Global Air Pressure

The atmospheric pressure is also known as barometric pressure, which is the pressure within the atmosphere of Earth. The standard atmospheric pressure symbol is atm and atm is defined as the unit of atmospheric pressure having a value of 101,325 Pa, here Pa stands for Pascal. The other values of Global Atmospheric Pressure are 1,013.25 hPa and 1,013.25 mbar where these values are equal to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi.

With the help of this page, you gain an in-depth understanding of what are pressure belts, along with this, you will get to know the list of idealized global air pressure belts across the globe.  Also, this page will throw light on the recent highlights on idealized global air pressure belts across the globe.

What are Pressure Belts?

The horizontal pressure turbulence that is identically spread across the earth's atmosphere, i.e., just above the earth's surface due to seasonal and spatial variations of energy received by the earth at different places are called pressure belts. A low-pressure belt stretches from 0 to 5° North and South of the Equator.

Pressure Belts Of The Earth: Important Points On Understanding Of How It Occurs

• A column of air generates weight in the form of pressure on the earth’s surface.

• This weight of the air column at a given place and time is what we call air pressure or atmospheric pressure.

• Now, we measure the atmospheric pressure by an instrument called a barometer.

• The unit for measuring the atmospheric pressure is force per unit area. Other than this, the unit used for measuring pressure in millibar, where one millibar is equivalent to the force of around one gram per square centimeter.

Factors Controlling The Pressure System Across The Earth’s Surface

Now, let us go through the factors that Control The Pressure System Across The Earth’s Surface:

• Thermal Factors 

When air is heated, it expands, as a result, its density decreases. Naturally, this leads to low pressure. On the other hand, cooling causes a contraction. Therefore, contraction leads to an increase in the density, and therefore, leads to high pressure.

• Thermal Facts Reason

The results of thermal lows and highs are the formation of equatorial low and polar highs, respectively.

• Dynamic Factors

Apart from temperature variations, the formation of pressure belts can be described by dynamic controls forming out of pressure gradient forces and the Earth’s rotation (also called the Coriolis force).

Now, What is Pressure Gradient? Let us understand it:

What Is Pressure Gradient?: The Relationship With The Atmospheric Pressure

The pressure gradient is the change of atmospheric pressure between two points on the earth’s surface per unit of time.

(Image Will be Updated Soon)

On the weather chart, the pressure gradient is marked by the spacing of isobars.

Close spacing of isobars pertains to a strong pressure gradient, however, a wide spacing shows there is a weak pressure gradient.

Vertical Distribution Of The Air Pressure

The vertical distribution is the columnar distribution of atmospheric pressure where the mass of air above the air column compresses the air under it, as a matter of fact, its lower layers become denser than the upper layers.

Now, as the lower layers of the atmosphere reach a higher density, they exert more pressure.

On the other hand, the higher layers are less compressible because of which they have low density and low pressure.

The air pressure of a given place and at a given time can be determined by the following three factors:

• The temperature of the air, 

• The amount of water vapor available in the air, and 

• The gravitational pull of the earth

Since the above three factors vary with a change in height, therefore, we find a variation in the rate of decrease in air pressure with an increase in altitude.

Also, rising pressure signifies fine, settled weather while descending pressure indicates unstable and cloudy weather.

Horizontal Distribution Of The Air Pressure

The following three factors are responsible for the variation in the air pressure:

• Air Temperature

• Rotation of the Earth, and

• The pressure of water vapor

Air Temperature

Earth never gets heated uniformly because of unequal distribution of insolation, distinctive heating, and cooling of land and water surfaces.

In equatorial regions, the air pressure remains low, while higher in polar regions.

As a matter of fact, low air pressure in equatorial regions occurs when hot air rises there with a gradual decrease in temperature thereby causing thinness of air on the surface.

However, in the polar region, cold air is very dense hence it falls, and there is a pressure rise.

The Earth’s Rotation

• The earth’s rotation produces a centrifugal force (an apparent outward force on a mass when it is rotated).

• The force generates deflection of air from its original place, leading to a decrease in pressure.

• The Earth’s rotation created low-pressure belts of the subpolar regions and the high-pressure belts of the sub-tropical regions.

The Presence of Water Vapour

• Air, carrying a large amount of water vapor, has lower pressure and that with a lower quantity has higher pressure.

List Of Idealized Global Air Pressure Belts Across The Globe

Below, you can find the four idealized global air pressure belts across the globe:

• One Equatorial Low

• The two Sub-tropical Highs

• The two Sub-polar Lows

• The two Polar Highs

(Image Will be Updated Soon)

Equatorial Low-Pressure Belts

Equatorial Low-Pressure Belts are low-pressure belts that extend from 0 to 5° North and South of the Equator.

