Atmospheric Pressure, also known as barometric pressure, is an important topic in Geography for competitive exams like IAS and UPSC, along with State Civil Service Exams, PSC, SSC, Bank Exams, PO, etc. Here are the NCERT study notes on the topic to help you prepare for your exams.

Atmospheric Pressure

• Atmospheric Pressure is nothing but a column of the weight of air in a particular unit area from the mean sea level to the top of the atmosphere.
• Atmospheric pressure is also called barometric pressure.
• The pressure is measured in force per unit area.

• The atmospheric pressure is expressed in the ‘millibar’’ or ‘mb’ unit.
• The atmospheric pressure can also be measured with the help of a mercury barometer.
• To check the pressure the height of the column of the mercury should match the weight of the column of the atmosphere in the mercury barometer, and we can also measure the pressure with the help of an android barometer.
• The atmospheric pressure is stated as kilo-pascals at the application level.
• The mercury barometer or aneroid barometer is used to measure the atmospheric pressure.
• The pressure decreases rapidly with height in the lower level of the atmosphere.
• The horizontal pressure gradient force is much smaller than that of the vertical pressure.
• In the center, the lowest pressure is the isobars of the low-pressure is encircled by one or more isobars.
• In the center, the highest pressure is the isobars of the high-pressure is encircled by one or more isobars.
• Isobars are connecting the lines and it has equal pressure.

Units to Measure Atmospheric Pressure

There are seven different ways to measure atmospheric pressure.

• Millimeters (Inches of mercury)
• Pounds per square inch (psi)
• Dynes per square centimeter
• Millibars (mb)
• Kilopascals

Standard Sea Level Pressure in Units

• The atmospheric pressure on the surface of the earth in millimeters is 760 or 29.9 inches of mercury.
• The atmospheric pressure on the surface of the earth in pounds per square inch is 14.70
• The atmospheric pressure on the surface of the earth in dynes per square inch is 1,013.25 × 103.
• The atmospheric pressure on the surface of the earth in millibars is 1,013.25.
• The atmospheric pressure on the surface of the earth in kilopascals is 101.325.

Pressure Gradient

• The change in the rate of pressure regarding distance is called the pressure gradient.
• If the pressure gradient is negative it is known as the force density.
• The pressure gradient is expressed in units of pascals per meter.
• Both path and geometry of the column are not the same.
• The horizontal pressure gradient force is directed from higher toward lower pressure.
• To make wind, the pressure gradient acts as the main force in the air.

Pressure Belts

• There are two patterns over the earth: Alternate high and Low pressure.
• There are seven pressure belts in the atmospheric pressure.
• There are two subtropical highs in north and south and two subpolar lows in north and south and the two polar highs in north and south except for the equatorial low.
• The pressure belts oscillate with the movement of the sun.
• They move southwards in the winter season and they move northwards in the summer in the northern hemisphere.
• On being light the equatorial region gets too much heat and warmth. The rise in the flow of air in the equatorial region generates a low pressure.
• Equatorial Low: The low equatorial is found near the equator. The pressure in the sea level is low.

• Subtropical Low: The subtropical low is present in the high-pressure areas, this reason is present in 30 degrees north and 30 degrees south.
• Subpolar Lows: This region is present in the low-pressure belts particularly in 60 degrees north and 60 degrees south.
• Polar Highs: The Polar highs present are near poles which have high pressure.

FAQs

Question: What is atmospheric pressure?

Question: How is atmospheric pressure measured?

Question: Why does atmospheric pressure decrease with altitude?

Question: How does atmospheric pressure affect weather?

Question: What is the relationship between atmospheric pressure and wind?

MCQs

1. Which of the following instruments is used to measure atmospheric pressure?

A) Thermometer

B) Barometer

C) Hygrometer

D) Anemometer

2. Atmospheric pressure is highest at which of the following locations?

A) At sea level

B) At the top of Mount Everest

C) At the equator

D) At the North Pole

3. Which of the following factors causes atmospheric pressure to decrease with altitude?

A) Increase in the density of air molecules

B) Decrease in the weight of the air above

C) Increase in the gravitational pull

D) Decrease in the volume of air

4. What is the standard atmospheric pressure at sea level?

A) 1013.25 hPa

B) 1000 hPa

C) 1020 hPa

D) 1050 hPa

5. In which region would you expect to find areas of high atmospheric pressure?

A) The equator

B) The poles

C) The middle latitudes

D) The tropics

GS Mains Questions and Model Answers

Q1: Explain the concept of atmospheric pressure and how it influences weather patterns.

