April 11, 2021

Wind, the Invisible Force

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Wind is invisible. However, we can definitely feel the presence of the wind. It is because the movement of the wind can be seen indirectly through swaying leaves, fluttering hair, and a spinning pinwheel.

The power of the formless wind has even changed the history of mankind. In the sixteenth century, England, losing in a battle, won an unprecedented victory over Spain called Invincible Armada. The strong winds and currents that blew right at the perfect time worked in favor of the English taking down the Spanish ships. Wind is invisible but has a big impact on our lives, even affecting the rise and fall of a nation. What is wind?

Wind, the movement of air

Simply put, wind is the flow of air, which is created due to the difference in atmospheric temperature and air pressure. Warm air becomes less dense and lighter and rises just like a balloon with helium which is lighter than air. Then cold air moves to where the warm air was in order to fill in the empty space, and this is called the wind.

Just as water flows downward, air moves from high to low atmospheric pressure. High atmospheric pressure is where air pressure is higher than the surroundings, and low atmospheric pressure is where air pressure is lower than the surroundings. Where the atmospheric pressure is high, the wind blows outward from the center, and where the atmospheric pressure is low, the wind blows from the surroundings to the center.

Even now the atmosphere continues to move. You can clearly see it just by looking at the wind that touches the skin, clouds flowing somewhere, and the weather that changes multiple times a day. However, when you look at the whole view of the earth, you can see that there is a consistent flow in the motion of the atmosphere. This global flow of air is called the atmospheric circulation.

Atmospheric circulation starts with solar energy. Because of the round surface of the earth, the amount of sunlight reaching the earth’s surface differs, depending on its latitude. Therefore, the closer it is to the equator, the more solar energy reaches its surface. Thus, near the equator, warm air rises, creating an empty space, and the surrounding cold air moves to that space. As the risen mass of air cools while rising, it becomes heavier and heavier, which causes it to come back down, and the air that has come down spreads to all directions, and moves to the areas where there are air shortages. Air circulates through this process. So, the energy imbalance by different latitudes eventually leads to a large-scale convection.

Global Winds

If the earth didn’t rotate on its axis, the wind would only move from the polar regions which have high atmospheric pressure to the equator which has low atmospheric pressure. However, on the rotating earth, the direction of the wind bends. In the Northern Hemisphere, the wind blowing northward blows diagonally as if it were forced to the right in its direction. On the earth, three main circulations exist between the equator and poles, influenced by the earth’s rotation, distributions of oceans and lands on the surface. The winds blowing on the surface as a result of these are the trade winds, the westerlies, and the polar easterlies.

The trade winds blow in the lower part of the troposphere between the earth’s equator and 30 degrees latitude, and they blow from northeast to southwest in the Northern Hemisphere. The name of the trade winds is derived from the time when ships like Spanish galleons traded using these winds in the Age of Exploration. These winds are very useful for sailing to the Continent of America from Europe. The westerlies which blow in the middle latitudes are winds that blow from the west to the east, and the wind tends to blow from the west to the east in the countries that are in the zone influenced by the westerlies.

Atmospheric circulation protects lives by distributing energy

Of course, if the earth only absorbed energy, the overall temperature of the earth would keep rising but such a thing doesn’t happen since the earth also emits energy. The amount of radiant energy the earth emits is almost consistent, regardless of latitude. Therefore, in the low latitude region, the amount of energy emitted is smaller than the amount of energy absorbed, resulting in excessive energy. In the high latitude region, the amount of energy emitted is higher than the amount of energy absorbed, causing energy to be insufficient.

The difference in the amount of sunlight received by each region is greater than most people would think, so if heat did not travel around the earth, there might have been few places where people could live. Places with a lot of heat will keep heating up and melt, and places with less heat will continue to be colder and everything will freeze.

Then how does energy circulate above the earth? Heat spreads thanks to atmospheric convection, which moves the air and wind on the earth’s surface. If air does not spread heat as it travels, there will be a very severe energy imbalance depending on the regions. As the trade winds, the westerlies, and the polar easterlies blow constantly and cause friction against the ocean, they create surface ocean currents which flow in certain directions. As the atmosphere and sea water circulate and transport the excessive heat energy from low latitudes to high latitudes where there is a shortage of energy, the earth can keep an energy balance.

Even at this moment, air and sea water are moving diligently and spreading heat evenly throughout the earth. The earth’s atmospheric circulation, along with the ocean’s circulation, contributes to the distribution of energy on the earth.

Earth needs typhoons

Apart from this general heat circulation process, there is also a unique auxiliary device in the earth’s atmosphere that can move a large amount of energy at once. It is typhoon. A typhoon is a tropical cyclone that travels to high latitudes with strong winds and heavy rains, as the warm air in low latitude regions receives a massive amount of vapor from the sea. Typhoons are called with different names, depending on the location where they occur. It is called a typhoon in the western Pacific region, and a hurricane in North and Central Americas, and a cyclone in the Indian Ocean.

A typhoon gets a lot of heat in the process of receiving vapor. As it moves to high latitudes in this state, it can move the remaining heat in low latitudes very quickly and efficiently. Like this, a variety of air flows are constantly being made in the atmosphere surrounding the earth, and the heat too circulates along the moving air.

Typhoons cause great damages with strong storms almost every year. However, it is an essential weather phenomenon because it sucks up the hot air of the tropics and scatters it in high latitudes, maintaining proper temperature and heat balance of the earth. It also plays an important role in providing water resources which eliminates water shortage problems and also in resolving the red tide phenomenon of the sea by stirring and circulating seawater.

What if the heat energy received from the sun did not circulate on the earth? The earth’s surface would have experienced a tremendous difference in temperature between day and night and between different latitudes. Fortunately, the earth is dispersing heat energy through convection of the atmosphere and oceans. This atmospheric circulation allows the earth’s energy to balance over a long period of time so that living creatures can live on the earth like a mom’s warm embrace.

​References
Hong Jun-eui and 3 others, Living Science Textbook 1, Humanist, 2011
Kim Byeong-chun and Park Il-hwan, Interesting Weather and Climate Change Story, Gana Publishing Company, 2014
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