However, because of the vertical rays of the sun in the equatorial region, there is intense heating. As a result, air expands and rises as convection currents lead to the development of low pressure. We also call this low-pressure belt a doldrums because equatorial regions are a zone of total calm without any breeze.

Sub-tropical High-Pressure Belts

At about 30°North and South of the Equator, there lies an area where the ascending equatorial air currents fall. This area is of high pressure, also called the area of the Horse latitude.

In this region, the wind blows from high pressure to low pressure. Therefore, the winds from the subtropical region shift towards the Equator as Trade winds, and other wind blows towards Sub-Polar Low-Pressure as Westerlies.

Pressure Belts In July

• During summers, in the northern hemisphere, with the apparent northward shift of the sun, the thermal equator (the temperature belt) is located to the north of the geographical equator.

• Also, rising pressure signifies fine, stable weather while descending pressure points towards unstable and cloudy weather.

(Image Will be Updated Soon)

Pressure Belts In January

During winters, the above conditions reverse, i.e., the pressure belts move south of their annual mean locations. Opposite conditions occur in the southern hemisphere. The amount of shift is, however, lower in the southern hemisphere because of the prevalence of water.

Likewise, the distribution of continents and oceans has a marked effect on the distribution of pressure. Since in winters, the continents are cooler than the oceans and they develop high-pressure centers, while, in summers, they are relatively warmer and generate low pressure, which is just the reverse with the oceans.

(Image Will be Updated Soon)

Circum-Polar Low-Pressure Belts

• Circum-Polar Low-Pressure Belts are positioned between 60° and 70° in both the hemispheres.

• In the Sub-tropical region, the falling air gets divided into the following two parts:

• The first part storms towards the Equatorial Low-Pressure Belt, while the other towards the Circum-polar Low-Pressure Belt.

• This zone is affected by the ascent of warm Sub-tropical air over cold polar air blowing from poles. As a result of the Earth’s rotation, the winds surrounding the Polar region blow towards the Equatorial region.

• Centrifugal forces functioning in this area generate the low-pressure belt, which is called the Circum-polar Low-Pressure Belt, where this region is marked by violent storms in winter.

Polar High-Pressure Areas

• Polar high-pressure areas are at the North and South Poles, stretching between 70° to 90° North and South, the temperature at this place is extremely low.

• The cold falling air results in the rising of high pressures across the Poles.

• These high-pressure polar areas are known as the Polar Highs and are characterized by permanent IceCaps.

The Coriolis Effect

Whenever the air tries to move from high to low pressure in the atmosphere, there is an effect that diverts the air so that it follows the pressure contours and this effect is the Coriolis Effect.

The Coriolis deflection affixes the prime constraint on how many cells the planet’s atmosphere divides into. Coriolis force is stronger and is preferred for more rapid rotation. It is the size of the planet and rotational speed (to a lesser extent, the depth of the atmosphere) that determines how many of these. Earth’s atmosphere divides into 3 cells.

For Jupiter, it is much more, as it is twelve times larger in diameter and yet has a day only 12 hrs long. Coriolis Force is very strong.

Atmospheric Circulation

Atmospheric Circulation is the wide-reaching air movement and together with ocean circulation is the means by which thermal energy is reshifted on the Earth’s surface.

The two types of atmospheric circulation are as follows:

• Latitudinal circulation – The wind belts circumscribing the planet are organized into three cells in each hemisphere:

•        The Hadley cell, 

•        The Ferrel cell, 

•        The polar cell. 

These cells persist in both the northern and southern hemispheres.

• Longitudinal circulation – Latitudinal circulation is a.k.a the Walker circulation occurs as a result of the highest solar radiation per unit area (or the solar intensity) falling on the tropics. The solar intensity decreases with the increase in latitude and reaches zero at the poles. Longitudinal circulation, thereby, happens because of the heat capacity of water, its absorptivity, and its mixing. 

• Since water absorbs more heat than the land does, but its temperature does not rise as exponentially as does the land. Furthermore, temperature variations on land are higher than on the water.

Idealized Global Air Pressure Belts Across the Globe: Facts and Information

• The average air pressure above sea level is around 14.7 pounds per square inch or 14.7 psi.

• According to Vanos, most people feel comfortable with a barometric pressure of 30 inches of mercury (inHg). However, when the pressure rises to 30.3 inHg or above, or descends to 29.7 or lower, the heart attack risk increases.

• The moving of pressure belts happens when there are systematic differences in the Earth's land temperature that affect the air pressure, and also the significant patterns of pressure that exist over time are called pressure belts/wind belts. Wind belts rely on the temperature, so any change in temperature can shift the belts and also change wind patterns.