below it. The opposite is also true.

If the southern axis points toward the Sun, it will be summer in the southern hemisphere and winter in the north.

Scientists have found that the Earth’s northern axis points the most toward the Sun on 21 June. After six months, it points the most away from the Sun, typically on 21 December. These two dates mark the beginning of summer and winter for the northern hemisphere and the beginning of winter and summer for the southern hemisphere.

Why do the north and south poles only have two seasons?

As we have just mentioned, the Earth’s axis takes six months to move from pointing toward the Sun to pointing away from the Sun. So since the north pole is at the top of the planet, it will be exposed to the Sun and, therefore, experience summer for six months straight. During that long period, the Sun never sets.

Meanwhile, the south pole is away from the Sun and thus has winter simultaneously with the north pole summer. Likewise, the Sun never rises at the south pole in winter.

After six months, everything is inverted. The north pole is away from the Sun and does not receive sunlight. So it experiences winter, while it is summertime in the south pole.

That said, one cannot help but wonder, if the north and south poles get exposed to the Sun for six months non-stop, does that not make them the hottest places on Earth during that time?

Well, that question does make sense. But the answer is no. They are never hot or even warm. In fact, both poles remain freezing during their summers.

And this is all caused, again, because of the Earth’s curvature.

Since Earth is a sphere and the north pole is at the top of that sphere, the Sun’s energy is spread out over the curve of the Earth. So yes, the north pole gets six months of sunlight but way less heat. That is why the north pole in the midst of summer has an average temperature of 0°C.

In winter, when it is entirely dark at the north pole, the average temperature drops to -40°C!

The same thing applies to the south pole during its summer and winter. During summer, the south pole is lit but receives little heat. So, its mean temperature over these six months is -28.2°C. On the other hand, it gets super freezing in winter, with the average temperature dropping to -60°C.

What makes the south pole colder than the north pole?

The south pole being colder than the north pole in both summer and winter has nothing to do with Earth’s tilt but more with the nature of both poles.

The south pole is located on a landmass surrounded by water, the Southern Ocean. This landmass is Antarctica, and Antarctica, which is 14.2 million square km, is covered with a super thick ice sheet. This great ice sheet has a 2.16 km average thickness, an area of 8.53 million square km, and a volume of 30 million cubic km.

On the other hand, the north pole is in the Arctic Ocean, which is surrounded by land. This land is the northernmost regions of Russia, the United States, Norway, Canada, and Denmark. Ice sheets, icebergs, and glaciers float on the Arctic Ocean, collectively, making an area of about 6.61 million square kilometres.

So there is much more ice at the south pole than at the north pole. And that is why the south pole is way colder than its northern counterpart.

Why are days longer in summer and shorter in winter?

Well, this, again, caused by the Earth’s tilt.

When the north pole is pointing toward the Sun, the northern hemisphere receives more sunlight in summer than the southern hemisphere. The Sun rises early and sets late, making days longer. As a result, the summer day length increases to 15 hours while the night lasts only nine hours.

But again, since the Earth is round, not every region in the northern hemisphere has a 15-hour-long summer day. The more up north you go, the longer the summer day gets.

For instance, a summer day in Sweden is usually 18 hours long. But the same day in Egypt, which is way to the south of Sweden, lasts for 14 hours only, despite both countries being in the northern hemisphere.

When the southern hemisphere points away from the Sun, it receives less sunlight. So days become shorter and the nights get longer. Typically, a winter night is 15 hours long while the day, from sunrise to sunset, only lasts for nine hours.

Are the seasons changing?

Well, yes. More precisely, summer and winter are.

Scientists have found that summer is getting longer. In the northern hemisphere, it used to be 78 days on average but jumped to 95 days in only 60 years. Some scientists expect summer to extend for six months. Conversely, winters are continuously shortening.

Additionally, both seasons are currently witnessing a noticeable change in temperatures. So winters are getting colder, and summers are becoming hotter and hotter.

And this is all happening because of global warming.

In our previous lesson about the atmosphere, we learned that using fossil fuels, such as coal, gas, and oil, over the past 150 years has distributed the Earth’s balance.

The burning of these fossil fuels releases carbon dioxide (CO2) and methane (CH4) into the air. These two gases absorb the heat from the Sun, lock it in the atmosphere, and prevent it from going back to space. They are called greenhouse gases because they turn the Earth into a gigantic, hot greenhouse!

So as heat is trapped in the atmosphere, the temperature rises. This has been going on for a century and a half now. That is why both summer days and nights have become hotter over the years.

Conclusion

And so we get to the end of today’s journey, in which we studied the year’s four seasons.

In this article, we learned that it is not the distance between the Earth and the Sun that causes the seasons but rather the Earth’s tilt. This also causes the Earth’s two hemispheres to have two different seasons simultaneously.

Then, we moved on to the north and south poles and understood why they experience only two seasons, winter and summer, and why each of them lasts for six months. Thanks to the Earth’s roundness which spreads the heat from the Sun, the north and south poles never get hot during summer despite being exposed to the Sun for six months each.

After that, we explored the nature of the two poles and understood why the south pole is typically colder than the north one. Finally, we got to know why days are longer in summer and shorter in winter as well as the changes that these extreme seasons are experiencing thanks to global warming.

We hope you enjoyed today’s lesson as much as we enjoyed writing it for you. If you still have not checked our lesson about our planet’s dazzling atmosphere, go ahead and read it here.

And until another journey, keep learning.

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