Maarg Manthan · Topic 5.7

Atmospheric Pressure: Vertical and Horizontal Distribution

Geography › Climatology · Topic 5.7

Atmospheric pressure is the weight of the column of air above a place, measured in millibars (hPa); the average sea-level value is 1,013.25 mb. It falls quickly with height, by about 1 mb for every 10 metres near the surface. Horizontally, pressure varies with temperature: warm air rises and gives low pressure, cold air sinks and gives high pressure. The result is a world pattern of alternating belts: the equatorial low, subtropical highs, subpolar lows and polar highs.

6 min read · UPSC Prelims · Geography · Climatology

At a Glance

Sea-level pressure 1,013.25 mb
Vertical change Falls about 1 mb per 10 m near the surface
Equatorial belt Low pressure (doldrums)
Subtropical belts High pressure (horse latitudes)
Subpolar belts Low pressure
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the study of temperature and introduces the pressure that results from it. It supplies the pressure belts that cause the winds studied in the topics that follow. Previous topic: Inversion of Temperature. Next topic: Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force.

What Is Atmospheric Pressure?

In short: air has weight, and atmospheric pressure is the force this weight exerts on a surface.

The air around the Earth is held by gravity and has weight. Atmospheric pressure is the weight of the column of air above a unit area of the Earth’s surface, extending to the top of the atmosphere. At sea level the average pressure is 1,013.25 millibars, or 1,013.25 hectopascals (the two units are equal), which equals the pressure of a column of mercury 760 mm high, or 29.92 inches, as first shown by Torricelli in 1643. Pressure is measured with a barometer. A mercury barometer balances the weight of air against a column of mercury, and an aneroid barometer uses a sealed metal box that changes shape with pressure. The meteorological unit is the millibar, and in the modern SI system it is the hectopascal. A rising barometer usually predicts fair weather, and a falling one predicts rain or storms.

How Does Pressure Vary with Height?

Pressure decreases rapidly with altitude, since the air above becomes less and the air is compressed by gravity near the surface, so the air is densest at the bottom. Near the surface the pressure falls by about 1 millibar for every 10 m of height, but the rate slows at higher levels. About half of the atmosphere’s mass lies below 5.6 km, and about 99 per cent below 32 km.

Height	Approximate pressure
Sea level	1,013 millibars
About 5.5 km	About 500 millibars
About 8.8 km (Mount Everest)	About one-third of sea level pressure, around 337 millibars
About 16 km	About 100 millibars
About 50 km	About 1 millibar

This is why climbers need oxygen at great heights: the percentage of oxygen is the same, but the pressure, and therefore the amount of oxygen taken in with each breath, is much less. The vertical decrease of pressure is much greater than any horizontal variation, which is why weather maps use pressure reduced to sea level, so that the effect of altitude does not hide the differences caused by weather.

What Causes Horizontal Differences of Pressure?

Temperature: warm air expands, becomes lighter and rises, producing low pressure at the surface; cold air is denser and sinks, producing high pressure. This is why the equator has low pressure and the poles have high pressure.
Water vapour: humid air is lighter than dry air because water vapour molecules are lighter than nitrogen and oxygen, so moist air has lower pressure. Low pressure is therefore linked with clouds, humidity and rain.
Rotation of the Earth and the movement of air: subtropical high pressure belts are caused by the sinking of air, which is a dynamic effect, as explained in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.
Gravity: varies slightly with latitude, as in Motions of the Earth: Rotation and Revolution and Their Effects.

The highest pressure ever recorded at sea level was 1,085.7 millibars, at Tosontsengel in Mongolia in December 2001, and the lowest was about 870 millibars in the eye of Typhoon Tip in 1979, according to the World Meteorological Organization.

What Are Isobars and the Pressure Gradient?

Isobars are lines drawn on a map that join places having the same pressure, reduced to sea level. A map with isobars shows the pressure systems.

Feature on the map	Meaning
Closed isobars with pressure falling inward	A low pressure area (cyclone); air spirals in and rises, so cloud and rain
Closed isobars with pressure rising inward	A high pressure area (anticyclone); air sinks and spirals outward, so clear weather
Closely spaced isobars	Steep pressure gradient; strong wind
Widely spaced isobars	Weak gradient; gentle wind or calm

The pressure gradient is the rate of change of pressure with distance, and it is the force that starts the winds, as described in Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force. A wind always blows from high pressure to low pressure, because air tries to equalise the difference. As an analytical point, India’s own weather forecasts rely on isobar maps: the heat low over northwest India in summer, which draws in the monsoon, is a low-pressure area identified on such maps.