Answer: Atmospheric pressure refers to the weight of the air above the Earth's surface, which exerts a force on everything it contacts. The standard atmospheric pressure at sea level is 1013.25 hPa or 1 atm. Atmospheric pressure decreases with altitude, as there is less air above a given point. Weather patterns are significantly influenced by atmospheric pressure. Low-pressure areas are typically associated with rising air, which cools and condenses to form clouds, leading to precipitation and stormy weather. Conversely, high-pressure areas are characterized by descending air, which warms and dries, preventing cloud formation and leading to clear, calm weather. The movement of air between regions of high and low pressure creates winds, further influencing weather systems. Thus, the distribution of high and low-pressure zones is crucial in shaping the global and local climate.

Q2: How does atmospheric pressure affect human activities, particularly in aviation and meteorology?

Answer: Atmospheric pressure has a significant impact on human activities, especially in aviation and meteorology. In aviation, aircraft rely on atmospheric pressure for altitude calculations and airspeed measurements. As altitude increases, atmospheric pressure decreases, which affects the performance of aircraft engines and the efficiency of their flight. Pilots must adjust for changes in pressure, which are measured by altimeters, to maintain the correct altitude.

In meteorology, atmospheric pressure plays a critical role in forecasting weather. High-pressure systems are typically associated with clear skies and stable weather, while low-pressure systems indicate potential storms and unsettled weather. Meteorologists use pressure data from barometers to track the movement of air masses and predict weather changes. Sudden changes in atmospheric pressure can also be indicators of severe weather events like cyclones or tornadoes, making pressure a key factor in weather prediction.

Q3: Discuss the significance of atmospheric pressure in the formation of winds and their impact on global weather systems.

Answer: Atmospheric pressure plays a vital role in the formation of winds, which in turn influence global weather systems. Winds are created by differences in atmospheric pressure; air moves from areas of high pressure to areas of low pressure. This movement of air is driven by the pressure gradient force, where the greater the difference in pressure over a given distance, the stronger the wind. Local wind patterns are created by smaller-scale pressure differences, such as sea breezes or mountain winds.

On a global scale, the uneven heating of the Earth's surface creates large-scale pressure systems, such as the trade winds, westerlies, and polar easterlies. These winds are driven by the temperature differences between the equator and the poles, which lead to the formation of high and low-pressure belts around the Earth. The movement of air masses between these pressure systems is responsible for the transfer of heat and moisture, significantly affecting global weather patterns, including precipitation and temperature distribution. For example, the trade winds help to carry moisture from the oceans to land, influencing rainfall patterns in tropical regions. Thus, atmospheric pressure and the resulting winds are fundamental to the Earth's climate system and play a key role in weather formation.

Previous Year Questions on Atmospheric Pressure

1. UPSC CSE 2020

Question: "Explain how atmospheric pressure affects the weather, and discuss its role in the formation of winds."

Answer: Atmospheric pressure plays a crucial role in determining weather patterns and the formation of winds. High-pressure systems, where air is descending, are generally associated with clear skies and calm weather, as the sinking air prevents cloud formation. Conversely, low-pressure systems, where air is rising, lead to the cooling and condensation of moisture, resulting in cloud formation and precipitation. The movement of air from areas of high pressure to low pressure causes winds, with the strength of the wind depending on the difference in pressure (the pressure gradient). These winds are essential in transferring heat and moisture around the planet, influencing both local and global weather systems. The interaction of pressure systems creates weather patterns, such as cyclones, which can lead to severe weather conditions.

2. UPSC CSE 2019

Question: "Discuss the relationship between atmospheric pressure and weather forecasting. How do meteorologists use pressure data in predicting weather?"

Answer: Atmospheric pressure is an essential parameter in weather forecasting, as it is closely related to weather patterns. Meteorologists use barometers to measure atmospheric pressure and track changes over time. A sudden drop in atmospheric pressure often signals the approach of a low-pressure system, which is typically associated with stormy or rainy weather. Conversely, a rise in pressure indicates the approach of a high-pressure system, which usually brings clear, dry weather. By monitoring the movement of pressure systems, meteorologists can predict changes in weather conditions. For instance, the approach of a cyclone is often preceded by a rapid decrease in pressure, allowing meteorologists to issue warnings in advance. By tracking pressure data over large areas, meteorologists are able to create weather forecasts that help in planning and preparedness for different weather conditions.