Atmospheric Pressure: Even though the Earth’s atmosphere is divided into several layers, the part that sustains all life we know extends upward only to about 18,000 feet – just over 3 miles. Whether you are aware of it or not, Atmospheric Pressure affects your everyday life. Weather patterns and forecasts around the world are affected by Atmospheric Pressure. Atmospheric Pressure, also known as Barometric Pressure, is the force exerted upon the Earth by the blanket of air that surrounds it. What does it mean to us, and how does it affect us? We will ponder upon these questions in this article.

What is Atmospheric Pressure?

We define atmospheric pressure as the pressure exerted by the column of air above the earth’s surface.

The atmosphere is composed of gases surrounding the earth. As long as there is a gravitational force acting on it, it will have the same weight as any other object. Atmospheric pressure is defined as the weight of a column of air of a unit area that reaches the top of the atmosphere. It was first measured using a mercury barometer, so it is also called barometric pressure.

Unit of Atmospheric Pressure

Pressure is defined as the force per unit area. The SI unit of atmospheric pressure is pascal. A pascal \(\left({pa} \right)\) is defined as the pressure exerted by a force of \(1\,N\) over an area of \(1\,{{\text{m}}^2}.\)
\(1\,{\text{pascal}} = \frac{{1\,{\text{newton}}}}{{1\,{{\text{m}}^2}}}\)

Measurement of Atmospheric Pressure

Evangelista Torricelli invented a device called the barometer to measure atmospheric pressure. It consists of a one-metre glass tube closed at one end, filled with mercury, and then dipped into a bowl of mercury. The level of mercury inside the tube settles at \(76\,{\text{cm}}\) when we are at sea level. The space above the mercury column is a vacuum which is also known as Torricelli Vacuum.

\(p = \rho hg\)
where,
\(p\) is the pressure in pascal.
\(\rho \) is the density of mercury. It is \(13.59\,{\text{g}}/{\text{cc}}\)
\(h\) is the height of the column of mercury in millimetres. At sea level, it \(760\,{\text{mm}}.\)
\(g\) is the acceleration due to gravity. It is \(9.81\,{\text{m}}/{{\text{s}}^2}\)
\(p = 13.59 \times 760 \times 9.81 = 101321.6\,{\text{pascals}}\)
\( \Rightarrow P = 101.32\,KPa\)

Variation of Atmospheric Pressure With Altitude

The atmospheric pressure is not the same throughout. At sea level, the pressure is \(760\,{\text{mm}}\) of mercury, and it reduces as we move upwards.

Relation Between Atmospheric Pressure and Weather

Winds are created when air flows from a high-pressure area to a low-pressure area. The heat from the sun warms the air, which in turn becomes less dense and rises upwards, creating a low pressure region. The air from the surrounding high-pressure areas rushes in to fill this void. Low pressure causes windiness, storms, and precipitation. Calm weather is an indication of high pressure.

Cyclones, hurricanes, typhoons are a result of low pressure. The tropical regions on either side of the equator get uniform heat from the sun almost year-round. As a result, the warm air rises to create low pressure. Tropical storms and cyclones are a result of this.

The Indian monsoon is also a product of atmospheric pressure variation. Generally, April and May are the hottest months in India. The warm air creates low pressure causing the moisture-laden cold, heavy winds from the high-pressure air above the Indian Ocean to rush to fill this low-pressure area. This brings in good rains and is called the monsoon, a cyclic phenomenon that usually begins in June every year.

Effect of Atmospheric Pressure on Human Body

We know that the pressure of air reduces as we go up. As a result, the oxygen level also reduces considerably, which causes altitude sickness. People who go to high altitude regions from the plains take some time to get used to the low pressure. This is called acclimatization. Mountaineers must carry bottled oxygen while climbing high mountains.

People with joint pains and arthritis also feel pain during changes in atmospheric pressure. Low atmospheric pressure is also the reason for headaches in some people. High atmospheric pressure can cause an increase in blood pressure.

Atmospheric Pressure – Sample Problems

Q.1. If we use water in place of mercury in a barometer, what will be the height of the water column?
Ans: Water density is \(1000\,{\text{g}}/{\text{ml}}{\text{.}}\)
Air pressure is \(101321\,Pa.\)
\(P = \rho hg\)
\(h = \frac{P}{{\rho g}}\)
\( \Rightarrow h = \frac{{101321}}{{1000 \times 9.81}} = 10.32\,{\text{m}}\)
If water is used instead of mercury, we need a tube of more than \(10\,{\text{m}}\) to measure pressure.

Q.2. At a certain place, the level of mercury in the barometer tube is \(74\,{\text{cm}}.\) If the density of mercury is \(13600\,{\text{kg}}\,{{\text{m}}^{ – 3}}\) and acceleration due to gravity \(g = 10\,{\text{m}}{{\text{s}}^{ – 2}},\) then what is the value of atmospheric pressure at that place?
Ans: The atmospheric pressure is given by
\(P = \rho gh\)
\( \Rightarrow P = 13600 \times 10 \times 0.74 = 100640\,Pa\)

FAQs on Atmospheric Pressure

Q.1. What is the difference between air pressure and atmospheric pressure?
Ans: Atmospheric pressure is the pressure exerted by the atmosphere downwards due to the earth’s gravity. Air pressure is exerted by the air surrounding us in all directions.

Q.2. Does atmospheric pressure affect the boiling point of water?
Ans: Yes. The boiling point of water is \({100^ \circ }{\text{C}}\) at sea level. At higher altitudes, the atmospheric pressure reduces. Therefore, the boiling point also reduces.

Q.3. Can atmospheric pressure affect the body?
Ans: Yes. At higher altitudes, it is less and contains less oxygen. Therefore, altitude sickness is caused in mountaineers due to low pressure and low oxygen.

Q.4. What is the instrument used to measure atmospheric pressure?
Ans: Mercury barometer or aneroid barometer is used to measure atmospheric pressure. Aneroid barometer does not use mercury.

Q.5. What is the normal atmospheric pressure?
Ans: We consider normal atmospheric pressure at sea level. It is \(760\,{\text{mm}}\) of mercury at sea level.