What Are the Pressure Belts of the World?

In short: if the Earth were uniform, the pressure would form seven alternating belts of low and high pressure between the equator and the poles.

Belt	Latitude	Origin	Features
Equatorial low pressure belt	About 5 degrees north to 5 degrees south	Thermal: intense heating makes air rise	Calms and light variable winds, called the doldrums; heavy convectional rain; the Intertropical Convergence Zone lies here
Subtropical high pressure belts	About 30 to 35 degrees north and south	Dynamic: air that rises at the equator cools and sinks here	Calm, dry, clear weather; the “horse latitudes”; the great deserts lie beneath
Subpolar low pressure belts	About 60 to 65 degrees north and south	Dynamic: air is drawn away by rotation and circulation	Stormy weather; meeting of warm and cold air; temperate cyclones
Polar high pressure belts	About 90 degrees north and south	Thermal: very cold dense air sinks	Cold, dry, outblowing winds

The name doldrums comes from sailors who found sailing ships becalmed in the weak winds there. The horse latitudes are said to be named after the horses that were thrown overboard from sailing ships stranded in the calm to save water. The vocabulary is part of the standard Prelims questions on the world pressure system.

Why Is This Topic Important?

Foundation Cause of winds
Numbers Units and values asked
Weather maps Isobar interpretation

What Came Next?

The next topic explains the forces that act on the wind: pressure gradient, friction and the Coriolis force. Next topic: Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force.

Key People and Terms

Barometer Instrument to measure pressure
Millibar Unit equal to one hectopascal
Isobar Line of equal pressure
Pressure gradient Change of pressure with distance
Cyclone Low pressure system
Anticyclone High pressure system

Exam Corner

Points to Remember

Sea level pressure is 1,013.25 millibars.
1 millibar equals 1 hectopascal.
Pressure falls about 1 millibar per 10 m near the surface.
Wind blows from high to low pressure.
Closely spaced isobars mean strong wind.

Do Not Confuse With

Isobar and isotherm Pressure versus temperature
Cyclone and anticyclone Low versus high pressure
Pressure and density Weight of air versus mass per volume

Memory Hook

A thousand and thirteen at sea; ten metres up, one less; high to low the wind will go.

Mains Angle

Questions ask for pressure factors and weather. These points add depth.

Vertical Falls fast with height
Horizontal Temperature and moisture
Winds Gradient drives them

A Question You May Face

An original practice question, not a past paper question.

“Explain the factors that cause variations in atmospheric pressure and show how they lead to the movement of air.”

How to Answer

Define Pressure and units.
Vertical Fall with height.
Horizontal Temperature, vapour.
Wind Gradient force.

GS Relevance

Prelims: pressure at sea level, vertical variation, isobars, pressure belts. GS Paper 1: atmospheric circulation.

Frequently Asked Questions

What is atmospheric pressure?

Atmospheric pressure is the weight of the column of air above a unit area of the Earth's surface. At sea level it averages 1,013.25 millibars, or hectopascals, equal to 760 mm of mercury, and it is measured with a barometer.

How does pressure change with height?

Atmospheric pressure decreases rapidly with height, because the air above becomes less and the air is densest near the surface. Near the ground it falls by about 1 millibar for every 10 m, and by about 5.5 km only half the sea level pressure remains.

What are isobars?

Isobars are lines on a weather map joining places with the same atmospheric pressure, reduced to sea level. Closed isobars show high or low pressure systems, and the spacing of isobars shows the pressure gradient: close isobars mean strong winds, wide ones mean light winds.

Why does warm air cause low pressure?

Warm air expands, becomes less dense and rises, which reduces the weight of air over the surface and creates low pressure. Cold air is denser and sinks, producing high pressure. That is why the hot equator has low pressure and the cold poles have high pressure.

Why is humid air associated with low pressure?

Humid air has a lower density than dry air at the same temperature, because water vapour molecules are lighter than the nitrogen and oxygen they replace. Moist air therefore tends to rise, which gives low pressure at the surface, and cloud and rain are common.

PYQ Practice — Statement Analysis

1 Atmospheric pressure decreases with increasing altitude.

True

The air above becomes less.

2 Winds blow from low pressure to high pressure.

False

They blow from high to low.

3 One millibar equals one hectopascal.