Atmospheric Pressure, Isobars and Vertical Variation
Geography › Pressure, Winds and Atmospheric Moisture · Topic 7.1
Atmospheric pressure is the weight of the column of air above a unit area, about 1,013.25 millibars or hectopascals at sea level, measured with a barometer. It falls rapidly with height, by about 1 millibar for every 10 m near the surface, and varies horizontally with temperature, water vapour and rotation. Isobars join places of equal pressure, and closely spaced isobars mean strong winds.
5 min read · UPSC Prelims · Geography · Pressure, Winds and Atmospheric Moisture
At a Glance
- Sea level pressure 1,013.25 millibars (hPa), 760 mm of mercury
- Instrument Barometer
- Isobars Lines of equal pressure
- Vertical change About 1 millibar per 10 m near the surface
- Half of the pressure Lost by about 5.5 km
- Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture
Where Does It Fit?
This is the first topic of the category on pressure, winds and atmospheric moisture. The previous category covered the atmosphere and its temperature, and this one begins with air pressure, the cause of winds. The next topic covers the pressure belts of the world. Previous topic: Vertical Distribution of Temperature and Temperature Inversion. Next topic: Pressure Belts and Pressure Cells.
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 Pressure Belts and Pressure Cells.
- Gravity: varies slightly with latitude, as in Rotation of the Earth and Its 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 Forces Affecting Wind Movement. 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.
Why Is This Topic Important?
- Foundation Cause of winds
- Numbers Units and values asked
- Weather maps Isobar interpretation
What Came Next?
The next topic describes the pressure belts and pressure cells. Next topic: Pressure Belts and Pressure Cells.
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
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.
The air above becomes less.
2 Winds blow from low pressure to high pressure.
They blow from high to low.
3 One millibar equals one hectopascal.
They are the same unit.
4 Closely spaced isobars indicate a weak pressure gradient.
They indicate a steep gradient and strong winds.
Related Topics
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