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. This value decreases with increasing altitude. i.e. with increasing altitude, the atmosphere becomes thinner, and the pressure reduces. Though the atmosphere exerts pressure on every surface, we are not sensitive to this ever-present burden of pressure. This is because our bodies have evolved to this pressure.

Read More: Pressure Belts

Atmospheric Pressure Measurement

To measure the pressure exerted by the atmosphere, an instrument called a “Barometer” is used. The atmospheric pressure is an indicator of weather. Hence, meteorologists use barometers to predict short-term changes in the weather.

A barometer is a device that measures atmospheric pressure in units known as atmospheres or bars. The average air pressure at sea level at 15 degrees Celsius is equal to one bar, sometimes known as one atmosphere, or 1 atm.

A drop in Atmospheric Pressure indicates a low-pressure system has been initiated. Low pressure indicates that there is not enough force or pressure to push the clouds or the storm away. Thus cloudy, rainy or windy weather prevails. When the atmospheric pressure rises, it means the clouds have been driven out, allowing for a bright sky and the arrival of 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. The normal rate of decrease in air pressure is calculated to be 34 millibars per 300 meters increase in altitude. With increasing altitudes in the mountains the pressure decreases, this can be explained by the increased time taken by rice and difficulty in breathing at higher altitudes.

The mass of air above the air in a column compresses the air under it hence lower layers are denser than the upper layers. Therefore, lower layers of the atmosphere have higher density and exert high 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. The distribution of atmospheric pressure over the Earth can be represented on the maps with the help of the isobars. “Isobars” are imaginary lines that connect points having equal values of pressure. The spacing in the isobars determines the rate and direction of change in air pressure. The change in air pressure is also called the pressure gradient. Closely spaced pressure gradients indicate steep pressure gradients while wide spacing indicates gentle pressure gradients.

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 low-pressure belts of the subpolar regions and the high-pressure belts of the subtropical regions are formed because of Earth’s rotation. The Earth’s rotation also results in convergence and divergence of air. Convergence and divergence result in 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 relationship between pressure, temperature, and density can be summarized by an equation called the ideal gas law i.e., P = nRT where P is pressure, n is density, R is the constant of proportionality, and T is temperature. This equation says that pressure “P” will increase if density remains constant, but temperature “T” increases, and that pressure will increase if the temperature remains constant, but density “n” increases.

The relationship between pressure, temperature, and density is straightforward within a closed container (a sealed jar, for example). However, the atmosphere is not a closed container in which any of these variables can be easily held constant and so the cause-and-effect relationships are actually quite complex. The above equation can help us examine some of the possibilities while checking how pressure varies with density, temperature, vertical movement of air, etc.

Read More: Insolation

Pressure and Density Relationships

The mass of matter in a unit volume is known as density. While the density of solids is constant throughout space (whether on Earth, the Moon, or somewhere else), the density of liquids fluctuates. The density of gases varies greatly with the location of the fluids. Gas density changes easily because gas is free to expand as far as the environmental pressure allows.

A gas’s pressure is inversely related to its density. The greater the pressure, the denser the gas. Gravity holds the atmosphere to the Earth, preventing gas molecules from escaping into space. The gas molecules in the atmosphere are more densely packed together at lower altitudes. Because the density is higher, there are more molecular collisions and higher pressure at lower altitudes. Since the air is less dense at higher elevations, the pressure also decreases.

Read More: Weathering

Pressure and Temperature Relationships

If air is warmed, the molecules become get energized, (the kinetic energy of the molecules increases with increasing temperature), and their speed increases. This increase in speed in the molecules leads to an increased number of collisions thus resulting in the development of greater pressure. Therefore, if other conditions remain the same, an increase in gas temperature produces an increase in pressure. Thus,  the air pressure will be high on warm days and low on cold days.

However, such is not usually the case; warm air is generally associated with low atmospheric pressure and cool air with high atmospheric pressure. As a result of the decrease in density, the increase in temperature may be accompanied by a decrease in pressure.

Read More: Physical Weathering

Dynamic Influences on Pressure

Surface air pressure can be influenced by “dynamic” factors as well. In other words, air pressure may be influenced by the movement of the air—especially the vertical movement of air. Air when heated up at the surface, tends to rise up (as the density of air reduces) and a vacuum is created. The vacuum created by the rising air is then occupied by the surrounding air which rushes in to fill the void. This triggers a convergence of air which can be seen near the warm surface. The warm air that rises up starts condensing as it climbs up the altitude.

The condensation of air in the atmosphere makes the air heavy (as the density of air increases) and the divergence of air triggers. Thus, the air is associated with different rates of air convergence and divergence at the surface and in the upper troposphere. Generally, descending air leads to relatively high pressure at the surface and ascending air leads to relatively low pressure at the surface.

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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