“I think it’s going to rain, ’cause my knees hurt.”
Whenever my grandmother said her knees hurt, it rained without exception. When cloudy days continue, some people say they feel down or their bodies ache. It is not just their imagination. When somebody says, “I’m a little under the weather,” his body is actually responding to weather changes.
When atmospheric pressure, weighing on our bodies, becomes low, the pressure inside the knee joint becomes high relatively, which causes pain in the knees. Moreover, when atmospheric pressure is low, it becomes cloudy and the sky becomes darker. As a result, people get less sunlight comparatively. Then, serotonin, a chemical produced in your brain that makes you feel happier and calmer, decreases, and melatonin, a hormone associated with sleep onset, increases. That’s why you come to feel down for no particular reason. Such physical response to influence by meteorological factors is called meteorotropism. It happens because atmospheric pressure takes effect. Then, what is atmospheric pressure that it decides our moods and body conditions?
Atmospheric pressure is the pressure exerted by the weight of air in the atmosphere of the earth. We do not sense the weight intuitionally. To visualize the weight of air, Evangelista Torricelli, an Italian physicist, employed mercury that is 13.6 times denser than water. He filled a tube about one meter long, sealed at the top with mercury, and set it vertically into a basin of mercury, making sure no air would go in. There were two forces working: the weight of mercury going down and atmospheric pressure that pushed mercury in the basin so that it would go up in the tube. As a result, the column of mercury fell to about 76 ㎝ [30 in], leaving the top of the tube vacuum. No more change in the height of the column of mercury in the tube means that the two forces were balanced. In conclusion, atmospheric pressure in the outside was strong enough to push mercury up to 76 ㎝ in the tube. When atmospheric pressure becomes higher, the column will be taller, and when atmospheric pressure becomes lower, the column will become shorter. This fluctuation in the column’s height enables atmospheric pressure to be measured. It is the mercury barometer. When water, lighter than mercury, is used, it forms a column of about 10.3 m [33.8 ft].
You may take this as a physical phenomenon, irrelevant to your everyday life. However, as mentioned in the beginning, atmospheric pressure affects not only your body but also many things. You cannot drink something with a straw, if there is no atmospheric pressure. When you suck liquid through a straw, the pressure inside the straw becomes lower. Then, the atmospheric pressure outside the straw becomes higher relatively and pushes the liquid into the straw. Then, can liquid be sucked as long as the straw permits? It is only possible up to around 10 m [32.8 ft] no matter how strong you are, because the liquid is pushed by air. On the same principle, the vacuum pump cannot draw water more than 10.3 m. If you want more, you need to use a compressor.
The typical place where we can feel atmospheric pressure is inside the airplane. After a plane takes off, it gains height. Then you may experience “airplane ears”—feeling your ear deafened or hurt. It happens because the pressure inside the body remains the same, whereas the outside atmospheric pressure becomes lower. Such unequal pressures push the eardrum outward, causing pain. If you swallow or yawn at that time, it activates the muscle that opens the Eustachian tube that connects the middle ear with the nasopharynx. Once opened, a little droplet of air can pass from the nose and throat to the middle ear, thereby relieving pressure, with a little pop.
One atmosphere (atm in short) is as heavy as the pressure applied by an object weighing 1 ㎏ on the area of 1 ㎠. If we suppose the area of your palm is roughly 50 ㎠ [7.75 in²],, the weight of air you are holding up with your palm amounts to 50 ㎏ [110 lb]. It is like we are carrying a column of water of about 10 m on our shoulders every day. But why can we not feel it? We don’t feel the weight of the air because there is an outward force in our body as much as the atmosphere presses us.
However, if atmospheric pressure goes higher than 1 atm, we will feel the pressure. When you go inside the water, you can feel water pressure. For every 10 m you go down, water pressure becomes higher by 1 atm. Deep-sea creatures that live in the 10 ㎞ [6.2 mi] deep sea put up with 1,000 atm. If a man goes down that deep without any protective gear, he will be pressed flat. Nevertheless, deep-sea creatures swim around the sea freely despite the tremendous pressure. It’s because they fill the spaces in their body with water or oil instead of air. That way, they can build enough internal pressure to achieve equilibrium with external pressure. It is for the same reason that an empty iron ball gets crushed in the deep water, while an aluminum can filled with water doesn’t.
With an apparatus, a man can dive 30 m [98 ft] deep. Scuba diving is an example. However, if he dives deeper than that, noticeable symptoms begin to occur. Around 30 m deep in the water, scuba divers experience nitrogen narcosis, a condition where they cannot reason properly and fail to respond to emergency, and their memories get dim. As a diver goes down deeper in the water, the pressure becomes higher, and as a result, more air dissolves and goes into his blood. Among them, nitrogen causes the anesthetic effect. If he swims up to less than 30 m deep, such symptoms soon disappear.
However, if he comes out of the water too suddenly, it can be more dangerous because of decompression sickness, also known as divers’ disease. If he comes to the surface quickly, air pressure goes down and dissolved gases come out of solution into bubbles inside the body. And these bubbles burst capillaries, and if they damage major blood vessels such as in the brain, it can cause even death.
Contrary to the deep sea, atmospheric pressure is low and oxygen is thin in the high altitude. When a man climbs up, oxygen in his body becomes thin abruptly, which causes fatigue, headache, stomach illness, dizziness; and in serious cases, difficulty in breathing and death. This is called altitude sickness.
What about in space, higher up above? Space is vacuum—0 atm. There, only the force inside the body pushing outward exists, which is dangerous for the body. That doesn’t mean that the body pops right away, though. In 1965, a man was in a vacuum chamber for a space suit test at NASA. Accidentally, the pressurization hose became disconnected and the space suit was depressurized to a near vacuum. The person lost consciousness in about 15 seconds, but he regained it soon after repressurization of the chamber began. He later said his last memory before blacking out was of the saliva on his tongue starting to boil. Thanks to the skin covering his body, he could survive exposure to vacuum conditions for a short time, but it must have indeed been a frightening moment.
When the unprotected human body is exposed to vacuum, the first thing he would notice is the lack of oxygen, and after a while, he would lose consciousness. His skin and the tissue underneath will begin to swell as the water in the body starts to vaporize in the absence of atmospheric pressure and the decrease of boiling point. And eventually, his body will freeze. For these reasons, astronauts wear the Extravehicular Mobility Unit [EMU] when they need to perform extra-vehicular activity although it looks inconvenient and awkward. The internal pressure of the EMU is around 0.3 atm, which is lower than the normal atmospheric pressure (1 atm). So they have to adapt themselves to the new environment by lowering the atmospheric pressure inside the spaceship before taking a spacewalk.
Mars, the nearest planet to Earth, is composed mostly of carbon dioxide, and its atmospheric pressure is about 0.01 atm. On the other hand, the pressure of Venus’s atmosphere is 90 atm, and the planet is composed primarily of carbon dioxide much denser and hotter than that of Earth. It is like more than 900 tonnes are pressing down on 1 ㎡. Earth’s atmospheric pressure is 1 atm and its atmosphere is made up of gases indispensable for our life. Despite the weight of air, we live without feeling any inconveniences. What do living things that are created to endure the perfect amount and weight of atmosphere tell us?
[H]e established the force of the wind . . . Job 28:23–25
- Reference
- Science Teachers Association, 101 Questions Even Science Teachers Are Curious About (in Korean, 과학선생님도 궁금한 101가지 과학질문사전), Book Mentor, 2010
- Kim Tae-il, Living Science Textbook (in Korean, 살아있는 과학 교과서), Humanist, 2011