Atmospheric Pressure, Definition, Measurement & Distribution

Atmospheric Pressure Definition

Atmospheric Pressure is the force exerted by the weight of these gas molecules on a unit of area on the earth’s surface. The pressure exerted by the atmosphere is about 14.7 pounds per square inch on the earth’s surface. Atmospheric pressure is highest at sea level and decreases as you go higher. This means there are fewer air molecules at high altitudes compared to low altitudes. We can measure atmospheric pressure using a barometer, which shows the height of a mercury column that balances the weight of the air above it. There are five units to measure atmospheric pressure:

• Millimeters (or inches) of mercury
• Pounds per square inch (psi)
• Dynes per square centimeter
• Millibars (Mb)
• Kilopascals

Read More: Pressure Belts

Atmospheric Pressure Measurement

A barometer is used to measure atmospheric pressure, which helps predict the weather. Meteorologists rely on barometers to see short-term changes. Atmospheric pressure is measured in units called atmospheres or bars. At sea level and 15 degrees Celsius, the average pressure is one bar (or one atmosphere, 1 atm). When atmospheric pressure drops, it indicates a low-pressure system, leading to cloudy, rainy, or windy weather. Conversely, when pressure rises, it means the clouds are clearing, resulting in bright skies and cool, dry air.

Read More: Heat Zones of Earth

Atmospheric Pressure Vertical Distribution

The distribution of atmospheric pressure in a columnar manner is known as the Vertical Distribution of Pressure. There is a decrease in air pressure with an increase in altitude but may not follow the same rate of reduction. Air pressure normally decreases by about 34 millibars for every 300 meters you go up in altitude. As you climb higher in the mountains, the air pressure drops, which is why it can take longer to breathe. The air above compresses the air below it, making the lower layers denser than the upper layers. This means that the lower parts of the atmosphere have higher density and exert more pressure.

Read More: Heat Transfer Methods

Atmospheric Pressure Horizontal Distribution

In common terms, the distribution of atmospheric pressure over the Earth’s surface refers to the Horizontal Distribution of Pressure. Atmospheric pressure around the Earth can be shown on maps using isobars. Isobars are imaginary lines that connect points with the same pressure. The distance between these lines shows how quickly and in what direction air pressure changes. When isobars are close together, it means there’s a steep pressure gradient; when they are far apart, the gradient is gentle.

The horizontal distribution of atmospheric pressure is not uniform across the world. It varies across time and places. The major factors influencing horizontal pressure are:

• The Temperature of the Air
• The Rotation of the Earth
• Presence of Water Vapour

Read More: Structure of the Atmosphere

Temperature of Air

Earth does not heat up uniformly because of unequal distribution of insolation. Due to differential heating and cooling of land and water surfaces. The inverse relationship between air temperature and air pressure within the atmosphere leads to regional variations in pressure. The hotter the air temperature, the lower the air pressure and vice-versa in an open system. The equatorial low-pressure system is considered to have formed because of the thermal heating and resulting pressure variations.

Rotation of the Earth

The centrifugal force generated due to the Earth’s rotation results in the deflection of air from its original place thereby causing a reduction in the pressure difference. The rotation of the Earth creates low-pressure areas in the subpolar regions and high-pressure areas in the subtropical regions. This rotation causes air to converge (come together) and diverge (spread apart), leading to low and high pressure, respectively.

Presence of Water Vapour

The presence of water vapour in higher quantities in the air results in low pressure while water vapour in lower quantities results in high pressure. Continents are cool in winter, thus the air has a lower capacity to hold water vapour thus developing high-pressure centres. In summer the continents are warmer and tend to form low-pressure centres as hot air has a higher capacity to hold water vapour or moisture.

The horizontal distribution of air pressure across the latitudes is characterized by high or low-pressure belts. This is however a theoretical model because they are not always found on Earth in the way it has been theorized.

Read More: Isotherms

Atmospheric Pressure Factors Influencing

The pressure, temperature, and density of a gas are all related to each other.

The Ideal Gas Law

The ideal gas law, expressed as P=nRT, explains the relationship between pressure (P), density (n), and temperature (T). It means that if density stays the same and temperature increases, pressure will rise. Similarly, if temperature remains constant and density increases, pressure will also rise. In a closed container, this relationship is simple, but the atmosphere isn’t closed, making these relationships more complex. The equation helps us understand how pressure changes with density, temperature, and the movement of air.

Read More: Insolation

Pressure and Density Relationships

Density is the mass of matter in a specific volume. While solid density remains constant everywhere (like on Earth or the Moon), liquid density can change. Gas density varies a lot depending on where it is because gases can expand easily based on surrounding pressure.

Gas pressure is inversely related to density: higher pressure means denser gas. Gravity keeps the atmosphere close to Earth, stopping gas molecules from escaping. At lower altitudes, gas molecules are packed more closely together, leading to higher pressure. At higher elevations, the air is less dense, so the pressure decreases.

Read More: Weathering

Pressure and Temperature Relationships

When air is warmed, the molecules gain energy and move faster. This increased speed leads to more collisions, which raises the pressure. So, if everything else stays the same, higher gas temperatures mean higher pressure. That’s why air pressure tends to be higher on cold days and lower on warm days. However, warm air usually has low pressure, while cool air has high pressure. This happens because warmer air is less dense, which can lead to lower pressure despite the increase in temperature.

Read More: Physical Weathering

Dynamic Influences on Pressure

Surface air pressure can also be affected by “dynamic” factors, like the movement of air. When air is heated at the surface, it rises because it becomes less dense, creating a vacuum below. This vacuum pulls in surrounding air to fill the space, causing air to converge near the warm surface. As the warm air rises, it cools and condenses, making it heavier and increasing its density. This condensation causes air to diverge. Generally, when air descends, it creates high pressure at the surface, while rising air leads to low pressure.

In short, differences in air density, temperature, and movement all have an effect on atmospheric pressure. It is important to understand these linkages, but it is often difficult to predict how a change in one variable will influence the others in a specific instance. Nevertheless, some useful generalizations about the factors associated with high-pressure and low-pressure areas near the surface can be made:

• Strongly descending air produces high pressure at the surface—a dynamic high.
• Very cold surface conditions produce high pressure at the surface—a thermal high.
• Strongly rising air leads to low pressure at the surface—a dynamic low.
• Very warm surface conditions result in relatively low pressure at the surface—a thermal low.

Surface pressure conditions usually can be traced to one of these factors being dominant.

However, in reality, the pressure distribution across the globe is not uniform. They vary from region to region and from time to time. In order to understand the distribution of the pressure belts and the factors influencing them, we can categorize the study of pressure belts as a horizontal component and a vertical component.

Read More: Mass Movement

Atmospheric Pressure UPSC

Air pressure is an important factor in weather, particularly when it comes to creating or changing atmospheric conditions. It is also one of the key variables used to produce accurate weather forecasts. The pressure created by the weight of the air in the Earth’s atmosphere is known as air pressure. It is also known as barometric pressure, after the instrument that is used to measure air pressure.

Air has weight because it is not empty, even if it is not visible. It contains small particles of nitrogen, oxygen, argon, carbon dioxide, and other gases. Because of the gravitational force of the Earth, the weight of the particles in the air creates pressure. The barometer is a device that measures air pressure. This article will discuss an important phenomenon known as Atmospheric Pressure in the context of the UPSC IAS Exam.

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